CHAPTER XXX.

II. MEDULLOSEAE.

The term Medulloseae was first employed by Goeppert and Stenzel[236] for a family of Palaeozoic plants that appears to have reached its maximum development in the Permian period: the oldest representatives so far discovered are of Upper Carboniferous age. Our knowledge of the family is chiefly derived from a study of the anatomical characters of stems, and it is therefore on this basis that any grouping of genera or species should be attempted. Although there is little information with regard to the reproductive organs of _Medullosa_, the type-genus, it is certain that the Medulloseae are Pteridosperms differing from members of that group included in the Lyginopterideae in the presence of more than one stele in the stem, in the habit of the fronds, and in the structure of the rachis, as also in the structure of the seeds, though these organs bear a fairly close resemblance to the seeds of _Lyginopteris_ and _Heterangium_. The fronds of the Lyginopterideae are of the _Sphenopteris_ type while in the case of such species of _Medullosa_ as afford evidence of connexion between stems and leaves the latter have the characters of _Neuropteris_, _Alethopteris_, _Odontopteris_, _Linopteris_, and other form-genera usually included in the Neuropterideae. Dr Lotsy[237] speaks of _Lyginopteris_ and _Heterangium_ as members of the Sphenopteridophylla and assigns species of _Medullosa_ either to the Neuropteridophylla or to the Pecopteridophylla, the latter subdivision including species with fronds of the _Alethopteris_ type. There is, however, little doubt that other forms of leaves, such as _Odontopteris_ and possibly _Taeniopteris_, were borne on Medullosan stems. It is undesirable except in the absence of more trustworthy criteria to make use of so protean a feature as leaf-form as a basis of classification. The name Neuropterideae has been frequently employed for Pteridosperms other than the Lyginopterideae on the ground that the foliage of _Medullosa_ is represented by species assigned to form-genera included in the Neuropterideae. It is, however, preferable to restrict the family-name Neuropterideae to fronds and to speak of the second family of Pteridosperms as the Medulloseae, including the genera _Medullosa_, _Sutcliffia_, and _Rhexoxylon_.

=MEDULLOSA.=

Some species of _Medullosa_ probably resembled in habit _Angiopteris evecta_ and the larger Marattias; they had short and relatively thick stems clothed with the large decurrent bases of long compound fronds superficially like those of some recent Ferns and the leaves of the Cycad _Bowenia_. It is probable that, as Zeiller[238] has pointed out, the fronds of _Medullosa_ and of other Pteridosperms had a greater tendency than those of true Ferns to a dichotomy of the rachis. In other types the stems reached a considerable length and leaves and branches were separated by several feet of bare stem. The large size of the leaf-stalks in proportion to the diameter of the stem as shown by such species as _Medullosa anglica_ and _M. Leuckarti_ (fig. 416) suggests either a short and thick main axis or, in the case of long stems bearing scattered leaves, a plant that supported itself partially at least by a habit of growth comparable with that of tropical Aroids or other lianes. While _Medullosa anglica_ with its contiguous leaf-bases affords an example of the first type, the occurrence of stems of a Permian species, _M. stellata_, 3½ metres long without branches or leaf-scars, suggests the habit of a liane; similarly a specimen of _Medullosa Leuckarti_ in the Chemnitz Museum bearing a few spreading petioles but little narrower than the stem and given off at a wide angle would seem to favour the view that some species were ill adapted to be mechanically self-supporting plants. The longest piece of stem that has come under my notice is a specimen of _M. stellata_ in the Chemnitz Museum reaching a length of nearly 8 metres: some species attained a diameter of about 50 centimetres.

_Medullosa_ is always polystelic: the plan of the vascular system varies considerably as regards both the number and form of the steles, but there is a uniform type of structure within the limits of each stele that recalls the single stele of _Heterangium_. The steles consist of a central region composed of primary xylem, originally surrounded by phloem, which in its mesarch or exarch structure agrees with the vascular tissue of some species of _Gleichenia_ or _Lygodium_. To this central region a cambium added secondary xylem and phloem either in the form of a cylinder of uniform breadth, or more frequently the centrifugally developed xylem exceeded in amount the secondary conducting tissue added to the inner side of the primary region. Apart from anatomical details a Medullosan stem with its several steles, each with secondary tissue, embedded in parenchymatous ground-tissue resembles the stems of some Dicotyledonous climbers such as _Thinouia scandens_, species of _Serjania_ and _Paullinia_[239].

Anatomically the main features of the stelar system of _Medullosa_, neglecting the secondary xylem and phloem, are in closer agreement with the stems of Ferns than with those of any other plants. It has been shown that the genus _Heterangium_ bears a close resemblance to _Gleichenia_ in the structure of the primary stele (fig. 418, C): one of the oldest types of _Medullosa_, _M. anglica_, may be described as a _Heterangium_ with three steles and may be compared with a dictyostelic Fern in which the irregular vascular framework is made up of three main strands. In certain types of _Medullosa_ (fig. 416) the ground-plan of the vascular system recalls that of a solenostelic Fern, while in others the greater complexity suggests comparison with such Ferns as _Matonia_, _Angiopteris_, _Psaronius_, or _Cyathea_; ‘it is as though Nature were at the Carboniferous moment in the midst of a series of amazing engineering experiments, most of which were either buried deep in Palaeozoic oblivion, or permitted to survive only as vestigial relics and atavistic ghosts’[240]. Though many Medullosae resemble Ferns there is an important difference between the two groups in the origin of the various plans of Medullosan stelar systems: in Ferns the leaf is the determining factor in the evolution of stelar arrangement, while in _Medullosa_ the occasional interruption of a solenostele or the development of an apparently complex dictyostele are features independent of the leaf and leaf-traces. In the structure of the secondary xylem and phloem and in root-structure _Medullosa_ agrees with recent Cycads. The genus is in short a generalised type with filicinean and cycadean affinities. In the possession of seeds borne on modified pinnae of compound fronds, _Medullosa_ resembles both _Cycas_ and the Lyginopterideae. The seeds exhibit a fairly close agreement with those of _Lyginopteris_, _Heterangium_ and recent Cycads, but they appear to have advanced further towards the cycadean type than is the case with the closely related seeds of the Lyginopterideae. The microsporophylls are very imperfectly known but they were undoubtedly much less advanced and more fern-like than the megasporophylls.

The genus _Medullosa_ is recorded from the Permian strata of Saxony, France, and Bohemia[241]; also from the Coal Measures of England, and the discovery of petrified petioles of _Myeloxylon_, the type borne on Medullosan stems in European species, may be taken as evidence of the existence of the genus in North America during the Carboniferous period[242].

The name _Medullosa_ was applied by Cotta[243] to three types, _Medullosa elegans_, _M. stellata_, and _M. porosa_, from the Rothliegende of the Chemnitz district. The first of these was recognised by Brongniart[244] as a distinct genus for which he proposed the designation _Myeloxylon_ and this was afterwards identified by Renault, Williamson, and other palaeobotanists as a petiole and not a stem. Further reference is made to _Myeloxylon_ on a later page. Cotta spoke of _Medullosa_ as the most puzzling of the genera dealt with in his ‘Dendrolithen,’ and in spite of the many additions to our knowledge the position of this Palaeozoic genus is still a fertile source of speculation. The generic designation _Medullosa_ is applied to stems, with or without petioles; petioles or rachises of fronds that frequently occur apart from stems are referred to the genus _Myeloxylon_. The leaves of _Medullosa_ include several well-known species of Carboniferous and Permian genera such as _Alethopteris_, _Neuropteris_ and others that have in recent years been transferred from the Filicales to the Pteridosperms. In a few instances seeds have been found in organic connexion with Medullosan foliage, and there can be no reasonable doubt that _Trigonocarpus_, some forms of _Rhabdocarpus_, _Pachytesta_, and other seeds represent the integumented megasporangia of _Medullosa_ or some closely allied genus.

Before attempting to summarise the salient features of _Medullosa_ a description of a few selected types will serve to place us in a better position to consider the genus as a whole. The British species are placed first on the ground that they are both geologically the oldest though, historically, the most recently described, representatives of the genus; and in the organisation of the stem they are simpler than the continental species. Their resemblance to _Heterangium_ serves to some extent to bridge the gap between the majority of species of _Medullosa_ and the simpler types of Pteridosperms represented by _Heterangium_ and _Lyginopteris_.

_Medullosa anglica_ Scott[245].

Prior to the discovery of this species the genus _Medullosa_ had not been recorded from Britain. A section in the Williamson collection recognised by Scott as that of a _Medullosa_ had been identified by Williamson as a large _Heterangium_ stem. An undescribed specimen was found by Arber[246] in the Binney collection at Cambridge which afforded some additional information as to the structure of the roots.

The specimens on which Scott’s thorough description is based were obtained by Messrs Wilde and Lomax from the Lower Coal Measures of Lancashire. The stem of this oldest species has the habit of a tree-fern and is almost completely invested by the stout decurrent bases of the petioles of large spirally disposed compound fronds with a phyllotaxis of ⅖, the leaves of the same orthostichy being separated from one another by a vertical distance of approximately 10 cm.

A transverse section of a slightly flattened stem is shown in fig. 416, A, the bases of three petioles give to it an angular form. Its dimensions are approximately 10 × 4 cm. The ground-tissue of two of the petioles is continuous with that of the stem, while that of the third leaf-stalk is cut through near its separation from the stem and its adaxial face is already defined by a hypodermal band of stereome, _d_. The surface of the stem is characterised by fine longitudinal ribs caused by the slightly projecting stereome in the outer cortex, and from the narrow furrows between the leaf-bases adventitious roots emerge in vertical series. The position of an interfoliar furrow is shown by a small arrow in fig. 416, A. There are three steles, 2–3 cm. × 6–10 mm. in diameter: each agrees very closely in structure with the single stele of _Heterangium_. _Medullosa anglica_ may be described as a polystelic _Heterangium_ and as having the same relation to _Heterangium_ as regards the stelar system as _Primula auricula_ bears to the monostelic _Primula_. The central core of the stele (the black patches in the diagram, fig. 416, A) consists of an anastomosing system of tracheal groups embedded in an irregular parenchymatous reticulum. The large primary tracheids reach a diameter of 150μ and have multiseriate pitting: at the periphery of the primary xylem there is a more definite grouping of tracheids as in _Heterangium_, and the slightly internal (mesarch) protoxylem elements are associated with scalariform and densely spiral tracheids (fig. 416, B, C) narrower than the more internal reticulate elements. The secondary xylem is manoxylic as in Cycads, tracheids in 2–4 radial series alternating with medullary rays 1–3 cells broad and usually of considerable depth (fig. 416, B). The principal rays are continuous with the parenchymatous matrix of the central core. Thick-walled tubular elements, no doubt of the nature of sieve-tubes, form a conspicuous feature in the phloem.

The three steles occasionally divide and fuse with one another. The tissue between the steles is crushed and disorganised and in the living plant was probably small in amount. In the imperfectly preserved inner cortical region there is a sinuous band of secondary parenchyma (periderm; fig. 416, A, _c_) developed from a deep-seated phellogen; in older stems this formed the superficial tissue after the fall of the leaves. There is no definite boundary between the cortex of the stem and the petiole-bases except when the hypoderm cuts across the cortex preparatory to the separation of a leaf-stalk. The stem-cortex and the ground-tissue of the petioles consist of parenchyma with numerous secretory canals, not sacs only as in _Heterangium_, and are abundantly supplied with scattered vascular bundles of collateral and exarch structure.

[Illustration: Fig. 416. _Medullosa_ stems. A-C, _Medullosa anglica_; A, transverse section; _a_, accessory vascular strand; _b_, accessory strand enclosed by periderm; _c_, band of periderm encircling steles; _d_, sclerenchyma between leaf and stem. B and C, longitudinal sections. (After Scott.) D, _Medullosa stellata_; _a_, star-rings; _p_, ‘partial pith.’ (After Weber and Sterzel.) E, _Medullosa Solmsi_. (After Weber and Sterzel.) F, Medullosa stellata, from a specimen in the British Museum (No. 13767). G, _Medullosa stellata_ var. _cortica_; _v_, leaf-bundles. (After Weber and Sterzel.) H, I, _Medullosa Leuckarti_. (After Weber and Sterzel and Solms-Laubach.) K, _Medullosa stellata_ var. _gigantea_; _a_, concentric stele; _b_, _b_, later cylinders of centrifugal tissue. (Adapted from Weber and Sterzel.) L, _Medullosa Solmsi_ var. _lignosa_. M, _Medullosa porosa_. (L, M, after Weber and Sterzel.)]

The leaf-traces are furnished by the peripheral tracheal groups at the free surface of the primary portion of each stele: each trace is at first concentric and consists of primary xylem with one or more protoxylem strands near the outer surface and is completely or partially enclosed by secondary xylem and phloem. In the course of its passage to the leaf a leaf-trace loses its secondary tissues, which were added by the cambium during the traverse of the zone of secondary wood, and divides into small collateral bundles consisting mainly of spiral and scalariform tracheids. The collateral bundles accompanied by some narrow fibres are of the _Myeloxylon_ type (fig. 420), the xylem being wholly centripetal. In the behaviour of the leaf-traces and in the vascular system of the petioles _Medullosa_ differs from _Heterangium_ and _Lyginopteris_. Each leaf-base is supplied by sets of vascular strands which pass into it from the stem at different levels; a large leaf-base reaching 4 cm. in diameter receives as many as 70–80 bundles. The hypoderm is like that first described in the French species _Myeloxylon Landriotii_[247] and often spoken of as the _Sparganum_ type of hypoderm. The branching of the rachises points to a compound frond, and the occurrence of numerous linear pinnules with revolute margins (fig. 420, D) in association with the stem suggests that the ultimate segments were of the _Alethopteris_ form. This inference receives confirmation from the occurrence of petrified specimens of undoubted _Alethopteris_ rachises with the structure of _Myeloxylon_. It is practically certain that the leaves borne on the stems of _Medullosa anglica_ are those long known as _Alethopteris lonchitica_ (Vol. +ii.+ A, p. 553, fig. 364).

An interesting feature in the stems is the occurrence of cortical vascular strands (fig. 416, A, _a_, _b_), reaching a diameter of 7 mm., containing scattered tracheids in a parenchymatous core surrounded by secondary xylem and phloem. These cauline bundles are almost identical both in structure and distribution with the accessory steles in the stem of a recent _Cycas_, and the agreement is emphasised by the presence of short square-ended tracheids in the primary xylem.

The roots branch freely and may attain a diameter of more than 1 cm.: they are generally triarch and the triangular primary xylem is enclosed by secondary xylem except opposite the protoxylem. The cortex is like that of _Lyginopteris_ roots and a conspicuous double layer of superficial tissue is another feature common to both (_cf._ fig. 410). The exceptionally well preserved specimens described by Arber[248] show very clearly the thick zone of periderm which forms the covering of older roots, and in some of the sieve-tubes groups of dark brown patches show the form and arrangement of the sieve-plates.

_Reproductive organs._ We have as yet no precise information in regard to the reproductive organs of _Medullosa anglica_, but there can be little or no doubt that the fronds bore seeds that have long been known under the generic name of _Trigonocarpus_. Many years ago Mr Hemingway noticed the almost constant association of the fronds of _Alethopteris lonchitica_ with _Trigonocarpus_, and Dr Kidston’s discovery[249] of seed-bearing _Neuropteris_ pinnae considerably strengthened the evidence derived from mere association. The structure of _Trigonocarpus_ is described later (p. 117) in a section devoted to reproductive organs attributed to _Medullosa_. Nothing is known as to the microspore-bearing organs.

While in the structure of each of the steles _Medullosa anglica_ agrees very closely with _Heterangium_, it differs from that genus in the presence of three steles and in the structure of the petioles which are much less fern-like than the simpler petioles of _Heterangium_ and _Lyginopteris_. From the continental species the British species is distinguished by its simpler stelar system, though there is a close correspondence as regards individual steles.

_Medullosa pusilla_ Scott.

This species, briefly referred to by Scott in 1909[250] and fully described in a recent paper[251], is founded on material from the Lower Coal Measures of Colne, Lancashire. It agrees in essential features with _Medullosa anglica_, but differs in the following particulars: the linear dimensions of the stem are about one quarter those of a typical stem of the older species; the leaf-traces possess little or no secondary xylem and the relatively large decurrent leaf-bases have a narrower and simpler hypoderm. The stem has a tri-stelar vascular system enclosed in a ring of internal periderm, and each stele (3 mm. in diameter) consists of a roughly triangular strand of reticulate tracheids and a small amount of scattered parenchyma. The protoxylem is either exarch or, as in _M. anglica_, mesarch, the exact position being difficult to determine in the available material. The secondary xylem closely resembles that of _M. anglica_.

Scott suggests the possibility that _Alethopteris decurrens_ may be the foliage of _Medullosa pusilla_. It is possible that there is no specific difference between _M. pusilla_ and _M. anglica_, but on the present evidence the employment of a distinctive name is desirable.

_Medullosa centrofilis_ de Fraine.

This species was founded by Miss de Fraine[252] on a petrified stem from the Lower Coal Measures of Lancashire. The maximum diameter of the flattened stem including four decurrent leaf-bases is 5 cm. The vascular system consists of an outer group of four steles, reduced to three by fusion in the upper part of the specimen, enclosing a central smaller stele or star-ring (fig. 417). It is the presence of the star-ring that distinguishes this type from the other two British species and forms a connecting link with certain continental Medullosae. The peripheral steles agree with the steles of _M. anglica_ but, as in _M. pusilla_, there is some doubt as to the exarch or mesarch position of the protoxylem. In the structure of the xylem the central stele conforms to the rest of the vascular system and a strand of protoxylem is preserved that is almost certainly exarch. There is evidence that the peripheral steles occasionally anastomose, but the central stele follows an independent course at least in the piece of stem examined. Leaf-traces are furnished by the primary xylem of the outer steles, and they appear to be without secondary tracheids as in _M. pusilla_. A zone of secondary cortex encloses the vascular system as in the other British stems: it is pointed out by Miss de Fraine[253] that this tissue, usually described as a deep-seated periderm, must have differed from cork in that there is no sign of drying up or decay in the tissues external to it. The leaf-bases are of the usual _Myeloxylon_ type. In size this species is intermediate between _Medullosa anglica_ and _M. pusilla_.

[Illustration: Fig. 417. _Medullosa centrofilis._ Transverse section showing the deep-seated ‘periderm’ (broken line) surrounding four steles. (After de Fraine.)]

_Medullosa stellata_ Cotta.

Cotta[254] described _Medullosa stellata_ as a stem characterised by the occurrence of several many-rayed stellate columns (‘vielstrahlige Sternsäule’) in a pith enclosed by a double cylinder of secondary xylem. The so-called pith is the central ground-tissue of the stem and the double ‘striated ring’ of Cotta is a cylindrical stele identical in structure with each of the steles of _Medullosa anglica_ but having a tubular form instead of forming a relatively broad and short band (_cf._ fig. 416, D and A). Goeppert[255] in his _Permian Flora_ gave a detailed account of the species, some of his sections being cut from Cotta’s material, and by the employment of varietal epithets emphasised the range of variation within the limits of the type. Goeppert and Stenzel[256] and, several years later, Weber and Sterzel[257] adopted the same plan as a convenient method of drawing attention to differences in anatomical characters. As Schenk[258] pointed out, there is a considerable risk in the case of small pieces of stems of attaching excessive importance to structural variations, and it is by no means improbable, as he said, that differences which are the expression of states of preservation or stages in development have been incorrectly regarded as distinguishing marks of individual plants. It is, however, convenient to recognise some of the more striking deviations from the type-species by speaking of the different forms as varieties though, as Weber and Sterzel fully admit, such varieties and even some of the species must be looked upon as provisional. Weber and Sterzel give expression to the provisional nature of their grouping by classifying the species with their varieties into form-cycles. Under the form-cycle _Medullosa stellata_ five more or less well defined forms are recognised, the type-species being _Medullosa stellata_ var. _typica_[259].

_Medullosa stellata_ var. _typica_.

Part of a transverse section of a cylindrical stem is represented diagrammatically in fig. 416, D. Very little of the cortex is preserved: a parenchymatous axial region with scattered secretory canals contains four oval or cylindrical vascular steles, the stellate columns of Cotta or star-rings of later authors. These are of the same nature as the small central stele in the English _Medullosa centrofilis_. The central region of the stem in this specimen is completely surrounded by a narrow cylinder of inversely orientated secondary xylem and phloem (fig. 416, D), the phloem being on the inner side of the xylem. Beyond the xylem is a parenchymatous band containing scattered groups of primary xylem tracheids with spiral, scalariform, and reticulate pitting, and this zone, which is usually designated the ‘partial pith,’ is succeeded by a second and broader, normally orientated, cylinder of secondary xylem and phloem. In this section the two concentric cylinders separated by the partial pith form a solenostele like that of several recent Ferns except in the presence of secondary tissue. The term ‘partial pith’ applied to the tissue between the two cylinders of secondary tissue is misleading: this tissue (fig. 416, D, _p_) is the primary xylem of the stele and is homologous with the primary portion of the stele of _Heterangium_ and of the steles of _M. anglica_. In many sections the continuity of the tubular stele is broken. In a section in the British Museum cut from one of Cotta’s specimens[260], 6 × 3·5 cm. in diameter to the outer edge of the vascular tissue, the cylindrical stele is interrupted at two places. An example of the interrupted type of stele is shown in fig. 416, F, and in fig. 416, H: the latter belongs to a distinct species. The complete type of cylindrical stele is exceptional and occurs occasionally at different levels in the stem. An important point is that the frequent breaks in the cylinder are not connected with the exit of leaf-traces and do not, therefore, correspond to the foliar gaps in the solenostele or dictyostele of a Fern.

The secondary xylem is of the cycadean type (fig. 418, B, D) like that of _Heterangium_ and _Lyginopteris_ and several other stems. Each of the star-rings in the axial region consists of a parenchymatous core with scattered primary tracheids enclosed by secondary vascular tissue (fig. 418, B). The star-ring shown in fig. 418, B, from a Chemnitz stem illustrates the characteristic cycadean character of the secondary xylem with broad medullary rays: some of the innermost elements are in contact with the primary tracheids. The phloem is rendered conspicuous by the black contents in some of the elements. Both the star-rings and the larger peripheral steles are constructed on the same plan and agree with the steles of _M. anglica_. The star-rings occasionally branch and anastomose with one another and with the encircling stele. The star-ring in fig. 416, D at _a_ is about to give off a small strand.

Leaf-traces are furnished by the primary xylem at the edge of the ‘partial pith’ of the outer stele: as a leaf-trace passes outwards through the outer cylinder of secondary xylem the cambium invests it with secondary xylem and phloem, but as it passes through the cortex of the stem it becomes reduced to its primary elements, and by successive branching gives rise to small collateral bundles which enter the petioles. The piece of stem shown in fig. 416, G, illustrates the exit of leaf-traces from the stele and their subsequent division into several small bundles, _v_, which are scattered in the cortex with strands of sclerenchyma. In a specimen identified with _Medullosa stellata_, Schenk[261] found part of a leaf-base attached to the stem: its vascular system was of the _Myeloxylon_ type, the bundles being identical with those in the cortex of the stem seen in fig. 416, G.

In some stems of _M. stellata_ the outer, centrifugally developed, portion of the main stele is very much broader than in the example represented in fig. 416, D. The diagrammatic sketch reproduced in fig. 416, F, represents a section of a Chemnitz specimen in the British Museum[262] in which the axial region containing several star-rings is almost enclosed by an inner zone of secondary xylem, and beyond the narrow primary xylem (black in the sketch) the rest of the block consists exclusively of secondary xylem 5·5 cm. broad. This example illustrates a common tendency in _Medullosa_ towards a large excess of centrifugal over centripetal secondary vascular tissue. A similar specimen of _Medullosa stellata_ is figured by Mougeot[263] from the Vosges showing a considerable development of centrifugal xylem comparable with that in the British Museum stem. Weber and Sterzel[264] describe stems of _Medullosa stellata_ showing slight periodic swellings which it is suggested, though there is no evidence in support of the opinion, may be connected with reproductive organs.

* * * * *

_Medullosa stellata_ var. _corticata_[265]. The specimen referred to this variety, represented in fig. 416, G, has already been quoted as affording data with regard to the origin and behaviour of the leaf-traces. In this type of stem the outer portion of the main stele is narrower than in _M. stellata_ var. _typica_ and the stele never forms a complete tube. The star-rings in the centre of the stem are more numerous than in the type-species of the genus. In the axial region of some stems included in the form-cycle to which _M. stellata_ belongs there may be flatter and tangentially elongated vascular strands in addition to the cylindrical star-rings; these are termed plate-rings.

* * * * *

In _Medullosa stellata_ var. _lignosa_[266] the outer xylem reaches a breadth of 4 cm. and the star-rings are reduced to one. The form _M. stellata_ var. _gigantea_[267] (fig. 416, K) is of special interest as an example of a stem reaching a diameter of nearly 50 cm. and having as many as 43 large and small star-rings in the axial region. A large tubular stele like that of the type-species (fig. 416, D) surrounds the central region, but in this form the cylindrical stele _a_ is succeeded by concentric cylinders of normally orientated xylem and phloem (fig. 416, K, _bb_) produced by successive cambiums either cortical or pericyclic in origin. This type of stem presents a striking resemblance to stems of _Cycas_ and _Macrozamia_ except in the possession of a double cylindrical stele consisting of both centripetal and centrifugal secondary xylem and phloem separated by a zone of primary xylem (partial pith).

_Medullosa gigas_ Renault.

This species was founded on a piece of stem from the Permian of Autun[268], consisting almost entirely of secondary xylem, which Brongniart had previously placed in his genus _Palaeoxylon_[269]. The secondary xylem reaches a diameter of 45–50 cm. and in the portion of the central region preserved there are a few vascular strands like the star-rings of other species. The considerable development of secondary xylem indicates a form of stem similar to some forms of _M. stellata_ (_e.g._ fig. 416, F), but as the available data are insufficient for accurate determination Renault’s specific name is retained. Renault describes the internal xylem cylinder (_i.e._ the centripetal xylem) as very slightly developed or as hardly visible, a feature in which the French specimen shows a nearer approach to the structure of a recent Cycad.

_Medullosa porosa_ Cotta.

The second of Cotta’s species[270], which has been fully investigated by Weber and Sterzel, is constructed on the same plan as that of _M. stellata_, but the stem is distinguished by the greater number of star-rings and, more especially, by the presence of an outer system of vascular strands in the axial region (fig. 416, M): these form a frequently interrupted cylinder of anastomosing strands characterised by the feeble development of secondary xylem and phloem or by the absence of this tissue on the outer face of the strands. The component parts of this outer series occasionally fuse with the internal star-rings.

_Medullosa Solmsi_ Schenk[271] var. _typica_ Web. and Ster.[272]

This type has a large axial region containing several very small star-rings enclosed by two concentric zones of separate plate-rings (fig. 416, E) each consisting of a complete flattened cylinder of secondary xylem and phloem enclosing primary xylem. As the complete cylindrical stele of the stem of _Medullosa stellata_ shown in fig. 416, D, was compared with the solenostele of a Fern, so in this stem (fig. 416, E) the vascular cylinder may be compared at least superficially with a dictyostele. From the inner circle of plate-rings strands are given off in the form of star-rings and these pass through the gaps in the outer system, eventually breaking up in the cortex into numerous collateral bundles. In another form of this species, var. _lignosa_ (fig. 416, L), the axial region is enclosed by a circle of plate-rings like those in the type-form, but these are succeeded by a circle of very asymmetrically developed and large steles with the outer xylem and phloem much broader than the inner. Moreover in this form additional cylinders of normally orientated vascular tissue are added as in _M. stellata_ var. _gigantea_ and in some recent Cycads. It is noteworthy that the secondary wood of _Medullosa Solmsi_ is rather more compact than in other species, a feature in which it to some extent agrees with the South African genus _Rhexoxylon_.

_Medullosa Leuckarti_ Goeppert and Stenzel.

In this species[273], also from the Permian of Saxony, the central region including some star-rings is surrounded by sinuous flattened concentric steles (snake-rings) agreeing anatomically with the steles of other species and characterised by the comparatively small breadth of the secondary xylem and phloem (fig. 416, H). Leaf-traces are given off, as in _M. anglica_ and other species, from the outer edge of the primary xylem. In some forms there is a single set of snake-rings; in others there is a double series. Fig. 418, D, shows part of the secondary xylem of a stele of this species from Chemnitz: the tracheids are in some places continuous with the primary xylem, and on the outer edge of the secondary wood is a cylinder of phloem. A section of _Medullosa Leuckarti_ figured by Goeppert and Stenzel[274] shows some radial rows of very thick-walled elements in the secondary phloem which they describe as bast sclerenchyma, but Solms-Laubach[275] believes them to be sieve-tubes. Precisely similar elements are figured by Scott[276] in _M. anglica_ and as this author suggests the thick walls are probably not an original feature. The structure of the primary xylem is more clearly seen in fig. 418, C, and the relation between primary and secondary xylem is shown in fig. 416, I, where the position of the protoxylem may be either exarch or mesarch. The protoxylem is only occasionally recognisable but some of the peripheral primary tracheal groups are undoubtedly mesarch. External to the stele, a part of which is reproduced in fig. 418, D, are strands of stereome elements and beyond them a band of radially elongated cells that may be ‘periderm’: still farther out there are some imperfectly preserved vascular bundles that are leaf-traces. This species is important as affording a complete demonstration of the organic connexion between the stem and petioles of the _Myeloxylon Landrioti_ type which indicate that the fronds were probably Alethopteroid.

[Illustration: Fig. 418. A, _Myeloxylon radiatum_, part of petiole. B, _Medullosa stellata_; star-ring; × 14. C, D, _Medullosa Leuckarti_; C, primary xylem; D, part of primary stelar tissues and secondary xylem and phloem; × 6. (A–D, Kidston Coll., 1014, 1307, 1238.)]

The specimen on which the diagrammatic drawing reproduced in fig. 419 is based has been investigated by Weber and Sterzel[277] and by Solms-Laubach[278]. The figure is a slightly simplified version of that given by Weber and Sterzel; it represents the stem of _Medullosa Leuckarti_ as a transparent object, the two lower transverse sections, B and C, being seen in perspective through the longitudinal faces. The steles are shaded obliquely in the longitudinal sections, and in the three transverse sections, A, B, C, the primary xylem (partial pith) is black and the enclosing secondary vascular tissue radially shaded. The whole block is 9 cm. in length and 6 cm. broad. Only a part of the axial region is shown internal to the peripheral snake-rings and in it are the star-rings _S_, _S_, _b_, and _c_. Outside the main steles is the narrow cortex _R_ and portions of leaf-bases _I–IV_. The lowest section, C, shows part of a peripheral snake-ring with a slight swelling at _f_ on its inner side which, as seen in sections B and A, foreshadows the separation of the star-ring _S_ and the consequent break in the continuity of the snake-ring (_d_, _e_, sect. B). In section A the gap is closed: in the longitudinal section between B and A the star-ring _S_ is seen to form two branches, _a_ and _b_, the branch _a_ closing the gap between _d_ and _e_ in section B. These sections demonstrate the formation of a star-ring from the main peripheral stele and the formation of additional star-rings by branching.

[Illustration: Fig. 419. _Medullosa Leuckarti._ Diagrammatic sketch of stem showing longitudinal and transverse sections: for explanation see text. (After Weber and Sterzel.)]

Numerous vascular bundles destined for the leaves are scattered in the cortex. The course of the decurrent leaf-base _I_ is shown on the longitudinal faces, its boundary being marked by crowded stereome strands (of the _Myeloxylon Landrioti_ type); other leaf-bases are represented by _II_, _III_, and _IV_.

In habit _Medullosa Leuckarti_ differs from such a type as _M. stellata_ in its relatively shorter and stouter stem and in the shorter internodes.

+Leaves and Reproductive Organs.+

i. _Leaves._ It has already been stated that in some cases petioles occur in organic connexion with Medullosan stems, notably in _M. anglica_ and _M. Leuckarti_: in the exceptionally rich collection in the Chemnitz Museum, which forms a fitting memorial of the work of the late Prof. Sterzel, there is a stem of _M. Leuckarti_ bearing large petioles of the type known as _Myeloxylon radiatum_. The occurrence of vascular bundles in the cortex of other species of stem identical with those in the attached petioles points to a uniform type of leaf-structure so far as regards the petioles and rachises of _Medullosa_. While it is clearly unnecessary to distinguish by a special generic title the petrified portions of fronds known to belong to certain species of stems, the frequent occurrence of detached petioles necessitates some distinctive term. The name employed is _Myeloxylon_: the genus was instituted by Brongniart in 1849 for Cotta’s species _Medullosa elegans_ the petiolar nature of which was suspected by Binney in 1872.

_Myeloxylon_ (Medullosan leaf-stalks).

1832. _Medullosa elegans_ Cotta. 1865. _Stenzelia_ Goeppert. 1876. _Myelopteris_ Renault[279]; 1877, _Aulacopteris_ Grand’Eury[280].

There is a very close agreement in general anatomical structure between the numerous specimens of _Myeloxylon_ from the Permian strata of Saxony and France and the Coal Measures and Millstone Grit of England[281]; the genus is also recorded from the Upper Carboniferous of Kansas[282]. Two well-defined types instituted by Renault are, however, readily distinguished by the form of the hypodermal stereome strands. _Myeloxylon_ may be defined as follows: Oval or cylindrical branched axes, reaching a diameter of 15 cm., bearing pinnae having the characters of _Alethopteris_, _Neuropteris_, _Odontopteris_, and some other genera that were formerly classed as Ferns. Below a single-layered epidermis, in which stomata have been recognised, occur a few layers of parenchyma: this superficial tissue, which is rarely preserved, is succeeded by a hypodermal region consisting of parenchymatous tissue and numerous vertical groups of narrow thick-walled fibres arranged as radial plates or circular, oval, or reniform strands (the _Sparganum_ type of cortex). In the hypoderm as in the ground-tissue generally secretory canals, often accompanied by stereome, are a characteristic feature. The vascular system is represented by a considerable number of collateral bundles scattered through the ground-tissue and especially abundant in the outer region: the bundles sometimes assume a more or less regular disposition in concentric circles. Each bundle consists of a small group of xylem tracheids, for the most part spiral or scalariform, though reticulately pitted elements are by no means rare, with a single protoxylem group on the outer face next the phloem (fig. 420, B, C). As a rule the xylem is wholly centripetal, but occasionally the exarch structure becomes mesarch by the occurrence of a few centrifugal tracheids. The phloem, rarely preserved (fig. 420, B), consists of narrow sieve-tubes with parenchyma, and the bundle as a whole is often partially enclosed by a sheath of fibres.

Superficially the anatomical structure is similar to that of the petioles of _Angiopteris_ or _Marattia_, and both Williamson[283] and Renault placed _Myeloxylon_ in the Ferns; but the collateral form of the vascular bundles, the position of the protoxylem, and the arrangement of the hypoderm tissues, are cycadean features.

_Myeloxylon radiatum_ (Renault).

[Illustration: Fig. 420. A, B, _Myeloxylon radiatum_; A, petiole ⅔ nat. size; B, vascular bundle. C, _Myeloxylon_ sp. vascular bundle; _px_, protoxylem. D, _Medullosa anglica_; section of pinnule. (A, B, after Zeiller; D, after Scott.)]

This type is characterised by the radially elongated stereome of the hypoderm. Prof. Zeiller[284], who has given a very clear and concise description of _Myeloxylon_, is disposed to regard Cotta’s _Medullosa elegans_ as a specifically distinct form on the ground that there are two concentric zones of stereome in the hypoderm; but this feature is shown only in one of Cotta’s figures, and Weber and Sterzel[285] point out that a doubting of the hypoderm zone may be caused by accidental juxtaposition of two faulted pieces of peripheral tissue. The drawing reproduced in fig. 420, A, shows the structural plan of an unusually large petiole from the Permian of Autun: a portion of the outer tissue is seen in fig. 418, A. The vascular bundle, fig. 420, C, from a Millstone Grit specimen[286], shows the centripetal nature of the xylem and fragments of phloem in the outer half of the bundle, with imperfectly preserved fibres abutting on the xylem. The characteristic hypoderm is shown also in fig. 418, A; the double xylem strand on the left illustrates a common feature caused by the branching of vascular bundles. Several secretory canals are scattered in the ground-tissue. The pinnules of _Myeloxylon radiatum_, or at least of some specimens, have been shown by Renault to be of the _Neuropteris_ type.

_Myeloxylon Landrioti_ (Renault)[287].

In this species the distinguishing feature is the occurrence of the hypodermal stereome in the form of circular, oval, or reniform strands in place of the radial plates of _M. radiatum_. It is this form of petiole that was borne by the stems of _Medullosa anglica_ and _M. Leuckarti_. In _M. anglica_ the pinnules (fig. 420, D) are of the _Alethopteris_ type, almost certainly _A. lonchitica_. Renault and Zeiller have described French specimens of _Myeloxylon Landrioti_ bearing pinnules like those of _Alethopteris aquilina_ and _A. Grandini_.

_Myeloxylon topekense_ (Penhallow).

The occurrence of _Myeloxylon_ petioles in the New World was recorded by the late Prof. Penhallow[288] who founded this species on some imperfectly petrified specimens from Upper Carboniferous strata at Topeka, Kansas. Enough material was available to show the _Myeloxylon_ characters, but the preservation is too imperfect to admit of a complete diagnosis. The hypodermal stereome shows a tendency to form tangentially extended strands in place of the more circular or radially elongated groups in the European species.

In addition to _Alethopteris_, _Neuropteris_ (including _Cyclopteris_) and _Odontopteris_ fronds, which are known to possess rachises with the _Myeloxylon_ features, there is reason to believe that the Permian _Callipteris_ fronds and possibly some of the older _Taeniopteris_ leaves may also belong to _Medullosa_[289]. It is, however, unsafe to assume that the occurrence of _Myeloxylon_ petioles necessarily denotes the existence of _Medullosa_. The French stem _Colpoxylon aeduense_ Brongn.[290] bore leaves with the same general anatomical features as those of a typical _Myeloxylon_, and there can be little doubt that other genera of the Medulloseae also possessed fronds constructed on the same plan as those known to have been borne by _Medullosa_. An interesting illustration of an injured organ that had produced a wound-cambium is afforded by a _Myeloxylon_ petiole from the Coal Measures described by Mr Holden[291].

+Reproductive Organs.+

Reference was made in volume +ii.+ to the reasons which led to the removal of several genera of Carboniferous and Permian fronds from the Filicales to the Pteridosperms, and in Chapter +xxix.+ of the present volume certain species of _Sphenopteris_ are described as the foliage of _Lyginopteris_ and _Heterangium_. It is with such genera as _Neuropteris_, _Alethopteris_, _Linopteris_ and others that we are now more especially concerned, as they represent some of the types of leaves borne by _Medullosa_ and other members of the Medulloseae. The absence of any specimens among the large number of these common genera bearing undoubted sporangia aroused suspicion as to the correctness of the generally accepted view that these fern-like fossils were the leaves of Palaeozoic Ferns. Subsequently the suspicion based on negative evidence was confirmed by researches into the anatomical structure of the leaf-bases, petioles, and fragments of pinnae attached to and associated with stems of _Medullosa_. It is only in a few cases that actual organic connexion between reproductive organs and Medullosan leaves has been demonstrated, but from such facts as are established it is safe to make the general statement that stems of _Medullosa_—a generic term that undoubtedly includes plants which, had we a fuller knowledge of them as complete individuals, would be assigned to more than one generic type—possessed fronds simulating in habit those of certain Ferns with some of the pinnae bearing seeds often of considerable size and in all cases of complex structure, agreeing in many respects with those of existing Cycads, while other fronds, or in some cases it may be other pinnae, bore microsporangia similar in form to the sporangia of Ferns.

_a. Microsporangia._

_Neuropteris._ A. _Neuropteris heterophylla[292]._

Several examples of supposed fertile specimens of _Neuropteris_ are recorded in palaeobotanical literature, but it was not until 1887 that any satisfactory specimen was discovered. In that year Kidston[293] described a specimen of _Neuropteris heterophylla_ from the Lower Coal Measures of Scotland in which slender forked branchlets bear small bodies at their tips some of which appear to represent four-valved organs (fig. 421, D), though the imperfect state of preservation renders impossible any definite pronouncement as to their structure. To the specimen are attached a few sterile pinnules, showing that it is a portion of a frond of _N. heterophylla_ characterised by the substitution of reproductive organs for pinnules. The subsequent discovery of seeds attached to pinnae of the same species afforded strong presumptive evidence, almost amounting to proof, of the microsporangial nature of the Scotch specimen. For this specimen, although no precise diagnosis is possible, Dr P. Bertrand[294] has proposed the generic name _Neurotheca_. In 1911 the Abbé Carpentier[295] described some small ovoid bodies, 1–1·5 mm. long, from the Coal Measures of France arranged in groups of 4 to 6 and in some cases said to be borne on a slender pedicel which he found in association with _N. heterophylla_ and compared with sporangia described by Lesquereux from the Coal Measures of Arkansas as _Sorocladus stellata_[296]. These supposed microsporangia have recently been assigned by Bertrand[297] to _Sphenophyllum_.

[Illustration: Fig. 421. A. _Potoniea adiantiformis_ Zeill. (very slightly enlarged). B. Microsporangia of _Potoniea_ (enlarged). C. _Linopteris obliqua_; side-view of microsporophyll. D. _Neuropteris heterophylla_, pinnules and microsporangia (?). E. _Linopteris obliqua_, surface-view of microsporophyll. (A, C, E, after P. Bertrand; B, after Carpentier; D, after Kidston.)]

B. _Neuropteris gigantea_, etc. _Potoniea_, Zeiller.

In 1899 Zeiller[298] instituted the name _Potoniea_ for some peculiar fertile leaves found in the Coal Measures of Heraclea consisting of a branched axis with cuneate segments, 7–10 × 6–8 mm., bearing numerous fusiform bodies, 1–1·5 mm. long, at the upper edge (fig. 421, A). These marginal bodies he regarded as sporangia and expressed the opinion that _Potoniea_ may be the fructification of some form of _Neuropteris_, _Mariopteris_ or _Alethopteris_. Carpentier[299] afterwards described similar though rather larger examples from the Pas-de-Calais coal-field as microsporophyll fragments of some Pteridosperm: he stated that similar specimens had been found by Kidston in England. In a later work Carpentier[300] described the sporangia as crowded in groups (fig. 421, B) in the substance of the thick lamina of _Potoniea_, and he connected the fertile segments with _Neuropteris gigantea_, _N. pseudogigantea_ Pot. and _Linopteris obliqua_ (Bunb.). Similar specimens are said to have been found in Holland and Silesia. Bertrand[301] also records the association of _Potoniea_ with _Neuropteris gigantea_ and _N. pseudogigantea_: he describes some specimens as belonging to _N. gigantea_ Sternb. while others, distinguished only by small differences, he attributes to _N. pseudogigantea_. Kidston[302] has recently drawn attention to the inconstancy of the characters mentioned by Potonié as distinguishing features of _N. pseudogigantea_, and he shows good cause for referring the examples so named to _N. gigantea_. The fertile lamina is almost orbicular in surface-view and attached to a slightly excentric pedicel; the microsporangia are borne on the lower surface and probably in groups as described by Carpentier.

_Neuropteris Carpentieri_ Kidston. Kidston[303] has recently described some fertile leaflets under this name from the Westphalian series of South Staffordshire which he identifies with French examples referred by Carpentier[304] to _Potoniea adiantiformis_ Zeill. The fertile pinnules are thick and sub-cyclopteroid in form; the upper surface bears densely packed, narrow and long, microsporangia, 4 × 0·5 mm., containing more or less spherical microspores 45–60μ in diameter; the ventral face of the lamina on the removal of the spores shows several strong veins. With these are associated sterile pinnules of the Neuropteroid type, and examples are described intermediate between the sterile and fertile leaflets. Kidston believes the specimens to be microsporophylls of some species of _Neuropteris_, but as the material does not suffice for identification with any known species a new name is proposed. It is pointed out that in some states of preservation the leaflets resemble Goeppert’s Permian species _Dictyothalamus Schrollianus_[305].

_Alethopteris. Linopteris._

No specimens of _Alethopteris_ fronds have so far been described that afford any information as to the nature of the microsporangia, and we have no means of knowing whether they were borne on naked pedicels as in _Neuropteris heterophylla_, or on modified pinnules as in _N. gigantea_.

Zeiller in 1888[306] described some fertile pinnules of _Dictyopteris Schützii_ Roem. from Commentry bearing two rows of long sporangia: he subsequently transferred this species to the genus _Linopteris_[307] and expressed the opinion that the sporangia occur singly and not in groups as he originally believed. Zeiller compares the fertile pinnules with the type _Crossotheca_. Bertrand[308], as the result of examining similar specimens, has suggested that the fringe of pendulous bodies regarded by Zeiller as sporangia may be tooth-like lobes of modified pinnules which served to protect microsporangia borne on the lower surface of the lamina. The nature of the impressions is not clear, though there is little doubt that they are microsporophylls. The fertile pinnae of _Linopteris obliqua_ (Bunb.) described by Carpentier[309] and Bertrand[310] closely resemble the microspore-bearing organs which have been referred to _Neuropteris gigantea_; they have the characters of _Potoniea_ and consist of oval laminae similar to the sterile pinnules but about half their size: the lamina was attached excentrically to a slender stalk (fig. 421, C, E) and traversed by numerous occasionally anastomosing veins. No actual microsporangia have been discovered in organic connexion with the lamina.

_b. Megasporangia._

_Neuropteris._

Kidston’s discovery of undoubted seeds attached to pinnae of _Neuropteris_[311] marked an important step in our more exact knowledge of the morphology of Medullosan sporophylls. Specimens from the ironstone balls (Coal Measures) of Coseley near Dudley showed seeds attached to portions of pinnae bearing pinnules of _Neuropteris heterophylla_. The seeds are approximately 3 cm. long and from 1·10 to 1·40 cm. broad; oblong and gradually tapering from the middle to a slightly curved and obtuse apical snout (fig. 422). The outer surface shows numerous longitudinal ribs which no doubt represent hypodermal fibres. The preservation of the seeds, which appear to be circular in section, is not such as to render possible a description of structural features. In their asymmetrical form the seeds agree with the genus _Platyspermum_ as recently defined by Arber, but Kidston’s specimens are in all probability radiospermic. Kidston compares the _Neuropteris_ seeds with _Rhabdocarpus tunicatus_ as figured from the Commentry coalfield[312] and with specimens from Gard named by Grand’Eury _R. subtunicatus_[313].

[Illustration: Fig. 422. _Neuropterocarpus Kidstoni_ (Arber). (After Kidston; × 3.)]

Additional proof of the occurrence of seeds on _Neuropteris_ fronds is furnished by examples from the Coal Measures of Holland described by Kidston and Jongmans[314]: these seeds are of the same general type as those from Coseley but nearly twice as large, and they were borne at the tips of a dichotomously branched pedicel of _Neuropteris obliqua_. Grand’Eury in 1904[315] recorded the association of radiospermic seeds with _Neuropteris_ fronds though no case of actual attachment was found. It is, however, noteworthy that he speaks of the frequent association with _Neuropteris_ of seeds characterised by six or rarely twelve longitudinal keels, a feature recalling the sclerotesta of _Trigonocarpus_ and allied seeds. Until petrified specimens are available it is impossible to refer the seeds of _Neuropteris_ to a generic type founded on structural features: the seeds described by Kidston are, as he says, very similar in external characters to species assigned to _Rhabdocarpus_, and there can be little doubt as to the generic identity of the _Neuropteris_ seeds and some of the impressions referred to _Rhabdocarpus_ which are characterised by a similarity in form, an apical snout that gives an asymmetrical appearance to the specimens and the presence of numerous longitudinal striations[316]. It is, however, by no means certain that these seeds possessed the morphological features of _Rhabdocarpus_ as described by Brongniart in petrified examples from St Étienne[317].

The seeds of _Neuropteris_ may, as Kidston suggests, agree anatomically more closely with _Pachytesta_[318], a type that Grand’Eury associates with _Alethopteris_ fronds. With a view to avoid the danger of incorrectly identifying petrified specimens and impressions that cannot be proved to belong to the same generic type, I have suggested the restriction of the name _Rhabdocarpus_[319] to seeds that do not furnish evidence as to internal structure, and the employment of the designation _Rhabdospermum_ for seeds that conform to those described by Brongniart as _Rhabdocarpus_. The seeds of _Neuropteris_ may be of the _Rhabdospermum_ type or they may agree anatomically more closely with _Trigonocarpus_ or _Pachytesta_; they are members of either the Trigonocarpales or the Cardiocarpales, probably the former group. Dr P. Bertrand[320] assigns to _Neuropteris gigantea_ some seeds of the _Hexapterospermum_ (= _Hexagonocarpus_[321]) type which occur in association with cupule-like organs. These supposed cupules resemble the _Potoniea_ leaflets with microsporangia also referred to the same species of _Neuropteris_; they are characterised by a laciniate edge and may be compared with the Indian fossil described by Zeiller as _Ottokaria bengalensis_[322] (fig. 433). Dr Arber[323] and Dr P. Bertrand[324] have independently proposed the generic name _Neurospermum_ for the seeds of _Neuropteris heterophylla_ and _N. obliqua_ in preference to _Rhabdocarpus_: the former author speaks of the seeds of _N. heterophylla_ as _Neurospermum Kidstoni_. The generic term _Neuropterocarpus_ used by Grand’Eury in 1904[325], though not defined by him, has priority and avoids the adoption of a new designation for seeds attached to _Neuropteris_ fronds. In his definition of _Neurospermum_ Arber makes no reference to the obliquity of the apical snout that is clearly shown in fig. 422. There is no evidence that _Neuropterocarpus_ possessed a cupular investment comparable with that of _Lagenostoma_. Additional instances of the association of seeds with _Neuropteris_ fronds are recorded by Renier from the Belgian Coal Measures, also by Bertrand and Chodat from France: Renier found seeds associated with _N. Schlehani_ Stur and with the same species of frond Bertrand found impressions of oval ribbed seeds. Prof. Chodat[326] has figured some fragments of _Neuropteris_ pinnules referred to _N. auriculata_ Brongn. from the Stephanian of France which show small seed-like bodies apparently in organic connexion with the lamina; but the specimens are too imperfect to afford any satisfactory evidence as to the nature of the reproductive organs.

Lotsy[327] has expressed the opinion that the bodies attached to _Neuropteris_ pinnae described by Kidston and other authors as seeds may possibly be vegetative buds, but if this were the case one would expect to find some evidence of the bud-nature in some at least of the specimens that have already been found.

It would seem that the microsporophylls of _Neuropteris_ were constructed on different plans, some being of the type described by Zeiller and other observers as _Potoniea_, while others bore sporangia on pinnae without any accompanying laminae; but our knowledge of the latter form represented by Kidston’s specimen of _Neuropteris heterophylla_ (fig. 421, D) is very incomplete. On the other hand the seeds appear to have been characterised by features suggesting a close affinity to _Trigonocarpus_ and pointing to membership of the same family.

_Alethopteris._

Although no specimens have been discovered showing actual connexion between fronds and seeds, it is practically certain that _Alethopteris_ leaves, or at least some species of the genus, bore seeds of the _Trigonocarpus_ type. The association of _Trigonocarpus Parkinsoni_ with _Alethopteris lonchitica_ is too frequent to be fortuitous and there is further evidence afforded by certain anatomical resemblances. In France other species of _Alethopteris_, _e.g._ _Alethopteris Serlii_ and _A. Grandini_, occur in association with _Pachytesta_[328], a large seed similar to _Trigonocarpus_, and _Trigonocarpus_ is found in the Pas-de-Calais coal-field with _Alethopteris Serlii_.

=TRIGONOCARPUS.= Brongniart.

The generic name _Trigonocarpum_ was instituted by Brongniart in 1828[329] for ovoid longitudinally ribbed ‘fruits’ from Upper Carboniferous strata, the type-species being named _T. Parkinsoni_. The generic name is often altered to _Trigonocarpon_: Williamson[330], who adopted this form, states that Brongniart substituted _Trigonocarpon_ for _Trigonocarpum_ in his _Tableau_[331], but in that work the original termination is used, the form _Trigonocarpon_, probably the result of a slip, appearing only in the index. In his later work on seeds Brongniart adopted the name _Trigonocarpus_, and in recent years this has been widely employed. Among other species named by Brongniart are two previously referred by Sternberg to _Palmacites_. Several examples of Brongniart’s genus were described by Lindley and Hutton, and in their description of _T. Noeggerathii_ the statement is made that a fractured specimen demonstrated that ‘the fossil in its ordinary state is an interior part divested of fleshy covering’[332]; this suspicion of the true nature of the nut-like fossils was afterwards proved correct by the investigations of Hooker and Binney[333] and by the later work of Williamson. The specimens on which the genus was founded are casts of seed-cavities and it is in this state that the seeds are usually preserved, often in large numbers, in the sandstones of the Coal Measures, as in the block shown in fig. 423 from the famous quarry at Peel near Bolton, Lancashire. Another type of preservation is represented by the seeds figured by Lindley and Hutton as _Carpolithes alata_[334], but the generic identity of the two states was not recognised until the discovery of petrified material afforded the clue. Figs. 424, 1, 425 illustrate the appearance of _Trigonocarpus_ when preserved as a carbonised impression showing a thick fleshy envelope enclosing an oval kernel with a hard wall prolonged upwards as a longer or shorter micropyle. Casts of the seed-cavity are represented in figs. 423; 424, 2, 3. The surface of these casts occasionally shows one or more short cylindrical projections which are probably extensions of the sand or mud into holes formed in the testa by boring insects. The view that _Trigonocarpus_ seeds are ‘obviously Palm fruits’ was not accepted by Hooker and Binney who inclined to regard them as the seeds of Conifers and compared them especially with the similar nuts of _Ginkgo_ seeds (_cf._ fig. 631, C). It was Mr Wild[335] who was first struck by the association of _Trigonocarpus_ and the petioles of _Medullosa_ (_Myeloxylon_) and by some resemblances in structure between the testa and the hypoderm of the petioles; though, as Scott and Maslen[336] point out, the agreement is not so close as Wild believed, his view of a possible connexion between the reproductive and vegetative organs has been confirmed. Williamson extended our knowledge of the genus by his account of _Trigonocarpus olivaeformis_ Lind. and Hutt., a form that is specifically identical with _T. Parkinsoni_ Brongn. This author also drew attention to the close resemblance between Brongniart’s three genera _Trigonocarpus_, _Hexapterospermum_, _Tripterospermum_ and expressed doubts as to the possibility of founding specific differences on casts of the _Trigonocarpus_ type without the evidence of anatomy. Our knowledge of the structure of _Trigonocarpus_ has in recent years been considerably extended by the researches of Oliver, Scott and Maslen, and Salisbury.

[Illustration: Fig. 423. Casts of the seed-cavity of _Trigonocarpus Parkinsoni_.

(Manchester Museum. ½ nat. size.)]

_Trigonocarpus Parkinsoni_[337] Brongniart.

The seeds of this species like all examples of the genus are radiospermic, that is radially symmetrical in contrast to the flattened or platyspermic seeds. The complete seed is elongate oval in form when preserved as an impression (fig. 425, A) and reaches a length of 4–5 cm.: the casts of the seed-cavity are ovoid and provided with three prominent ridges (fig. 424, 2, 3). The testa forms a thick covering differentiated into three regions, an outer flesh or sarcotesta, a sclerous shell or sclerotesta, and an inner flesh. Transverse sections show that the sclerotesta has three sharp longitudinal keels with corresponding furrows on the inner face, and between each pair of main ribs are 2–3 less prominent ridges, usually 12 in all (fig. 426). The sarcotesta consists of thin-walled parenchyma passing externally into a more lacunar tissue with a palisade-like hypoderm: the sclerotesta consists of thick cells which interlace and form an efficient protective shell. Both the sarcotesta and sclerotesta are continued into the apical region as the wall of the long micropyle, the sarcotesta being prolonged beyond the sclerotesta at the apex of the integument[338]. The micropyle is triangular in section and may exceed in length the whole seed (figs. 425; 426, A). Its form as seen in transverse section (fig. 426, B) suggests the presence of wings: this appearance may be deceptive and due to pressure or, more probably, it represents an original feature. The seed-body, that is the portion enclosed by the integument, consists of the nucellus, represented by a few crushed layers of cells, bounded by a well-defined epidermis; the nucellus is separated from the integument from the base of the seed upwards, an important feature in which this and some other Palaeozoic seeds differ from _Lagenostoma_ and the seeds of recent Cycads which are characterised by an integument adnate to the nucellus up to the level of the shoulder; the seeds of the Conifer _Phyllocladus_ afford an example of separation of integument and nucellus as in _Trigonocarpus_. The innermost layer of the nucellus consists mainly of tracheal tissue investing the large megaspore (fig. 426, A, C, _m_) which is preserved as a contracted membrane detached from the nucellus after the death of the seed. At the summit of the nucellus is a relatively small pollen-chamber (fig. 426, A, _Pc_) like a broad and low cupola bearing a terminal beak which extended some distance into the micropylar tube. No microspores have been found in this species, but Oliver[339] records the occurrence of multicellular microspores in _Trigonocarpus pusillus_. The pedicel of the seed had a central strand of sclerous tissue penetrated by a concentric vascular bundle which gives off six strands to supply the sarcotesta (fig. 426, C, _v_) and then passes into the nucellus where it forms a tracheal sheath (fig. 426, A, _nt_) surrounding the lower part of the megaspore and at a higher level breaks up into anastomosing strands of tracheids which reach up to the plane of insertion of the pollen-chamber.

[Illustration: Fig. 424. _Trigonocarpus._ 1. Section of imperfect seed showing a micropyle; _b_, sclerotesta; _c_, cast of seed-cavity; 2, 3, apical and basal view of seed-cast. (Approximately nat. size.) M. S.]

[Illustration: Fig. 425. _Trigonocarpus Parkinsoni._ A. Impression showing the nucule and sarcotesta. (= _Carpolithes alata_ Lind. and Hutt.)

B. Specimen with long micropyle. (A, nat. size; from the Lower Coal Measures of Kilmarnock; B, nat. size; Middle C. M. of Yorkshire. Kidston Coll., 1579 and 1062.)]

[Illustration: Fig. 426. _Trigonocarpus Parkinsoni_. A, longitudinal section; _Sa_, _Sc_, _if_, sarcotesta, sclerotesta, inner flesh; _mi_, micropyle; _Pc_, pollen-chamber; _v_, vascular bundles; _m_, megaspore and prothallus; _t_, tracheal disc; _nt_, tracheids in the nucellus. B, transverse section of the upper region showing the sarcotesta, sclerotesta (black) and the micropyle. C, transverse section of _T. Parkinsoni_. D, transverse section of _Trigonocarpus shorensis_. (A, C, after Scott; B, after Wild; D, after Salisbury.)]

A second species described by Scott and Maslen as _Trigonocarpus Oliveri_ has been further investigated by Salisbury who finds that it is an 8-angled seed which cannot be retained in the genus _Trigonocarpus_: its systematic position ‘must for the present remain uncertain[340].’ Dr Arber has recently described a new species of _Trigonocarpus_, _T. Moyseyi_[341], from the Nottingham Coal-field (Middle Coal Measures), similar to _T. Parkinsoni_ but much broader in proportion to its length: this species is founded on an impression without structure.

The species _Trigonocarpus Dawesi_ Lind. and Hutt.[342], from the Middle Coal Measures of Lancashire, was founded on casts differing in their large dimensions from those of _T. Parkinsoni_: specimens referred to this species were described by Fiedler[343] from Saxony in 1857 and Lesquereux[344] figures similar casts from the Upper Carboniferous and Permian rocks of North America.

_Trigonocarpus shorensis_ Salisbury.

This species, founded on specimens from the Lower Coal Measures of Shore, Lancashire[345], may exceed 4 cm. in length and has a breadth of 2·5 cm. In general plan it agrees with _T. Parkinsoni_ but there are certain well-marked differences: the micropyle is much shorter; the thick sarcotesta, attaining a breadth of 6 mm. at the base of the micropyle, is characterised by the presence of six peripherally placed vascular bundles (fig. 426, D, _v_) in contrast to the deeply embedded bundles of _T. Parkinsoni_. Below the epidermis of the sarcotesta is a hypoderm formed of radially disposed plates of sclerous tissue similar to that of _Myeloxylon_ and different from the palisade-like hypoderm of the type-species. Within the sarcotesta is a hard shell, the sclerotesta, characterised by three prominent ribs extending from base to apex and three shorter ribs which reach from the chalaza to about a third of the length of the seed. The fact that the sarcotesta and sclerotesta pass gradually into one another is a point in favour of the view that the integument is a single structure. There appears to be good evidence of the restriction of an inner flesh to the micropylar region, whereas this tissue in _T. Parkinsoni_ was probably continuous over the whole inner face of the sclerotesta. The sarcotesta is lacunar in its outer part as in some other types of Palaeozoic seeds, a feature probably connected with floating efficiency. _Trigonocarpus shorensis_ occurs in association with _Myeloxylon_ petioles, and there is a resemblance between the seed and the vegetative organs in the structure of the hypoderm as also in the structure of the secretory sacs which are particularly numerous in this species. Salisbury draws attention to the close resemblance between the form of _T. shorensis_ and the seeds found in organic connexion with pinnae of _Neuropteris obliqua_[346].

The species _T. corrugatus_ described by Renault[347] bears a close resemblance to _T. shorensis_.

+Other Genera founded in part on Reproductive Organs which may belong to the Medulloseae.+

=Codonotheca=, =Schützia=, =Whittleseya=, =Dolerophyllum=, =Ottokaria=, =Strobilites=.

=CODONOTHECA.= Sellards.

_Codonotheca caduca_ Sellards. This genus was founded on some spore-bearing bodies from the Coal Measures of Illinois[348]: nothing is known as to the plant which bore them, but Sellards is inclined to associate them with _Neuropteris decipiens_ Lesq.[349], a species abundant in the same coal-field. Whatever may have been the parent-plant it is probable, as the author of the genus believes, that _Codonotheca_ is the microspore-bearing organ of a Pteridosperm. As shown in fig. 427, 5, the form is that of a stalked cup consisting in the basal portion of a stout axis, the peripheral tissue of which is believed to have been fleshy, containing an axial rod of conducting tissue running up to the floor of the cup, _c_, and then dividing into six vascular strands, each of which forks into two branches. The upper part is composed of six linear segments united basally to form the sloping surface of the cup. On the inner face of each segment is a more or less well-defined depression covered with large elliptical spores ·29–·31 mm. long by ·18–·19 mm. broad (fig. 427, 6, 8). The presence of a median ridge (fig. 427, 8) indicates a bilateral origin. ‘There is no grouping of the spores or other indication of the location of the sporangia, which were doubtless more or less completely immersed in the tissue, the dividing wall disappearing at maturity.’ The spores are seen in fig. 427, 2, 3, on the inner face of the lobes. Some of the specimens have a fairly long pedicel: in the example shown in fig. 427, 1, the fleshy part of the basal portion is not preserved, only the more resistant vascular core. In a later account of these organs Sellards speaks of several lying by the side of a central stalk to which he thinks they were originally attached by slender pedicels. In view of Dr Benson’s interpretation of the morphology of _Telangium_ it is permissible to suggest that if a central sporangium in such a synangium as that of _Codonotheca_ developed a megaspore and the peripheral sporogenous lobes were sterilised, the result would be an arrangement not unlike the apical region of the seed _Physostoma_, the tentacles of which have been homologised with the canopy of _Lagenostoma_. There are obvious difficulties in the way of this, perhaps strained, comparison: the larger size of the spore-bearing linear segments of _Codonotheca_ led Sellards to regard each as a synangium rather than a single sporangium. But precise information as to the structure of the American fossils is not as yet available. If the association of _Codonotheca_ with _Neuropteris_ fronds has any significance it would favour a reference of these organs to the Medulloseae. In the absence of anatomical data it is impossible in some cases to distinguish microspore-bearing organs of the _Codonotheca_ type from small seeds enclosed in a lobed cupule or even seeds with a lobed integument: a case in point is the New Brunswick species _Pterispermostrobus bifurcatus_ Stopes[350].

[Illustration: Fig. 427. _Codonotheca caduca_. 1. The vascular tissue preserved as a conical base which was originally surrounded by fleshy tissue (_cf._ 5); I–III, VI, vascular bundles (× 2). 2, 3. Segments with spores; _c_, floor of cup (nat. size). 4. Vascular tissue as a cone in the middle of the fleshy base (nat. size). 5. Plan of the whole organ opened out flat (nat. size). 6. Spores (× 28). 8. A single spore showing median slit (× 84). (After Sellards.)]

A Spitzbergen, Culm, fossil recently described by Nathorst[351] as _Codonotheca (?) pusilla_ is briefly referred to under the genus _Pterispermostrobus_.

=SCHÜTZIA.= Geinitz.

This generic name was instituted by Geinitz[352] for some Permian fossils obtained by Bergmeister Schütz and regarded by the author of the genus as probably fertile branches of some Conifer. A more complete account was published by Goeppert[353] in his ‘Permian Flora,’ where the name _Anthodiopsis Beinertiana_ occurs on the Plates, printed before the publication of Geinitz’s description, but in the text the specimens are referred to _Schützia anomala_.

_Schützia anomala_ Geinitz.

The type-species, recorded from Bohemia and Silesia, is represented by fertile shoots consisting of a thick main axis bearing apparently two-ranked though probably spirally disposed short lateral branches, each of which terminates in a receptacle with numerous crowded linear-lanceolate bracts superficially resembling a partially expanded inflorescence of a Composite. Goeppert believed that the branches bore seeds and he refers to this species a number of detached, longitudinally striated and bluntly terminated, seeds. The same author describes other specimens from the same localities associated with _Schützia anomala_, which he names _Dictyothalamus Schrollianus_[354]: in habit these agree closely with _Schützia_ but the receptacles, the reticulate appearance of which suggested the generic name _Dictyothalamus_, bear a large number of small bodies regarded as seeds. The preservation of the fossils is not such as to enable us to determine their true nature but it is probable that _Schützia_ and _Dictyothalamus_ are not generically distinct. In his description of _Dictyothalamus_ Goeppert suggests that the two associated types may be the male and female shoots of one plant, but he speaks of seeds in both cases. Schimper[355], who unites _Dictyothalamus_ with _Schützia_, regards the latter as female and the former as male.

_Schützia Bennieana_ Kidston.

This species, described by Kidston[356] from the Calciferous series of Scotland, differs from _S. anomala_ in its much more slender axis and in the relatively narrower and less globular clusters of bract-like appendages. The principal axis bears three lateral branches with terminal clusters of acute and narrow linear scale-leaves. No seeds were found in association with the specimens.

_Schützia permiensis_ (Renault).

Renault founded this Permian species as _Antholithus permiensis_[357] on a specimen from Lodève; it consists of an incomplete inflorescence 6·4 cm. long bearing four lateral branches with stalks 1·5 to 2 cm. long terminated by clusters of small oval bracts 5 mm. long. Renault compares the fossil with the recent Conifers _Glyptostrobus_ and _Tsuga_, but it exhibits a much closer resemblance to _Schützia anomala_.

The genus _Schützia_, originally described from Permian strata, is recorded also from Westphalian strata in North Africa[358] as well as from Lower Carboniferous rocks in Scotland. The data at present available are insufficient to determine the morphological nature of the fertile branches: the evidence adduced by Goeppert in support of the occurrence of seeds is not convincing and the interpretation of the bract-like appendages is still an open question; they may have formed a cupular investment to seeds, but in the Scotch species the general appearance rather suggests that they may be microspore-bearing organs comparable with those of _Codonotheca_[359]. There are no adequate grounds for supposing _Schützia_ to belong to the Coniferales, a view advanced by some authors; it is much more likely to represent the fertile shoots of a Pteridosperm.

=WHITTLESEYA=. Newberry.

The genus _Whittleseya_, referred by many authors to the _Ginkgoales_, has no substantial claim to be regarded as allied to that group: its position is still uncertain, but the recent discovery of fertile specimens suggests the probability of a relationship to _Potoniea_ and an identification of Whittleseya as another form of microsporophyll of a Pteridosperm.

[Illustration: Fig. 428.

A, C. _Whittleseya elegans_. A, single leaflet, from Pennsylvania. (After Lesquereux; ⅚ nat. size.) C, diagrammatic sketch of part of a leaflet showing the teeth, the striated texture of the carbonised layer, and the vein-like markings on a lower surface. (After Thomas; enlarged.) B. _Whittleseya brevifolia_. (After White; the smaller figure ⅚ nat. size.)]

The generic name was given by Newberry[360] to some leaves, or possibly leaflets, originally described by C. Whittlesey from the Coal Measures of Ohio. Whittleseya is represented by species from several North American localities[361] in Ohio, Pennsylvania, Arkansas, Nova Scotia, and New Brunswick[362]; it occurs in Silesia and has recently been found in the English Coal Measures[363]. The genus is confined to Upper Carboniferous strata.

The leaves are fairly thick; the lamina is oblong, cuneate, broadly triangular or linear, usually rounded and truncate (fig. 428, A, C), generally dentate at the distal end, the proximal portion being gradually or abruptly contracted and occasionally prolonged into a short pedicel. The veins or ribs are parallel to the sides of the lamina and except near the base unbranched.

_Whittleseya elegans_ Newberry.

The type-species, from North America and Europe, is characterised by its shovel-like lamina from 3 to 6 cm. long closely resembling in shape some lepidopterous scales; the surface is ribbed, each rib corresponding to a tooth on the distal margin; on each of the parallel ridges are 4–5 longitudinal lines indicating either veins or stereome strands (fig. 428, A, C). The examination of preparations made by Dr Kidston from a leaflet of this species enables me to add a few facts with regard to the microspores. The spores, which cover almost the whole surface of the lamina, show a tendency to a more or less definite arrangement in longitudinal rows. Two types of cuticularised membrane are represented among the associated fragments: in some pieces of cuticle the cells are short and have straight walls while in others the preservation is inferior and the cells appear to be longer and narrower. One or both of these membranes probably belong to the sporangia. The oval slit, which is a striking feature on several of the spores (fig. 429), points to their bilateral nature and dehiscence along the major axis. A comparison of these spores with those obtained by Kidston from the English species _Whittleseya fertilis_ reveals a very close agreement both in size and shape and confirms the identification of the Staffordshire specimens as leaflets of _Whittleseya_. The large size of the microspores and the gaping oval aperture in some of them are features in which they agree closely with the spores of _Dolerophyllum fertile_ described by Renault[364]. In both cases the spores tend to be arranged in long groups and they are practically identical in form and in the nature of the exine; those of _Dolerophyllum_ are 280μ long while those of _W. fertilis_ reach a length of 220μ. In some of the _Whittleseya_ spores the exine has split as in the specimen shown in fig. 429, but in others there are two curved lines along which dehiscence has begun, a character in which the spores appear to be identical with those of _Dolerophyllum_ described by Renault who speaks of dehiscence by means of an operculum. There is, I venture to think, little doubt as to the very close affinity of the two types. The systematic position of _Dolerophyllum_ is not certainly established; if the generic identity of the leaves described as _D. Berthieri_ Ren. and the petrified specimens named _D. fertile_ is assumed, it is a legitimate inference that the genus is founded on fertile pinnules of a Pteridosperm with foliage of the _Neuropteris_ or _Cyclopteris_ form. It would seem probable that both _Whittleseya_ and _Dolerophyllum_ _fertile_ are microspore-bearing leaflets of Pteridosperms, possibly of some Medullosan plants. The leaflets of _Whittleseya_ agree in form fairly closely with those of _Potoniea adiantiformis_ Zeill. described on a previous page[365] as the male organs of a Pteridosperm.

[Illustration: Fig. 429. Microspores of _Whittleseya elegans_. A, a group of spores; B, a single spore. (Preparations made from an American specimen, No. 2314, in Dr Kidston’s Collection.)]

The specimens described by Lesquereux from Pennsylvania as _W. integrifolia_ and _W. undulata_ are less satisfactory than _W. elegans_. The Arkansas species _W. microphylla_[366], characterised by the obcuneate form of the lamina, is said to occur not only as detached leaflets but in loose bunches at the ends of slender axes, a circumstance favourable to the suggestion, based on the recently described English specimens, that the _Whittleseya_ leaves may be fertile pinnules of a Pteridosperm frond. Among other species attributed to Newberry’s genus is _W. brevifolia_ Wh. from Nova Scotia[367] with much smaller broadly triangular leaves 7 mm. long exclusive of the petiole and 8 mm. broad at the distal end (fig. 428, B). Dr Matthew[368] has also described a Canadian species _W. concinna_ from New Brunswick in beds assigned by Dr Stopes[369] to the Westphalian series.

_Whittleseya fertilis_ Kidston.

Since the discovery of _Whittleseya elegans_ in the Coal Measures of Staffordshire recorded by Mr Thomas, Dr Kidston has published an account of some specimens from the same district under the name _Whittleseya (?) fertilis_[370]: these consist of smaller cuneate scale-leaves or leaflets 1·4–2·4 cm. long and 8–9 mm. broad; the lamina has a dentate upper margin and is longitudinally ribbed. The scales occur in superposed pairs, closely fitting but not organically connected, at least in the state in which they are preserved; each pair forms a sporangium-like case enclosing numerous spores but the actual sporangia or synangia have not been preserved. Kidston describes the spores as 210–222μ in length, elliptical, and characterised in many cases by an oval slit; they are practically identical with the microspores of _W. elegans_.

=DOLEROPHYLLUM=. Saporta.

This name was proposed[371] primarily for a large ovoid petrified bud composed of rolled Cyclopteroid leaves from Permian rocks in the Ural Mountains, which had been previously described by more than one writer under different names and regarded as a young shoot of a Palm or other Monocotyledon. Eichwald[372], who published good drawings, called the fossil _Noeggerathia Goepperti_. Saporta connected with this species some leaf-impressions from the Permian of Bohemia described by Goeppert[373] as _Noeggerathia cyclopteroides_: in his family Dolerophylleae[374] the French author included other leaves which are probably not closely related to the type-species, _Dolerophyllum Goepperti_. The Dolerophylleae are spoken of by Saporta and Marion[375] as Progymnosperms. Before the publication of Saporta’s note Grand’Eury had instituted the genus _Doleropteris_[376] and the family Doleropteroideae; in the former he included several forms of leaves agreeing generally with Goeppert’s _Noeggerathia cyclopteroides_. Zeiller[377] adopts Grand’Eury’s designation for the Russian fossil in preference to _Dolerophyllum_, a choice justified by considerations of priority; but the latter name is retained in this account as it was assigned by Saporta to the specimen of greatest botanical interest, namely _Dolerophyllum Goepperti_, and because it does not suggest affinity to Ferns.

_Dolerophyllum Goepperti_ (Eichwald).

The type-species is from the Zechstein of Orenburg in the Urals and no specimens having precisely the same structure have been found elsewhere. Eichwald assigned it, with leaf-impressions of various kinds, to the Noeggerathieae and named it _Noeggerathia Goepperti_: it had previously been described by Kutorga[378] as _Aroides crassispatha_ and Unger[379] included it among the Palms as _Palaeospathe aroidea_. The species has been described also by Saporta and Marion and by Renault[380]. The following account is based on sections cut from a specimen in the British Museum[381] which, though assigned on the label (within a query) to France and named _Dolerophyllum Berthieri_, is undoubtedly Eichwald’s species from East Russia.

[Illustration: Fig. 430. _Dolerophyllum Goepperti_. Bud in surface-view, A; in longitudinal section, B, C; in transverse section, D. (British Museum; A, ⅔ nat. size.)]

The specimen (fig. 430) is 9 cm. long and 4·2 cm. broad: at the slightly contracted and broken base is a piece of immature axis (fig. 430, B, _a_) 12 mm. in diameter overtopped by a mass of closely packed leaves encircling one another like the bulb-scales of an Onion (fig. 430 A, C)[382]. Most of the leaves included in the bud were attached to the axis below the broken base. The curved, dichotomously branched, veins are seen on some of the pieces of lamina on the surface of the bud (fig. 430, A). The considerable breadth of the leaves is demonstrated by the longitudinal and transverse sections. In fig. C most of the laminae can be traced through the whole height of each of the steep-sided arches: a few overlapping margins are seen in fig. D. The veins are for the most part imperfectly preserved and appear as clear spaces at regular intervals in the brown mesophyll. The axis of the shoot consists of homogeneous parenchyma except near the sloping sides where narrow dark bands (fig. 430, B, _a_) mark the position of desmogen-strands of thin-walled elongated elements representing an early stage in the development of vascular bundles some of which have already produced spiral tracheids. Short secretory cells accompany the immature conducting elements. The lamina slightly exceeds 2 mm. in thickness in the broadest part: the mesophyll is composed of large parenchymatous cells of elliptical or spherical form often loosely attached owing to the well-developed system of intercellular spaces. The lower epidermis, assuming that the outer face of the rolled leaves is the morphologically lower surface, forms a uniform layer of palisade cells characterised by their free conical ends (fig. 431, A, B, _e_) which in some oblique sections appear as sharply pointed papillae with almost filiform apices; but while the cells were doubtless papillose like those of the epidermis of a velvety petal, the pointed form is due in part to the greater distinctness of the dark contents as compared with the lighter cell-walls. The upper epidermis is much less distinct; it consists of smaller flattened cells with occasional stomata. Renault[383] figures a specimen with stomata in a better state of preservation. The vascular bundles are rendered conspicuous by large secretory cells on the lower side, in the larger veins in the form of an arc or irregular group (fig. 432, A), but in the finer veins as single cells (fig. 431, A, B). These sacs resemble the tannin cells accompanying the veins in a leaf of _Ginkgo_ (_cf._ fig. 631, G). The xylem-elements are of two kinds, (i) elongated spiral and scalariform conducting elements, forming a vertical plate of a few rows in the larger veins (figs. 431, 432) or a small compact group in the more slender veins (fig. 413, A, B); (ii) much larger isodiametric cells with reticulate or spiral thickening resembling the transfusion-tracheids of Conifers or, perhaps more closely, similar elements in the leaves of _Lepidodendron_. These short tracheids are especially abundant on the flanks of the conducting tracheids (figs. 431, _t_; 432, A, _t_), but they sometimes form a complete investment. In the obliquely cut vein reproduced in fig. 431, D, the transfusion-tracheids are abundant: a few are enlarged in fig. 431, E. In the smaller veins (fig. 431, A, B) they are represented by the larger elements, _t_, on the sides of the conducting strands. The protoxylem lies close to the upper edge in the middle line (_px_, figs. 431, A; 432, A); it is difficult to determine its precise position, but it would seem to be slightly internal, the bundle being not quite endarch. No phloem was recognised in the British Museum specimen, but it presumably occurred, if present, where the black patch is shown in fig. 432, A. Renault describes some phloem in sections which he examined.

[Illustration: Fig. 431. _Dolerophyllum Goepperti_. Transverse sections of leaves. _e_, epidermis; _s_, secretory cells; _t_, transfusion-tracheids; _px_, protoxylem. (British Museum.)]

The mesophyll next the upper surface is in most cases represented by spaces between the veins which give a crenulated outline to the parenchyma (fig. 430, C, D); in some places the spaces contain remains of very loose and crowded cells suggesting the original presence of very lacunar tissue or possibly of thin-walled storage-cells. The confinement of stomata to what is assumed to be the upper surface may, as Renault and others have suggested, indicate leaves which floated on water, an inference opposed to the view that the gaps in the mesophyll mark the position of water-tissue.

No specimens have been described which enable us to correlate with certainty mature leaves or foliage-shoots with the petrified bud. It is, however, not improbable that the impression from Mount Pelé near Epinac named by Renault _Dolerophyllum Berthieri_[384] may be correctly referred to the same genus. The type-specimen consists of an axis, whether a rachis of a compound leaf or a shoot with simple leaves cannot be determined, bearing partially overlapping more or less orbicular leaves 18–20 cm. in diameter, with a _Cyclopteris_ venation. Among other leaves of unknown affinity referred to the same genus attention is drawn to _Dolerophyllum pseudopeltatum_ (Grand’Eury)[385] with an orbicular lamina reaching in some specimens 22 × 19 cm. Specimens of _Dolerophyllum pseudopeltatum_ are figured by Renault from the Commentry coal-field[386], some of which reach a diameter of 12 cm. The only British specimen of a leaflet of this type which I have seen is one in Dr Kidston’s collection from the Stephanian series, Glamorganshire. It is probable that some at least of the impressions assigned to _Dolerophyllum_ or _Doleropteris_ would be more appropriately included in _Cyclopteris_ or _Cardiopteris_ and may have been borne on the axis of large Pteridosperm fronds. Grand’Eury[387] has also called attention to the difficulty of distinguishing the larger _Cyclopteris_ leaflets from _Dolerophyllum_. Some of the Cyclopteroid leaflets figured by Roehl[388] on _Neuropteris_ fronds differ but slightly from those of _D. pseudopeltatum_. The shoot showing large leaf-scars figured by Saporta and Marion[389] as probably the axis of a _Dolerophyllum_ may well be a piece of _Cordaites_.

[Illustration: Fig. 432. A. _Dolerophyllum Goepperti_, section of vein. B. _Dolerophyllum Berthieri_; _s_, microspores. C. _Dolerophyllum fertile_. D. _Dolerophyllum fertile_, microspores. (After Renault.)]

_Microsporophylls assigned to_ Dolerophyllum.

Certain problematical fossils found in association with the sterile leaves of _Dolerophyllum Berthieri_ have been described by Renault as the male organs of that species. These are elliptical discs, 6 × 5 cm., with an excentrically placed stalk: embedded in a carbonised lamina are numerous rows of elliptical bodies, 410μ × 280μ, characterised by two curved longitudinal grooves on the surface and regarded by Renault as pollen-grains. The chains of these microspores radiate outwards from the neighbourhood of the stalk and cover most of the surface of the disc (fig. 432, B). Some silicified pieces of similar spore-bearing discs from Grand’ Croix named _Dolerophyllum fertile_[390] afford additional information as to these remarkable reproductive organs. The earlier account of this species by Renault is confirmed by Solms-Laubach[391] who examined the original sections. The peltate fleshy discs preserved as incomplete specimens consist of lacunar parenchyma 15–18 mm. thick traversed at right angles to the surface by numerous loculi (fig. 432, C), circular or oval in transverse section, containing large numbers of microspores, _s_, similar in size and form to those on the carbonised discs of the Mt Pelé specimen. Vascular strands occur between and parallel to the spore-chambers. The spores contain 8–10 cells (fig. 432, D) and Renault believes that dehiscence of the exine occurred along the two deep grooves which mark the limits of an operculum. He emphasises the peculiar structure of the microspores by speaking of them as prepollinia: in size and in the presence of internal cells (? male prothallus) they resemble the spores found in the pollen-chamber of a seed described by Renault as _Aetheotesta elliptica_[392] which he thinks may belong to a member of the Dolerophylleae. It has also been suggested that _Codonospermum_ may be a seed of _Dolerophyllum_[393]. An unconvincing specimen described by Saporta and Marion[394] as a seed-bearing bract is regarded by them as referable to _Dolerophyllum_, but the evidence for any connexion is far from satisfactory.

There is nothing definite to be said with regard to the affinity of _Dolerophyllum Goepperti_ or the microsporophylls represented by _D. fertile_ and the specimens associated with _D. Berthieri_. Renault considers that both sterile and fertile specimens belong to the same genus, which he assigns to a position between Pteridophytes and Cycads. As Solms-Laubach says, the evidence supplied by the structure of the veins of _D. Goepperti_ in favour of a cycadean alliance is not convincing. The type of vernation is unlike that of any known Cycad or indeed of any Gymnosperm: the large size of the leaves is another though weaker objection to this comparison, as the pinnae of _Bowenia_ (fig. 391) and especially those of some species of _Zamia_ (fig. 388), are of equal or larger dimensions. If, as seems probable, the xylem-strands are mesarch that is a point of contact with recent Cycads, but the bundle as a whole bears but a remote resemblance to that of a cycadean leaf and is much more like the veins of _Ginkgo_. The bud shown in fig. 430, A, is probably a young shoot and not merely a large compound leaf. If it were an unexpanded frond of _Neuropteris_ bearing _Cyclopteris_ pinnules we should expect to find indications of scattered desmogen-strands such as would occur in the _Myeloxylon_ type of rachis. The resemblance to most forms of _Cordaites_ is by no means close though a few leaves referred to that genus (_e.g._ _C. circularis_, fig. 468, B) are similar to those of _Dolerophyllum_[395].

The male organs are unlike those of any other plant: they may be described as sporophylls with microsporangia or perhaps synangia embedded in the mesophyll and containing microspores similar to those of some Pteridosperms or true Gymnosperms. Attention has been called to the close resemblance of the spores shown in fig. 432, C, D to those recently discovered by Kidston and referred to the genus _Whittleseya_ (fig. 429), and it is very probable that the striking similarity is an index of affinity.

=Ottokaria.= Zeiller.

_Ottokaria bengalensis_ Zeiller. A specimen of doubtful affinity from the Lower Gondwana (Karharbari beds) of Passerabhia, India, was originally described by Zeiller[396] as _Feistmantelia bengalensis_, but in a postscript he substituted the name _Ottokaria_ on the ground that _Feistmantelia_ had previously been employed by Lester Ward. Fig. 433 is drawn from the original specimen: it consists of a stalk attached in a slightly excentric position to an almost orbicular lamina, 2·5 cm. in diameter, with subacute marginal teeth and traversed by numerous radially disposed striations. Zeiller compares the fossil with _Whittleseya elegans_ and _Rhipidopsis ginkgoides_ and assigns it with some hesitation to the Salisburieae. An examination of the type-specimen led me to form the opinion that it may be a cupular organ of a Pteridosperm that enclosed a seed. The lamina is slightly concave and has the form of a shallow cup; moreover the surface-features resemble those of a bract rather than the regularly veined lamina of a foliage-leaf. The specimen bears a very close resemblance to one figured by Bertrand[397] as the cupule of _Hexapterospermum modestae_ which he connects with fronds of _Neuropteris gigantea_.

[Illustration: Fig. 433. _Ottokaria bengalensis._ (Nat. size; drawn from the type-specimen.)]

_Ottokaria_ occurs in association with fronds of _Glossopteris indica_ and with the large seeds described by Zeiller[398] as _Cardiocarpus indicus_. I have lately obtained some evidence in favour of assigning Feistmantel’s seeds _Carpolithes Milleri_[399] to the genus _Glossopteris_: among several specimens from the Lower Gondwana rocks of India I found an example showing a seed partially covered by a scale-leaf in its natural position which appears to be identical with scale-leaves of _Glossopteris_. It may be that the specimen represented in fig. 433 belongs to _Cardiocarpus indicus_, though this is a mere guess: my belief is that _Ottokaria_ is a cupular organ that enclosed the base of a seed borne on a Pteridosperm. There is little doubt that as additional data are obtained it will be found that Pteridosperms played no inconsiderable part in the vegetation of Gondwana Land.

[Illustration: Fig. 434. _Strobilites Milleryensis._ (Natural size; after Renault.)]

=Strobilites.= Schimper and Mougeot.

_Strobilites Milleryensis_ (Renault).

This species, from the Permian of France, was placed by Renault in _Cycadospadix_[400], but having regard to the fact that it differs essentially in habit from Mesozoic examples of that genus the provisional name _Strobilites_[401] is suggested. The type-specimens are long and narrow spikes or loose strobili, 8–16 cm. long and 2–2·6 cm. broad; a stout axis bears spirally disposed bracts 8–10 mm. long attached by a slender decurrent pedicel expanded distally into a fan-shaped laciniate lamina with a convex upper face, and there are said to be two seeds attached to the sides of each pedicel (fig. 434). The oval seeds appear to be platyspermic and resemble _Samaropsis fluitans_ Daws. Two of the strobili figured by Renault are attached at right angles to a second axis, a habit suggesting comparison with that of a large compound frond. Renault is inclined to regard these fertile shoots as cycadean and suggests a possible connexion with the Permian stems _Ptychoxylon_ or _Poroxylon_, both of which are known to have produced fairly numerous branches. In habit the spikes are similar to some of the longer examples of _Cordaianthus_, but their preservation is not sufficiently good to afford accurate information as to the relation of seed to sporophyll. _Strobilites Milleryensis_ is, perhaps, more likely to be the fertile branch of a compound frond of a Pteridosperm, and it is significant that the seeds have been found in association with _Callipteris_ leaves.

=COLPOXYLON=. Brongniart.

_Colpoxylon aeduense_ Brongniart. The genus was founded by Brongniart[402] on a piece of stem 15 cm. in diameter from the Permian of the Autun district and regarded by him as a distinct type, with certain resemblances to recent Cycads. A thick section in the British Museum, 13 cm. in diameter (fig. 435, A), illustrates the main anatomical features described by Renault[403], to whom our knowledge of the genus is chiefly due. There are two large steles of irregular outline closely resembling those of _Medullosa Leuckarti_ (_cf._ fig. 416, H); each consists of a band of secondary xylem with broad medullary rays and a narrow zone of phloem enclosing a central region composed of parenchyma, in which strands of primary tracheids, both reticulate and spiral, pursue a more or less horizontal course, associated with a few small groups of vertical xylem-strands at the inner edge of the secondary wood. The manoxylic nature of the wood is clearly shown in fig. 436; the continuous ink-line marks the position of the cambium and the dots show the internal protoxylem. Homogeneous parenchyma surrounds the steles and beyond this is crushed tissue containing large secretory canals and nests of stereome fibres either as separate groups or in contact with the canals (fig. 435, C). In the same peripheral tissue occur scattered collateral vascular bundles (fig. 435, D) identical with those of _Myeloxylon_. The outer cortex of the stem is marked off from the more homogeneous inner region by a fairly distinct line where there is some indication of periderm. The anatomical features are clearly shown in fig. 436, a photograph from a section in Dr Kidston’s collection. At _a_ is an imperfectly preserved vascular bundle with a crescentic group of secondary xylem which is probably a leaf-trace that has just emerged from the secondary cylinder. Renault speaks of these more or less circular strands as possibly connected with reproductive shoots, but it is more probable that they are homologous with the strands in the pericycle and inner cortex of _Medullosa_ and represent leaf-traces before division into smaller collateral strands. Renault describes the stem as possessing seven vascular cylinders in the apical region and suggests branching of the main axis as the cause of the increase in number: there is, however, no evidence to support such correlation. The two steles seen in fig. 435, A, become merged at a lower level into a single stele of sinuous form (fig. 435, B).

[Illustration: Fig. 435. _Colpoxylon aeduense_. A. Diagram of a transverse section of a specimen in the British Museum. (V. 9393. ½ nat. size.) B. Stele; much reduced from Renault’s figure. C, D. Secretory canal with fibres and collateral vascular bundle (Kidston Coll., 1946).]

[Illustration: Fig. 436. _Colpoxylon aeduense_. Transverse section of half of the stem; _a_, stele of branch. Slightly enlarged. (Kidston Coll., 1946.)]

Beyond the facts furnished by the leaf-trace bundles in the outer cortex and the occurrence of two large scars about 5 cm. in breadth on a stem figured by Renault, we have no positive information as to the form of the leaves or the structure of the reproductive organs. There is little doubt that the fronds were large and compound like those of most species of _Medullosa_. There is, however, some slight evidence that _Alethopteris Grandini_ Brongn. and seeds of the _Pachytesta_ type (fig. 497) were borne on _Colpoxylon_ stems; this rests solely on the association in the Loire coal-basin[404] of _Alethopteris_ fronds with stems presenting structural resemblances to _Colpoxylon aeduense_.

The striking resemblance between _Colpoxylon_ and _Medullosa Leuckarti_ has led certain authors[405] to propose the substitution of _Medullosa_ for _Colpoxylon_. The resemblances though close are hardly sufficient to warrant this course. In _Colpoxylon_ the stelar system is simpler; there is no central region with star- or plate-rings as in _Medullosa Leuckarti_ but, as in _Medullosa anglica_, the vascular tissue consists only of large steles without a medullary system. _Colpoxylon_ differs from _M. anglica_ in the reduction in some parts of the stem of the vascular system to a single stele and, moreover, the primary portion of the steles is much more parenchymatous in structure and contains more irregularly anastomosing tracheal strands than is the case in _M. anglica_.

The alteration in the pattern formed by the vascular system at different levels in some Medullosan stems, especially in _Colpoxylon_, may be compared with the varying disposition of the vascular strands in the thick dorsiventral rhizomes of _Polypodium heracleum_ Kunz. and _P. quercifolium_ L. In the rhizome of _P. heracleum_ there are two vascular systems, an outer, cortical, system in the form of a hollow cylinder composed of a lattice-work with polygonal meshes from which branches are given off to the roots, and a more complex medullary system that is concerned with the emission of leaf-traces. As shown by a series of drawings reproduced in an account by Klein[406] of the anatomy of these species of _Polypodium_, the inner system of steles consists of two cylinders connected towards the upper surface of the stem by a rounded arch of vascular strands; nearer the leaf-base the two cylinders meet and eventually a larger cylinder is produced partly from the upper halves of the two cylinders of the previous section and in part from the connecting arch: the remains of the two smaller cylinders become connected with the outer vascular system. These and other changes suggest comparison with _Colpoxylon_ as also with the stelar changes in the stem of _Ptychoxylon_. The comparison cannot be carried beyond the grosser features and is chiefly interesting as affording a further illustration of a similarity in plan between some recent Ferns and extinct Pteridosperms and other Palaeozoic genera.

[Illustration: Fig. 437. _Rhexoxylon africanum_. Transverse section of stem showing peripheral steles and central (_c_) stele. (After Bancroft; × 2.)]

=RHEXOXYLON=. Bancroft.

_Rhexoxylon africanum_ Bancroft. The genus _Rhexoxylon_ was instituted for a new type of stem represented by a single incomplete specimen from the Karroo series of South Africa: its precise geological horizon is not known but it may be referred provisionally to the lower or Palaeozoic portion of the series. Though our knowledge of the morphological features of the type-species is far from complete owing in part to the method of preservation of the specimen and in part to the destruction of the outer portion of the vascular tissue and the whole of the cortex, Miss Bancroft’s careful description[407] demonstrates the existence of characters which justify the employment of a new generic name. _Rhexoxylon_ is more nearly related to the Medulloseae than to any other group and is particularly interesting as the first recorded example of this group from the Southern Hemisphere.

Fig. 437 shows a transverse section (7 × 5 cm.) of the stem. The ground-tissue consists of fairly large-celled parenchyma with sclerous nests and a few bands of periderm. At the periphery of the stem are radially disposed groups of vascular tissue varying in size and to some extent in shape. Unfortunately the stem is incomplete and it is impossible to say how much vascular or other tissue originally existed beyond the present corroded edge. The vascular groups, or steles as they may legitimately be called, follow a vertical course through the length of the block (6·5 cm.) and afford only slight evidence of branching or anastomosing. A close examination of the steles shows that they consist of portions of two series, an inner and outer set; there is also a curved vascular band in the central ground-tissue (fig. 437, _c_) and some isolated and scattered patches of vascular elements. Each stele of the inner series is made up of two parts, an outer smaller and normally orientated group of secondary xylem and a larger inversely orientated inner group of identical structure. A single stele of the inner series is shown in fig. 438, B, C; the larger inner portion consists of slightly divergent rows of tracheids and uniseriate medullary rays and is separated from the smaller portion by a narrow space, _a_, occupied by crushed tissue which may correspond to the ‘partial pith’ or primary xylem of a Medullosan stele. The two groups of xylem are no doubt the products of two cambium arcs, the protoxylem of each group being situated on the flat inner face. The cambium and phloem are represented only by crushed brown cells on the curved outer edge of the xylem. The separate individuality of the two portions of each stele is indicated not only by the presence of the ‘partial pith’ but by the discontinuity of the tissue at the ends of the narrow space. The tracheids seen at _b_, fig. C, are in oblique longitudinal section and are probably being detached to form a leaf-trace. This type of stele may be compared with the steles of _Medullosa Solmsi_ (fig. 416, L) but those of _Rhexoxylon_ differ in the lack of continuity of the secondary xylem round the narrow band of crushed primary xylem. The other steles of the inner ring exhibit the same dual nature though with local modifications. In the stele seen in fig. 438, B, there is a close approach to a continuous cylinder of secondary xylem especially on the right-hand side. External to the inner series are several portions of normally orientated secondary xylem-groups (fig. 437): these probably represent a second series of steles separated from the inner series by a narrow crushed arc of tissue on which the protoxylem strands of the outer groups abut. The xylem of the outer steles agrees in its normal orientation with the outer and smaller part of the inner steles and, as there is no accompanying group of inversely orientated xylem corresponding to the larger mass of secondary xylem of the inner series, the outer strands are designated partial steles. The central stele consists of two curved irregular bands composed of vertically and obliquely running tracheids: the central part of this stele consists of crushed tissue that probably represents primary xylem like that between the two parts of each of the peripheral steles.

[Illustration: Fig. 438. _Rhexoxylon africanum_. A, radial wall of tracheid; B, C, peripheral steles; _a_, primary portion of stele; _b_, lateral strand. (After Bancroft.)]

_Rhexoxylon_ differs from the usual Medullosan type in the structure of the secondary xylem which is composed of tracheids with an Araucarian form of pitting: there are usually two alternate rows of contiguous pits (fig. 438, A) and occasionally one or three rows. The medullary rays are uniseriate and 3 to 15 cells in depth, a feature characteristic of coniferous wood and not of the wood of the Medulloseae.

In the absence of more complete information as to the anatomical structure of this stem and of all information as to the leaves or reproductive organs it is impossible to fix with precision the systematic position of the genus. It is, however, clear that _Rhexoxylon_ is closely connected with _Medullosa_ in certain features though in the structure of the wood it exhibits important peculiarities. The imperfectly known stem _Cladoxylon Kidstoni_[408] shows a fairly close agreement with the African plant in the form of the steles (fig. 460) but the pitting is scalariform.

=SUTCLIFFIA.= Scott.

_Sutcliffia insignis_ Scott.

The generic name _Sutcliffia_ was given by Dr Scott[409] to a peculiar type of stem from the Lower Coal Measures of Shore, Lancashire, in recognition of the valuable services rendered to Palaeobotany by the late Mr W. H. Sutcliffe, the owner of the colliery from which several new types of plants have been obtained. Two stems are recorded, the type-specimen and a more recently discovered stem, also from Shore, described by Miss de Fraine[410], which differs in several particulars from Dr Scott’s species. In view of the well-marked peculiarities of the second stem it is convenient to speak of it as _forma β_ instead of including it without a distinctive epithet in _Sutcliffia insignis_. We know nothing of the reproductive organs of the genus.

1. _Sutcliffia insignis, forma α._

This consists of a piece of stem approximately 12 × 7 cm. in diameter characterised by a broad cortex of parenchyma with secretory sacs and ducts and strands of mechanical tissue (fig. 439, A). Decurrent and massive leaf-bases form a prominent feature as in the stem of _Medullosa anglica_. The stele, though compressed before petrifaction, was probably not quite cylindrical but more or less polygonal or broadly triangular in section; it consists of groups of large primary tracheids (350μ in diameter) with numerous bordered pits (fig. 439, B, and fig. 440) embedded in an anastomosing system of parenchyma containing scattered secretory sacs, a type of protostele like that of _Heterangium_ and _Medullosa_ _anglica_ except in the possession of exarch protoxylem strands. The metaxylem tracheids contiguous to the external protoxylem elements have a dense spiral or scalariform type of pitting. In the lower part of the stem the primary xylem is enclosed by a cambium which has added a few secondary tracheids (120μ in diameter), but in the upper part of the specimen the cambium is only partially developed and the addition of secondary xylem has hardly begun (fig. 440). A narrow band of secondary phloem was recognised in places consisting of small-celled parenchyma with some sieve-tubes and medullary rays continuous internally with the parenchyma of the primary stele. In close association and occasionally in organic connexion with the surface of the stele are several tangentially elongated and large groups of vascular tissue associated with smaller oval strands varying considerably in size. These groups, designated meristeles (Fig. 439, A), are identical in structure with the main stele and are occasionally invested by a feebly developed zone of secondary xylem and phloem. The meristeles are detached at intervals from the parent stele around which they form by anastomoses an irregular network: the larger meristeles give off smaller strands and from these the actual leaf-traces are produced by subdivision. It appears, however, that in this type the meristeles are not completely used up in the production of the leaf-traces, portions of them behaving as cauline vascular strands. A protoxylem of a meristele still attached to the central protostele occupies an internal position, and at a higher level, as separation of the meristele is effected, the spiral tracheids occur on the inner face. New meristeles are given off at intervals from the main stele ‘to compensate for those parts of the reticulum which were used up in the formation of leaf-trace strands[411].’ The meristeles form the starting-point for the leaf-traces, an intermediate system between the main stele and the actual leaf-traces; they differ, therefore, from the parent leaf-traces of _Medullosa anglica_, which are completely used up by repeated subdivision. Moreover in _Sutcliffia_ the leaf-bundles are concentric and not collateral.

[Illustration: Fig. 439. _Sutcliffia insignia_ Scott. A, B, E, forma α. (After Scott.) C, D, forma β. (After de Fraine.)

A. Transverse section of stem; _a_, _b_, double rows of leaf-trace bundles. The larger black masses are the meristeles; the smaller patches represent bundles derived from the meristeles. B. Longitudinal section of a radially symmetrical bundle from a leaf-base. C. The junction between the primary and secondary xylem. D. Vascular system in transverse section: _m_, _m′_, meristeles; _s_, main stele; _e_, extrafascicular strands; _lt_, leaf-traces; _c_, secondary cortex. E. Vascular bundle from petiole showing a ring of large sieve-tubes surrounding the xylem, and portions of stereome strands.]

[Illustration: Fig. 440. _Sutcliffia insignis._ Part of the primary xylem showing on the upper edge the beginning of secondary growth. (Cambridge Botany School, 560.)]

A conspicuous feature of the stem of _forma α_ is the occurrence of two double rows of vascular strands stretching across the cortex (fig. 439, A, _a_, _b_). These are interpreted by Scott as downward continuations in the stem of the inner surface of leaf-bases. The outer cortex of the stem and leaf-bases has hypodermal strands of stereome which remain separate or rarely anastomose, and form a superficial zone exactly like that of some species of _Medullosa_. The leaf-trace bundles may be radially symmetrical or unilateral in the arrangement of the xylem which is in all cases completely surrounded by phloem. Fig. 439, B, shows part of a longitudinal section of a large leaf-trace bundle: spiral protoxylem elements (_px_) abut on the phloem (_ph_) and are succeeded to the left by narrow scalariform and large reticulately pitted tracheids. In the larger and radially constructed traces there are several protoxylem-strands distributed over the surface of the xylem, while in the smaller unilateral traces there may be one or two protoxylem strands. A characteristic feature of the xylem of the leaf-traces is the admixture of parenchyma with the tracheids (fig. 439, B, E) and another noteworthy character is the occurrence of large thin-walled tubes in the phloem described by Scott as sieve-tubes and compared with the large sieve-tubes in Marattiaceous leaf-bundles. Immediately internal to the hypoderm is a row of leaf-bundles (fig. 439, A) each of which is accompanied by stereome strands.

The petioles, which reach a diameter of 12 cm., contain numerous, occasionally anastomosing, concentric bundles. Nothing is known of the fronds as a whole beyond the fact that they are spirally disposed and had decurrent bases of large dimensions in proportion to the stem.

_Sutcliffia insignis, forma β._

This form, described by Miss de Fraine as _Sutcliffia insignis_[412], is represented by a stem rather smaller than the type-specimen described by Scott, though it is probably an older example of the same species. It is distinguished by a greater development of secondary xylem and phloem both on the main stele and the meristeles; it differs also in the absence of the greater part of the cortex and leaf-bases which have been cut off as the result of the formation of a deep-seated periderm (fig. 439, D, C). The meristeles are smaller and fewer than in the larger form and are distinguished by some other peculiarities. At _m′_ in fig. 439, D, a meristele is seen attached to the main stele. In _Sutcliffia insignis forma α_ the main stele is enclosed by an irregular network of subsidiary steles or meristeles and these form the points of departure of the leaf-traces, but the meristeles are not completely used up in the process of conversion into leaf-traces. In _Sutcliffia insignis forma β_ the meristeles agree in structure with the main stele except in the smaller amount of secondary tissue: they do not, however, form a network as in _forma α_ but occur as strands parallel to the central stele, ‘giving off leaf-traces and ultimately dividing up into smaller strands, often unequal in size, the primary wood of the meristeles being entirely used up in the production of radially symmetrical or unilateral bundles.’ In this respect, as Miss de Fraine points out, _forma β_ agrees more closely than _forma α_ with _Medullosa anglica_ and supports Scott’s view that _Sutcliffia_ is a primitive type of Medullosan stem. The leaves were given off at fairly long intervals as in some species of _Medullosa_: the leaf-traces are exarch and similar to those of the type-species. The most striking feature of the new stem is the presence of a vascular network (fig. 439, D, _e_) which encloses both the main steles and the meristeles; it consists of extrafascicular strands composed of normally orientated bands of secondary xylem and phloem often assuming a fan-like arrangement and occasionally almost concentric or inversely orientated. These strands are always accompanied by short, usually reticulate, tracheids on the inner margin of the xylem: similar isodiametric tracheids also occur in the pericyclic region. The extrafascicular strands are believed to be secondary structures phylogenetically independent of the meristeles and main stele, comparable with the successive cylinders or arcs of secondary xylem and phloem in some recent Cycads and in some species of _Medullosa_[413].

The stele is exarch and roughly triangular; except in the broader zone of secondary tissue it agrees with the protostele of _forma α_. The secondary xylem (fig. 439, C) possesses numerous medullary rays 3–4 cells broad and of considerable depth: the secondary phloem is characterised by the presence of thick-walled elements, presumably sieve-tubes, like those of _Medullosa Leuckarti_ and _M. anglica_.

The ground-tissue is rich in secretory tissue and the stem-surface, from which the leaf-bases have been detached, is limited by a wide zone of secondary tissue produced by a phellogen.

_Sutcliffia Williamsoni_ (Seward).

1876. _Myelopteris_ (_pars_) Williamson, Phil. Trans. R. Soc. Vol. 166, Pl. +ii.+ figs. 7, 8; Pl. +iv.+ fig. 17. 1893. _Rachiopteris Williamsoni_ Seward, Ann. Bot. Vol. +vii.+ p. 1. 1894. _Rachiopteris Williamsoni_ Seward, _ibid._ Vol. +viii.+ p. 207, Pl. +xiii.+ 1906. _Sutcliffia Williamsoni_ Scott, Trans. Linn. Soc. Vol. +vii.+ pt +iv.+ p. 62.

In an account of _Myelopteris_ (= _Myeloxylon_) published in 1876 Williamson included some sections of petioles from the Lower Coal Measures which I afterwards with his concurrence transferred to the genus _Rachiopteris_ as _R. Williamsoni_. In _Rachiopteris Williamsoni_ the vascular bundles are concentric and not collateral, and are further distinguished from those of _Myeloxylon_ by the association of parenchyma with the tracheids. In the arrangement of the bundles and in the structure of the ground-tissue the petioles of _Rachiopteris Williamsoni_ agree with those of _Myeloxylon_. An outstanding feature of the vascular strands of the former is the occurrence at fairly regular intervals in the peripheral part of the phloem of comparatively large tubes described by me as secretory canals on the ground that evidence was furnished of their development by the schizogenous separation of cells to form a central canal. The canals were compared with the large sieve-tubes of the Marattiaceae, but with the qualification that ‘their mature form and their manner of development are strongly suggestive of small secretory canals.’ Nothing was known as to the stem which bore these petioles until Scott’s discovery of _Sutcliffia_ with vascular bundles in the cortex and leaf-bases of the same type as those of _Rachiopteris Williamsoni_. As Scott says, there are a few distinguishing features which suggest that _R. Williamsoni_ is not specifically identical with the petioles of _Sutcliffia insignis_, though the agreement is such as to justify the substitution of _Sutcliffia_ for _Rachiopteris_. The tubular elements in the phloem which I considered to be secretory canals are regarded by Scott and Miss de Fraine as sieve-tubes.

Miss de Fraine gives an excellent summary of our knowledge of the genus and discusses, in the light of the additional facts furnished by the second stem, the position of _Sutcliffia_ in a phylogenetic series. The new form strengthens the comparison instituted by Scott between _Sutcliffia_ and _Medullosa_ and gives support to a close connexion between the Medulloseae and the Cycadaceae. Scott lays stress on the fact that the Medulloseae, except _Sutcliffia_, are polystelic, while the Cycads, at least the adult stems, are monostelic. The views of Worsdell and other botanists who believe that the Medulloseae and the Cycads are intimately related are discussed on another page: in reference to these views Miss de Fraine adds a caveat as to the danger of attaching excessive importance to evidence based on seedling anatomy when we are concerned with broad phylogenetic questions. The stele of a cycadean stem may be derived from a protostelic type such as that of _Sutcliffia_ by the gradual disappearance of the internal tracheids: in _Sutcliffia_, as in some Cycads and species of _Medullosa_, extrafascicular strands and cylinders are a characteristic feature and these may well have arisen independently of the central stele in response to physiological requirements. From such a type as _Sutcliffia_ evolution may have proceeded along two lines; in one direction new types were produced in which increasing complexity, as represented by a multiplication of steles, was an outstanding feature. These forms, illustrated by _Medullosa anglica_ and other more elaborate species, proved inefficient and were unproductive. Along another line the protostelic condition was maintained though in some cases extrafascicular strands or cylinders and cortical steles were superadded: it was this line that led to the recent Cycads.

_General considerations suggested by the anatomical features of_ Medullosa.

A comparison of the stems described under the generic name _Medullosa_ reveals a considerable range in the grosser anatomical features superadded to certain fundamental characters denoting a common origin[414]. If additional data were available giving us a fuller knowledge of individual plants differences between species would be more clearly defined and would provide adequate grounds for the institution of new genera for some of the types now included in the comprehensive genus _Medullosa_. A proposal by Dr Lotsy[415] to adopt the names _Pecopteromedullosa_ and _Neuropteromedullosa_ rests primarily on the relatively unimportant difference between the fronds associated with certain Medullosan stems and, as Scott[416] points out, this two-fold division if applied to such a species as _Medullosa Leuckarti_ would result in its separation from species which anatomically are clearly of the same generic type. In _Medullosa anglica_, one of the oldest and simplest types, there are three steles of equal importance, and each of them is practically identical with the single stele of _Heterangium_. Each stele—as indeed all Medullosan steles—consists of a strand of primary xylem enclosed by secondary xylem and phloem, and it is obvious that the development of a constantly increasing cylinder of secondary conducting tissue about three centres would lead to serious mechanical difficulty: a stem constructed on the plan of _Medullosa anglica_ or the smaller _M. pusilla_ could not increase the thickness of its secondary vascular tissue beyond a certain point without detriment to its efficiency. In some types this difficulty is partially overcome by the production of complete concentric cylinders of centrifugally developed conducting tissue external to an inner system of concentric steles agreeing individually with those of _Heterangium_ (fig. 415, B). _Medullosa anglica_, regarded from the point of view of the architectural efficiency of its vascular system, affords a much less promising _point d’appui_ for further evolution than some of the forms described under _Medullosa stellata_ in which the mechanical impasse is avoided by the adoption of the cycadean plan as represented by such genera as _Cycas_ and _Macrozamia_. The English species _Medullosa centrofilis_ (fig. 417) affords the first example of a characteristic Medullosan feature, namely the presence of a small concentric stele in the central region of the stem: this so-called star-ring differs not only in its smaller dimensions but in its more cylindrical form from the larger peripheral steles. In the later Permian species, _e.g._ _Medullosa porosa_ and _M. Solmsi_, the single star-ring of the older _M. centrofilis_ is replaced by a large number of precisely similar conducting strands. These star-rings are structurally comparable with the cortical steles of _Cycas_ and, in position, with the medullary system of bundles in a _Macrozamia_; they are essentially cauline and take no part in the emission of leaf-traces. _Medullosa Leuckarti_ (fig. 416, H) resembles in its vascular plan _M. centrofilis_, but in this larger stem there are several star-rings and the enlarged peripheral steles are more or less sinuous. In _Medullosa Solmsi_ (fig. 416, E) the star-rings are still more numerous and the main vascular system consists of a double series of concentric steles, each agreeing with the larger peripheral steles of _M. Leuckarti_.

Some of the forms included in _Medullosa stellata_ appear to be very different from _M. anglica_ and _M. Leuckarti_ (_cf._ fig. 416, F, A, H), but their similarity is apparent if we imagine _Medullosa anglica_ with only one main stele (with the addition of star-rings) which is stretched tangentially until it becomes a long and narrow plate-ring and is then rolled into a hollow cylinder like that in fig. 416, D. A modification of the tubular type of stele is seen in _Medullosa stellata_ var. _corticata_ (fig. 416, G) in which the vascular cylinder is broken up into two or more curved plate-rings, a change superficially similar to that by which a dictyostele is produced from a solenostele, but in _Medullosa_ it is not the overlapping of leaf-gaps that is the cause of the change. A striking feature in the stem of _Medullosa stellata_ represented in fig. 416, D and F (especially F), is the inequality in breadth of the centrifugal and centripetal xylem: this inequality is an expression of the difficulty caused by the presence of an internal as well as an external addition of secondary conducting tissue. A limit is set to the production of secondary centripetal xylem by the space available for extension, whereas there is unlimited room for increase in the case of the centrifugal tissue. This tendency to a greater development of xylem and phloem on the outer side of the primary portion of the steles is illustrated also in _M. Solmsi_ var. _lignosa_ (fig. 416, L) where the outer of the two series of peripheral steles has a much larger proportion of centrifugal xylem. In _M. anglica_ the secondary xylem on the inner side of the steles is not infrequently broader than the corresponding tissue on the abaxial side[417]; but this is exceptional in the genus. A further development of centrifugal conducting tissue without any corresponding development of centripetal tissue is effected in certain cases (fig. 416, K, L) by the addition of concentric cylinders of centrifugal xylem and phloem beyond the original concentric steles. The type illustrated by _Medullosa Solmsi_ var. _lignosa_ and _M. stellata_ var. _gigantea_ (fig. 416, L, K) at once suggests comparison with stems of _Cycas_, _Macrozamia_, and _Encephalartos_, the chief difference being the presence in _Medullosa_ of an inner series of concentric steles and a central ground-tissue containing star-rings, though the latter may be regarded as corresponding to the medullary system of bundles in _Macrozamia_. In _Macrozamia_ the central region of the stem is considered to be the pith of a monostelic stem, whereas in _Medullosa_ the stem is polystelic. In recent cycadean stems it is not uncommon to find patches of inversely orientated xylem and phloem internal to one or more of the cylinders of centrifugal vascular tissue. These abnormal developments are considered by Worsdell[418] to be relics of the inner portions of concentric steles possessed by the Medullosan ancestors of recent Cycads. This interpretation affords a means of bringing into closer relationship the polystelic Medulloseae and the monostelic Cycadaceae, the apparent simplicity of the latter being the result of the progressive loss of centripetal xylem and phloem, the normal cycadean cylinder being therefore regarded as a one-sided remnant of a concentric Medullosan stele. In other words, the Cycads are descended from polystelic ancestors. As further evidence in support of this view Worsdell points to the occurrence of concentric steles in the cortex of _Cycas_ and their occasional presence in the pith of other genera. Matte[419] has shown that in the seedling stem of _Encephalartos Barteri_ (fig. 396, K) there are three concentric steles each similar to a normal Medullosan stele: at a higher level in the axis the steles become ‘unrolled’ and assume the form of one-sided cylinders of centrifugal xylem and phloem.

In the peduncles of some recent Cycads, _e.g._ _Stangeria_, there is a tendency towards a somewhat irregular orientation of the collateral bundles that constitute the vascular cylinder, and tracheids occasionally occur internal to the protoxylem of the individual bundles[420]. Worsdell regards these features as evidence of a Medullosan ancestry. If the sinuous plate-rings of a stem of _Medullosa Leuckarti_ (fig. 416, H) were broken up into separate portions and wholly or in part deprived of the centripetal xylem, the result would be an arrangement of bundles comparable with that in a _Stangeria_ peduncle[421]. The scattered centripetal tracheids discovered by Scott in _Stangeria_ and other cycadean peduncles are interpreted by Scott and by Worsdell as relics of some ancestral centripetal xylem, but with this important difference in the point of view; Scott believes that they represent the almost completely aborted centripetal xylem of a single stele like that of _Lyginopteris_, while Worsdell sees in them fragmentary vestiges of the central primary xylem of two or more Medullosan steles. An abnormal seedling of _Araucaria Bidwillii_ described by Shaw[422] exhibits features analogous to those in some cycadean seedlings: within the normal stele a cambium forms an inversely orientated vascular cylinder which at a lower level becomes continuous with the outer centrifugal tissue, the whole vascular system being eventually represented by two concentric steles. The polystelic stage is a development of a monostelic condition, and the inner or inverted portion of each of the two concentric steles is derived from an inversely orientated cylinder in the central region of the root. This abnormal root does not, however, supply an argument in favour of the derivation of a monostelic type of stem from one that was polystelic, but it shows a close relation between the two plans in one organ. The seedling is not altogether normal in form apart from structure and it is not improbable that the anatomical abnormality is connected with some pathological cause.

It has been suggested[423] that Worsdell attaches too much phylogenetic significance to the irregularities in the disposition and form of the vascular bundles in the peduncle of _Stangeria_, and the criticism that insufficient allowance is made for the possible reaction on structure of the special physiological requirements of reproductive shoots is well founded. Granting an overestimate of the arguments drawn from the occasional occurrence of concentric vascular strands, a considerable body of evidence remains in favour of Worsdell’s main contention.

Mrs Thoday[424] has drawn attention to certain features exhibited by the inflorescence-axes of _Welwitschia_, particularly the occurrence of concentric and inversely orientated bundles, similar to those characteristic of the seedling of _Cycas siamensis_[425] described by Matte, and to anatomical characters occasionally present in adult cycadean stems and normally represented in _Medullosa_. She is of opinion that the occurrence in _Welwitschia_ of certain Medullosan features has a phylogenetic significance. The differences between the Gnetales, Medulloseae, and Cycadales are considerable, and it would seem unlikely that the anatomical resemblances described by Mrs Thoday are of great value as criteria of close relationship. The comparison of _Lagenostoma_ with gnetalean seeds is alluded to elsewhere. To the statement that the presence of concentric and inversely orientated steles in _Welwitschia_ are reminiscent of the polystelic Medulloseae, Mrs Thoday adds the qualifying remark that the occurrence of four concentric groups of vascular tissue in the hypocotyl of _Welwitschia_ is not sufficient to justify the conclusion that the ancestral type was polystelic. This reservation accords with the contention of Scott and other botanists, that the occasional occurrence in cycadean seedlings and adult stems of anatomical features suggestive of polystely does not in itself furnish an adequate reason for doubting that the apparent monostely of Cycads is phylogenetically what it seems to be, namely, an indication of monostelic ancestry. This brings us to the question of a possible monostelic ancestor. It may be that the Upper Carboniferous genus _Sutcliffia_ affords a clue to the problem of the origin of the polystelic type illustrated in various forms by _Medullosa_. The protostele of _Sutcliffia_ bears a close resemblance to each of the three steles of _Medullosa anglica_; the fact that _Sutcliffia_ is exarch and that _Medullosa anglica_ has mesarch xylem is of secondary importance, particularly as exarchy is represented within the genus _Medullosa_. The extrafascicular strands of xylem and phloem and the accessory strands are points in which _Sutcliffia_ and _Medullosa anglica_ agree and, as Miss de Fraine[426] adds, the meristeles of _Sutcliffia_ may be homologous with the leaf-trace strands of _Medullosa_. Scott[427] gave expression to the characters shared by these two types by describing _Sutcliffia_ as the most primitive of the Medulloseae. It is suggested that the protostelic axis of _Sutcliffia_ may be regarded as the starting-point of the monostelic Cycads, the central mass of tracheal tissue being replaced by a parenchymatous pith, while the extrafascicular and accessory strands arose independently of the central stele in response to increased physiological demands consequent on the increase in size of the stem. From the same starting-point evolution may have progressed along another line through such a type as _Medullosa anglica_ leading to the more complex Permian species of _Medullosa_. Chodat’s view[428] that the Medulloseae are Protocycadaceae, if we include _Sutcliffia_ as well as _Medullosa_ in the Medulloseae, is probably correct. There is clear evidence of a close bond of union between recent Cycads and the Medulloseae, and _Sutcliffia_ offers a possible means of deriving complex polystelic types from a monostelic ancestor.

Worsdell’s opinion[429] that the stele of _Lyginopteris_ affords evidence of derivation from a polystelic ancestor and is not homologous with the true monostele of _Heterangium_ is opposed to the undoubted signs of intimate connexion exhibited by these genera. The Lyginopterideae are, as Scott[430] says, a less advanced group than the Medulloseae and, it may be added, they are more remote from the modern representatives of the Cycadales. The Lyginopterideae and the Medulloseae are probably offshoots of a common stock, but the Medulloseae occupy a position farther removed from the filicinean ancestry than _Heterangium_ and _Lyginopteris_[431].

The relative meagreness of our knowledge of the reproductive organs of the Medulloseae gives precedence to anatomical data in phylogenetic considerations, but the evidence furnished by _Trigonocarpus_ and other seeds that may fairly be assigned to Medullosan plants is in harmony with the conclusions based on vegetative characters with regard to a close affinity between the Medulloseae and Cycads.

The comparative examination of recent Cycads naturally suggested by any attempt to compare the group as a whole with Palaeozoic types leads to some apparently contradictory results. The habit of the megasporophyll of _Cycas_ is usually quoted as a primitive attribute: the close resemblance in plan and in manner of occurrence on the stem between megasporophylls and foliage leaves recalls both Ferns and Medullosan fronds. On the other hand the production of eight body-cells in the pollen-tube of _Microcycas_[432] in place of the usual single cell may also be regarded as a primitive character. It is perhaps possible, as Miss Dorety[433] says, that the polyspermy may be a case of recurrence and not a direct inheritance. _Microcycas_ differs from _Cycas_ in having only one vascular cylinder, and if the presence of several concentric cylinders in _Cycas_ be interpreted as an indication of a closer connexion with a Medullosan ancestry, the _Microcycas_ type would represent a more advanced stage in evolution. Attempts to arrange plants according to a natural sequence are frequently frustrated by instances of unequal progress in the development of vegetative and reproductive organs; one or other set of members lags behind; some characters point to the retention of primitive traits while others indicate a marked progressive tendency. It is noteworthy that the Mesozoic Bennettitales are characterised by a greater simplicity of stem-structure than is the rule in recent Cycads, and both in their vegetative features and in the structure of the seeds they are further removed from the Medullosan type.

[Illustration: Fig. 441. _Steloxylon Ludwigii._ A, surface-view (longitudinal) of stem showing leaf-bases. (After Solms-Laubach.) B, longitudinal section showing anastomosing vascular strands. C, steles, the lower one showing the emission of a leaf-trace. (B, C, after Schenk.)]

II. A. =STELOXYLEAE.=

=Steloxylon.= Solms-Laubach.

_Steloxylon Ludwigii_ (Goeppert and Stenzel). The genus is founded on a piece of stem from Siberia, possibly of Permian age though not improbably older, which was originally described as _Medullosa Ludwigii_[434]. It is characterised by numerous cylindrical and band-like vascular strands forming an irregular anastomosing system (fig. 441, B) and by crowded spiral leaf-scars on the exposed face. The appearance presented by the transverse section figured by these authors, while suggesting comparison with _Medullosa_, reveals a distinctive character, namely the absence of a definite peripheral system of vascular rings such as forms a striking feature of the continental Medulloseae. A more complete description was afterwards published by Schenk[435] who recognised more fully the peculiar features and hinted at the possibility that the species might more appropriately be regarded as a member of a distinct group. Solms-Laubach[436] went a step further and instituted the generic name _Steloxylon_, and in a later publication gave a fuller account of the anatomical characters. The complete stem must have reached a diameter of approximately 13 cm. The homogeneous ground-tissue forms a matrix enclosing an anastomosing vascular system of cylindrical or oval steles (fig. 441, C). Each strand consists of a band of secondary xylem tracheids with one or several rows of circular or oval bordered pits on the radial walls and narrow medullary rays usually 1–2 cells broad and 1–4 cells deep, though occasionally deeper. No phloem is preserved. The tissue in the centre of each stele is very imperfectly preserved, but it is clear that the secondary xylem enclosed a central region (‘partial pith’) like that in the steles of a _Medullosa_, doubtless consisting of primary xylem and conjunctive parenchyma.

The stem is covered with leaf-bases of oval or circular section and between them are small organs, probably multicellular hairs (fig. 441, A). A leaf-base consists of an outer zone of strengthening tissue and a parenchymatous ground-tissue traversed by two or more small vascular strands which assume various forms. These petiolar strands are simply portions of the main vascular system which bend outwards at the periphery of the anastomosing network. The more noteworthy features in which _Steloxylon_ differs from _Medullosa_, particularly such species as _M. stellata_ and _M. Leuckarti_, are (i) the crowded and comparatively small leaf-bases in place of the massive decurrent petioles of _Medullosa_; (ii) the supply of the leaves by compact branches of the stelar network instead of the bundles detached as leaf-traces from a stem-stele of _Medullosa_ (the origin of a leaf-trace in _Steloxylon_ is shown in fig. 441, C); (iii) the absence of a peripheral system of vascular plate-rings and the irregular distribution of cylindrical and plate-steles in the ground-tissue. Nothing is known of the reproductive organs or leaves beyond the structure of the attached leaf-bases. The opinion expressed by P. Bertrand[437] that the fossil described by Stenzel as _Asterochlaena_ (_Clepsydropsis_) _kirgisica_ is the petiole of _Steloxylon_ was abandoned after the additional facts published by Solms-Laubach.

As regards the affinities of _Steloxylon_: the structure of the steles agrees closely with that of the star- and plate-rings of a _Medullosa_, while the pitting of the tracheids is more like that in _Medullosa_ than _Cladoxylon_. In the tendency to a more radial than tangential disposition of the band-like steles _Steloxylon_ recalls _Cladoxylon_ rather than _Medullosa_, but in _Cladoxylon_ the vascular system does not form an irregular network as in _Steloxylon_. The information as to the structure of the primary xylem is very meagre, but it points to a closer connexion with _Medullosa_ than with _Cladoxylon_. On the whole _Steloxylon_ may perhaps be defined as a genus allied to the Medulloseae in the anatomical features of the stem more closely than to other genera, but sufficiently distinct to be excluded from the Medulloseae as at present understood[438].