CHAPTER XXXII.

CYCADOFILICES.

In this chapter are included several types represented by stems, but which in the absence of definite information with regard to the reproductive organs cannot be assigned to the Pteridosperms.

=MEGALOXYLEAE.=

=Megaloxylon.= Seward.

This genus is represented by a single species founded on a piece of stem from the Lower Coal Measures of Lancashire discovered in the Binney Collection in the Sedgwick Museum, Cambridge[466]. Nothing is known as to the leaves, reproductive organs, or roots. The type-specimen consists exclusively of primary and secondary xylem.

_Megaloxylon Scotti_ Seward.

The type-specimen, reproduced natural size in fig. 446, A, B, consists of a fragment of stem which at first sight bears a close resemblance to _Cordaites_, but the apparent pith, 1·9 cm. in diameter, is the primary xylem of the stele and is enclosed by an incomplete cylinder of secondary xylem 2 cm. broad. The distinction between the lighter primary xylem and the darker secondary cylinder is especially well marked in the longitudinal section (fig. 446, B). The central region, shown on a larger scale in fig. 447, is occupied by groups of tracheids varying in size and shape associated with thin-walled parenchyma: the latter is represented by lighter patches in the section. The majority of the tracheids are characterised by their great breadth—in some cases ·4 mm.—and their isodiametric or even horizontally elongated and flattened form. Some of these large metaxylem elements are seen in longitudinal section at _m_ in fig. 448 where the shrinkage and partial decay of the parenchymatous tissue have resulted in the separation of transverse bands of xylem simulating the discoid pith of a _Cordaites_. The walls of these tracheal cells are covered with multiseriate pits. With the short and sometimes flattened xylem elements occur others of greater length, but these are chiefly met with in the more peripheral part of the central region where some of the tracheids are much narrower and have the form of ordinary water-conducting elements. On one side of the primary xylem in fig. 447 an oval and more compact group of narrow and longer tracheids is seen at _lt¹_; this is a leaf-trace about to enter the secondary-xylem cylinder on its outward course. The same leaf-trace is shown at _lt_ in fig. 446, B; as it descends the trace becomes less distinct and its elongated elements gradually merge into the general mass of metaxylem. A portion of this leaf-trace is seen in fig. 448, B, _lt_, close to the inner edge of the secondary xylem, _x²_, and abutting internally on the contracted tissue, _m_, which consists mainly of large and short tracheae with remains of associated parenchyma. The trace includes some conjunctive parenchyma interspersed with the tracheids: on its outer surface, that is on the abaxial edge of the ovate xylem strand as shown at _lt¹_ in fig. 447, there are six external protoxylem strands. In the peripheral region of the primary xylem of the section reproduced in fig. 447 there are several more or less well-defined leaf-traces, _e.g._ _lt²_, _lt³_; these differ from that seen at _lt¹_ in their greater tangential breadth and in the less compact arrangement of the tracheids. As each trace is followed downwards in the primary region of the stele it tends to become broader, especially in a tangential direction; the spiral protoxylem strands are more widely separated (fig. 448, A, _px_) and the elongated and comparatively narrow tracheids as they spread out fan-wise are reduced in length, finally passing over into the broad and short reticulately pitted cells. Each leaf-trace can be followed through approximately four internodes before its individuality is lost in the general mass of metaxylem. The disposition of the peripheral traces is such as to justify the conclusion that the phyllotaxis of the stem is ⅖.[467]

[Illustration: Fig. 446. _Megaloxylon Scotti._ A, B. Transverse and longitudinal sections. _x¹_, _x²_, primary and secondary xylem; _lt_, leaf-traces. Nat. size. C. Secondary xylem.]

[Illustration: Fig. 447. _Megaloxylon Scotti._ Transverse section of the primary portion of the stele with leaf-traces, _lt_, and the inner edge of the secondary xylem.]

[Illustration: Fig. 448. _Megaloxylon Scotti._ A, transverse section at the junction of the primary and secondary tissues; B, longitudinal section showing the inner edge of the secondary xylem, _x²_, a leaf-trace, _lt_, and the metaxylem, _m_.]

The secondary xylem (fig. 446, C) resembles that of _Lyginopteris_ and _Heterangium_ though it is less parenchymatous. The medullary rays are numerous and vary in breadth from 1 to 5 cells, while the tracheids, with multiseriate bordered pits on their radial walls, form bands 1 to 8 elements in breadth. There are no regular rings of growth but occasional arcs of narrow tracheids interfere slightly with the otherwise uniform structure of the wood. A leaf-trace in its oblique outward course through the wood becomes completely enclosed by a cylinder of secondary xylem and thus appears to be concentric. Owing to the absence of any tissue external to the secondary xylem of the stem, statements as to the subsequent behaviour of the leaf-traces on emerging from the stele are purely hypothetical. It is, however, not improbable that each concentric trace lost its secondary tissue and broke up into several collateral strands, a suggestion based on the behaviour of the leaf-traces in _Medullosa anglica_.

The most striking characteristics of _Megaloxylon_ are: (i) the structure of the primary xylem, particularly the unusual form of the majority of the metaxylem tracheids, a form obviously correlated with storage rather than with conduction of water; (ii) the gradual spreading of the leaf-traces and their absorption as they descend into the main mass of the xylem; (iii) the exarch structure of the primary xylem. Confining our attention to the primary region of the stele; a comparison is at once suggested with _Heterangium_. In _Megaloxylon_ the peculiarities are the substitution of the large storage-tracheids for the normal xylem-elements; the greater irregularity in the groups of metaxylem; and an exarch instead of a mesarch structure. In these last features the primary xylem agrees with that of recent species of the Schizaeaceous Fern _Lygodium_ and the external protoxylem is a character shared with _Rhetinangium_. The occurrence of short tracheids similar to those of _Megaloxylon_ in the inner portion of the stele of the Osmundaceous Fern _Zalesskya gracilis_ (Eich.)[468] may be quoted as an example of parallel modification but, as Scott[469] points out, the resemblance has no phylogenetic significance. The secondary xylem though less parenchymatous than in recent Cycads agrees more closely with the manoxylic than with the pycnoxylic type.

_Megaloxylon_ affords an interesting example of a combination of primary stelar anatomical features, comparable in the exarch position of the protoxylem with the stele of _Lygodium_, and secondary wood similar to that of _Lyginopteris_ and _Heterangium_. The large metaxylem tracheids may be regarded as derivatives of elements which in some ancestral type were structurally fitted for the rôle of water-transport and made up the xylem of a _Lygodium_-like stele with little or no secondary xylem. As the cambial activity increased and centrifugal xylem became a prominent feature, usurping the function of the centripetal xylem, the latter became modified and fitted for a new service.

=RHETINANGIEAE.=

=Rhetinangium.= Gordon.

_Rhetinangium Arberi_ Gordon.

The stem on which this genus is founded was discovered by Dr Gordon[470] in the Calciferous Sandstone series of Pettycur: a specimen collected by Dr Kidston in Berwickshire may be specifically identical with the Pettycur plant. We know nothing of the leaves or reproductive organs of _Rhetinangium_. The stem, approximately 2 cm. in diameter, was probably cylindrical; it possesses a single stele consisting mainly of a central primary region occupied by anastomosing groups of tracheids, 130–150μ in diameter, embedded in parenchyma containing numerous secretory sacs and ducts. In the peripheral region of the stele the groups of tracheids consist of narrower elements characterised by exarch protoxylem. Each peripheral group forms the base of a wedge of secondary xylem (fig. 450, _x²_), the primary medullary rays being in direct connexion with some of the parenchyma of the primary xylem. The secondary tracheids, 45–85μ in diameter, have multiseriate bordered pits on their radial walls and the rays are broad and deep as in _Heterangium_ and _Lyginopteris_. With the exception of the external position of the protoxylem, the stele of _Rhetinangium_ is practically identical with that of _Heterangium_, though in _Rhetinangium_ the primary tracheids form larger and fewer groups. The inner cortex is composed of thin-walled cells with many secretory sacs: there are no stereome elements. In the outer cortex (fig. 449, _cr_) radially disposed bands of stereome form a reticulum with narrow and very long meshes like that of Medullosan petioles.

The recently recorded occurrence of polydesmic petioles[471] in _Heterangium_ is of special interest from the point of view of the comparison of that genus with _Rhetinangium_ and the Medulloseae.

The very broad decurrent petiole-bases are a striking feature, the major diameter of the pulvinus-like base of the leaf-stalk exceeding that of the stem (fig. 449, _p_). Several xylem-strands from the peripheral region of the primary xylem go to form a single leaf-trace: these U-shaped strands of xylem destined for a leaf are connected laterally by parenchyma and form an irregularly corrugated band. Fig. 450 shows a petiole-trace still enclosed on each side by the secondary xylem cylinder _x²_.

[Illustration: Fig. 449. _Rhetinangium Arberi_ Gord. Transverse section of stem with large leaf-base; _cr_, outer cortex of stem; _p_, petiole. (After Gordon; × 1¾.)]

There are several protoxylem groups in a leaf-trace, one on the outer face of each xylem-strand. In the petiole the xylem groups are more intimately connected and the trace has the form of a flat band with abaxial protoxylem. There is no indication that a leaf-trace undergoes division into separate strands. The roots are described as tetrarch with well-developed secondary xylem.

[Illustration: Fig. 450. Transverse section of leaf-trace of _Rhetinangium Arberi_ showing the exarch structure; _x²_, secondary xylem. (After Gordon.)]

The primary xylem of _Rhetinangium_ agrees in its exarch structure with the Palaeozoic genera _Sutcliffia_, _Megaloxylon_ and _Stenomyelon_, also with _Lygodium_ and some other recent Ferns: the secondary wood is of the manoxylic type like that of _Lyginopteris_, _Heterangium_ and other genera. In the general structure of the stele _Rhetinangium_ agrees with _Sutcliffia_ and, except in the exarch structure of the primary xylem, with the steles of _Heterangium_ and _Medullosa anglica_[472]; but the structure and origin of the leaf-traces are characters which mark it off from _Sutcliffia_. The sclerenchymatous bands in the inner cortex of _Heterangium_ are unrepresented in _Rhetinangium_, and in the latter genus the abundance of secretory sacs and ducts is a characteristic feature, moreover in _Rhetinangium_, the leaf-trace consists of several groups of primary xylem-elements. Dr Gordon regards _Megaloxylon_ as the type which comes nearest to _Rhetinangium_; but the differences in the structure of the secondary wood and the marked contrast between the leaf-traces are too pronounced to justify a preference for _Megaloxylon_ over _Heterangium_ in the order of affinity. Gordon considers that the undivided leaf-trace of _Rhetinangium_ may represent a form transitional between the simple leaf-trace of _Lyginopteris_ and the much divided type in _Medullosa_. The external position of the protoxylem is a character to which too much weight may easily be attached: the difference in position between the protoxylem of _Rhetinangium_ and _Heterangium_ is in some examples of the latter genus hardly perceptible. Kubart[473] speaks of the stele of his species _Heterangium Sturi_ as being almost exarch. The inconstancy in the position of the protoxylem in the xylem of Osmundaceous stems and in the primary bundles of _Eristophyton_ and other Palaeozoic genera is worthy of consideration in this connexion.

[Illustration: Fig. 451. _Stenomyelon tuedianum_ Kidst. Transverse section of stem. The black patches represent leaf-traces. (After Kidston; × 1½.)]

=STENOMYELEAE.=

=Stenomyelon.= Kidston.

_Stenomyelon tuedianum_ Kidston. The specimens on which this monotypic genus is founded[474] were obtained from the Lower Carboniferous rocks (Calciferous Sandstone series) at Norham Bridge, Berwickshire, Scotland. They consist of petrified pieces of a flattened stem, a fragment of a rachis and portions of laminae: there is no evidence as to the nature of the reproductive organs. The original form of the stem is obscured by the destruction of a considerable part of the cortex and the consequent flattening of the whole with the production of wing-like extensions of the imperfectly preserved tissues enclosing the almost cylindrical stele (figs. 451, 452).

The stele consists of a bluntly triangular core of primary xylem, 3–4 mm. in diameter, composed almost entirely of reticulately pitted tracheids reaching a diameter of 160μ: a few parenchymatous cells occur in the peripheral region and a band of parenchyma extends from the middle of each of the three sides of the xylem to the centre of the stele, thus dividing the primary conducting tissue into three groups which are the expression of a phyllotaxis of ⅓. The tracheids near the outer face of each xylem-group are narrower than the others and have scalariform pitting. The secondary xylem first appears along the slightly concave sides of the primary stele, eventually enclosing the whole: it consists of tracheids with multiseriate pits on the radial walls and numerous deep medullary rays 1–6 cells broad. No phloem is preserved though it is probable that a narrow band was originally present. A characteristic feature is afforded by a zone of thick-walled cells, regarded as periderm, encircling the stele and formed by a deep-seated phellogen. On the outer face of this band there are projecting bosses, and similar sclerous nests are scattered in the cortex. The outer cortex has a _Sparganum_[475] type of hypoderm, that is long vertical strands of fibres alternating with parenchyma. The leaf-traces are formed from the blunt angles of the primary xylem; an angle becomes nipped off as a more or less cylindrical strand enclosed by a zone of secondary tracheids which is very narrow on the adaxial side (fig. 452). Protoxylem was recognised only in the leaf-traces and not in the rest of the stele. A pair of protoxylem strands occurs on the outer edge of a prominent angle of xylem before it becomes detached from the stele, and these form a single strand at a lower level. As a leaf-trace passes outwards, the exarch xylem strand becomes mesarch and there is a single protoxylem group except at a point near the bifurcation of a trace. In its passage through the cortex a leaf-trace divides repeatedly, the secondary xylem on the outer face of each strand being retained for a considerable time.

Our meagre knowledge of the nature of the leaves is based on incomplete fragments found in association with the stem. The leaf is believed to have been simple and characterised by a thick lamina with a hypodermal zone of sclerous strands and several vascular bundles.

[Illustration: Fig. 452. _Stenomyelon tuedianum_. Transverse section of stele. (After Kidston; × 7.)]

As Kidston and Gwynne-Vaughan[476] remark, _Stenomyelon_ is a very distinct type; while resembling _Sutcliffia_ in some respects it differs from that genus not only in the structure of the primary stele but in the absence of the system of meristeles which form so characteristic a feature of the latter genus.

=CYCADOXYLEAE.=

=Cycadoxylon.= Renault.

This generic term[477] is applied to a few types of Permian and Upper Carboniferous stems possessing a vascular cylinder, which may reach a considerable breadth, of secondary centrifugally developed xylem and phloem enclosing a large pith containing either a narrow, peripherally placed, and more or less continuous cylinder of inversely orientated conducting tissue or scattered bands of centripetal xylem and phloem. The secondary xylem is manoxylic, while the internal vascular tissue recalls that of _Ptychoxylon_ and to a less extent the inverted arcs that are rarely met with in _Lyginopteris_ stems. A brief diagnosis of two species may serve to illustrate the genus: a third species is included in _Cycadoxylon_, but it is founded on material too incomplete to admit of satisfactory diagnosis.

_Cycadoxylon Fremyi_ Renault.

This Permian species[478] is represented by a piece of stem 2–2·5 cm. in diameter (fig. 453, B) characterised by (i) a fairly broad parenchymatous cortex with secretory canals and several hypodermal nests of sclerous tissue, (ii) a cylinder of secondary xylem and phloem nearly the whole of which is centrifugal, (iii) a large pith containing several scattered narrower bands or arcs of centripetally developed xylem and phloem. The tracheids, with 4–6 series of hexagonal pits, form radially disposed rows, 1–4 elements broad, separated by broad and deep medullary rays. Renault does not mention the occurrence of any primary xylem as distinct from the secondary centrifugal xylem, but in a section which I examined some years ago in his laboratory there appeared to be a group of primary tracheids. There are no anastomoses between the main cylinder and the internal bands of inversely orientated tissue.

_Cycadoxylon robustum_ (Seward).

This species[479] is based on a piece of stem from the Lower Coal Measures of Lancashire first described by Williamson and identified as an unusually large example of _Lyginopteris_. Williamson and Scott, while recognising certain features in addition to the large size of the stem, which must have reached 14 cm. in diameter, expressed the view that ‘there is a presumption that it really belonged to a _Lyginodendron_, or to some plant of the same type of structure.’ The examination of additional material led me to adopt the name _Lyginodendron robustum_, though I suggested that possibly _Cycadoxylon_ might be the more appropriate genus. Subsequently Scott[480] proposed the substitution of _Cycadoxylon_ for _Lyginodendron_.

The type-specimen consists of secondary xylem agreeing closely in structure with _Lyginopteris_ and recent Cycads: the pith, 2·9 cm. in breadth, is incompletely preserved; there is a narrow band of centripetal xylem[481] at the periphery of the perimedullary region and close to the inner face of the main mass of wood (fig. 453, C; the black line marks the position of the centripetal xylem). Nests of sclerous tissue and secretory canals are scattered in the medullary parenchyma and deeper in this region are arcs of secondary parenchyma, possibly periderm. In places the centripetal and centrifugal xylem are in contact and occasionally the tapered ends of the rows of centrifugal tracheids merge into groups of primary xylem elements. The preservation in the central region is far from complete, and although the occurrence of primary xylem groups is probable it cannot be said to be positively established. At the inner edge of the centrifugal xylem and in tangential longitudinal sections a few leaf-traces are seen, but nothing is known as to the nature of the leaf-traces in their course beyond the stele nor have we any data with regard to the leaves or reproductive organs.

This older species differs from _Cycadoxylon Fremyi_ in the limitation of the centripetal xylem to the outer portion of the pith and in the presence of sclerous nests in the medullary region, though the latter character is probably of no great taxonomic value. _Cycadoxylon robustum_ approaches more closely to _Lyginopteris_, and although the differences are sufficient to justify a distinctive generic name, there can be little doubt as to a fairly intimate relationship between this type of _Cycadoxylon_ and _Lyginopteris_.

=Ptychoxylon.= Renault.

_Ptychoxylon_[482] _Levyi_ Renault. Like many Palaeozoic genera founded on anatomical features, _Ptychoxylon_[483] is represented only by stems, our knowledge of the leaves being confined to the leaf-traces in the stem which appears to have a phyllotaxis of ⅜. The stem of this Permian species has a diameter of 5–6 cm.: the comparatively broad cortex contains numerous secretory canals, but in place of hypodermal strands of stereome there is a superficial periderm. The vascular tissue, consisting of secondary xylem and phloem, assumes different patterns at different levels. There is an outer vascular cylinder of centrifugally developed xylem and phloem; the xylem is manoxylic and the tracheids have 3–5 rows of bordered pits on the radial walls. At intervals the continuity of the main stele is broken by the formation of leaf-gaps and before one gap is repaired a second may be produced, thus converting the cylinder into two crescentic and infolded bands (fig. 453, A). A striking character is the occurrence in the large parenchymatous central region of internal vascular bands or arcs varying in size and number at different levels and composed of centripetally developed secondary xylem and phloem. These internal bands differ from the outer and broader cylinder both in their inverse orientation and their limited vertical range. The connexion between the inner and outer vascular tissue and the alteration in plan of the conducting tissue at different levels are illustrated by fig. 453, 1–4, simplified from some of Renault’s figures of successive sections through a vertical distance of 4–5 cm. In section 1 the main cylinder is continuous except for a small gap where a leaf-trace is about to be given off: there are three internal vascular bands similar in structure to the outer stele but inversely orientated. At a higher level (section 2) the leaf-gap is larger and in it is a double leaf-trace of two collateral strands consisting of primary centripetal xylem and a fan-like group of secondary xylem and phloem. The free edges of the outer stele of section 1 have curved inwards and united with the two lateral medullary bands, while the lower internal band of section 1 has increased in extent and forms a discontinuous arc with the upper portions enclosed by the loops formed by the infolded ends of the outer vascular tissue. In section 3 a second leaf-gap has been formed in the outer stele and its invaginated ends have fused with the internal bands. In section 4 the first leaf-gap is closed and the invaginated bands of section 3 have broken up into an irregular circle of shorter bands. The section reproduced in fig. 453, A, shows the main cylinder in the form of two curved and flattened loops, each composed partly of the centrifugally developed xylem and phloem of the main stele and in part of the inversely orientated tissue of the inner bands. At a lower level the two bands _b_, _b_, will become detached as the upper leaf-gap is closed and form part of an inner cylinder like the discontinuous ellipse formed by the two bands _c_. The section of a branch-stele is seen at _a_.

[Illustration: Fig. 453. A. _Ptychoxylon Levyi_; transverse section of stem; _a_, stele of branch. (After Renault.) B. _Cycadoxylon Fremyi_; transverse section of stem. (After Renault.) C. _Cycadoxylon robustum_; centre of stem. 1–4, diagrams of sections of the stem of _Ptychoxylon_ at different levels. (Simplified from Renault.)]

_Ptychoxylon_ differs considerably from _Medullosa_, which Renault included in the _Cycadoxyleae_, in the plan of the vascular system: there is nothing corresponding to the ‘partial pith’ or primary region which forms the central portion of the plate- and snake-rings in _Medullosa_. The double leaf-trace and the absence of the _Medullosa_ type of hypoderm are other distinguishing features. The paired leaf-bundles suggest comparison with _Lyginopteris_ among other genera and, as Scott[484] points out, the internal arcs of inversely orientated tissue which sometimes occur in the peripheral region of the pith of _Lyginopteris_ (fig. 405, C, _c_) behave like the internal bands of _Ptychoxylon_ in occasionally joining the main cylinder at a leaf-gap; but the differences outweigh the resemblances. As regards the general arrangement of the vascular tissue in two irregular concentric circles and their connexions with one another, but not in the structure of the xylem and phloem, there is a similarity between this genus and such a Fern as _Matonia_. In the varying patterns formed by the vascular system at different levels in the stem _Ptychoxylon_ resembles the Ferns _Polypodium quercifolium_ and _P. heracleum_[485].

=CALAMOPITYEAE.=

=Calamopitys.= Unger.

In 1856 Unger[486] described several fragmentary petrifactions from Thuringian strata of Upper Devonian age, the majority of which he referred to the Calamarieae and the Rhachiopterideae. In an earlier publication[487] he gave a list of species including two families, the Haplocalameae and the Calamoxyleae, assigned by him to the group Calamarieae: in the Haplocalameae he placed the new genera _Kalymma_, _Calamosyrix_, _Calamopteris_, and _Haplocalamus_. These were subsequently examined by Graf Solms-Laubach and identified as portions of petioles, for the most part belonging to unknown stems. In his second family, the Calamoxyleae, Unger included the single genus _Calamopitys_ represented by the type-species _C. Saturni_. The type-specimens have been thoroughly investigated by Solms-Laubach[488] who instituted the family-name Calamopityeae and recognised a close anatomical affinity between _Calamopitys_ and _Lyginopteris_, a conclusion which led to the incorporation of Unger’s genus in the Pteridosperms. Further data have been supplied by Zalessky[489] and, more recently, by Scott and Jeffrey[490] who have recognised _Calamopitys_ in Lower Carboniferous beds in Kentucky.

_Calamopitys Saturni_ Unger.

Our knowledge of this and other species is confined to stems and petioles. One of the largest examples of the species is a piece of stem with a diameter of 1·5 cm.: the single stele consists of a parenchymatous pith enclosed by secondary xylem made up of tracheids with 4–8 rows of bordered pits and medullary rays more than one cell broad and of considerable depth. Between the inner edge of the wood and the pith are groups of primary xylem (fig. 454, B, _x_) which, like those in _Lyginopteris_, constitute the leaf-traces: each has a single internal protoxylem strand (fig. 455, B). The comparatively wide cortex consists of parenchyma with a hypoderm of the _Sparganum_ type. Each primary xylem-strand passes out as a single leaf-trace through the secondary xylem and on emerging divides into two as in _Lyginopteris_: these branch in the cortex and the two are replaced by six in the leaf-base (fig. 454, B–D). As seen in figs. 454, C, 455, A, the boundary between the stem proper and the decurrent leaf-base is marked by a line of stereome strands. The petioles of _Calamopitys Saturni_ agree generally in structure with the imperfect specimens on which Unger founded his genus _Kalymma_[491], so named in reference to the structure of the hypodermal zone. A specimen described by Solms-Laubach as a _Kalymma_ petiole occurs in organic connexion with a stem of _Calamopitys_ (fig. 454, C: a detached petiole is shown in fig. 454, D). The identification by White[492] of this attached petiole with Unger’s _K. grandis_ has been confirmed by Scott and Jeffrey. A fuller account of _Kalymma_ (fig. 456) is given on a later page, as the petioles so named belong to more than one species of stem.

[Illustration: Fig. 454. _Calamopitys Saturni._ B, stem with leaf-base; _x_, primary xylem and pith; black patches represent leaf-traces; C, stem with two leaf-bases; D, section of petiole. (After Solms-Laubach.)]

[Illustration: Fig. 455. _Calamopitys Saturni._ A, stem with leaf-bases; × 5. B, stele showing leaf-trace after emerging from the secondary xylem; × 16. (After Zalessky.)]

In _Calamopitys Saturni_ we have a plant agreeing with _Lyginopteris_ in the possession of secondary xylem of the manoxylic type and in the structure of the common primary bundles, while it is distinguished from _Lyginopteris_ by the greater number and by the structure of the bundles in the axis of the leaf.

_Calamopitys annularis_ (Unger)[493].

This species, originally assigned to the genus _Stigmaria_, has a more strongly developed primary vascular system and there is a more decided tendency towards the formation of a continuous zone of primary xylem on the inner edge of the secondary wood; but where the protoxylem tracheids occur the metaxylem elements form definite strands, like those of _C. Saturni_. It has been pointed out by Scott and Jeffrey that there is some evidence of the occurrence of tracheids in the parenchymatous pith of this species, an important feature distinguishing it from _C. Saturni_ and connecting it with _C. americana_. Information with regard to the behaviour of the leaf-traces is far from complete, but there are indications that each trace divides into two before emerging from the secondary xylem[494]. The leaf-traces in the cortex are concentric as in _C. Saturni_.

_Calamopitys americana_ Scott and Jeffrey.

This Lower Carboniferous species[495] from the Waverley shales of Kentucky is represented by portions of stems and leaf-bases and detached petioles. The secondary wood consists of tracheids, 30–60μ in diameter, with deep and broad rays; the small pits on the tracheids form 5–6 alternating series. Phloem and cambium are very imperfectly preserved. The outer cortex is of the same type as in other species. At the inner edge of the secondary xylem there is a ring of primary xylem strands of mesarch structure composed of rather larger tracheids, 80–120μ in diameter, separated from one another by narrow strips of parenchyma. So far the vascular tissue agrees with that of _C. annularis_. In the American species the axial region is not a parenchymatous pith but a protostele, consisting of parenchyma and a larger or smaller number of tracheal groups, the number being less in stems with a larger central region. The peripheral strands alone are concerned with the emission of leaf-traces, as in _Heterangium_. Each primary xylem strand divides into two as it leaves the perimedullary zone and passes through the secondary xylem as two bundles, each being accompanied by an arc of secondary tracheids which, in the cortical region, completely surrounds the primary elements. At a later stage the single protoxylem of each trace divides into two and before entering the leaf-base there is a further division. In some specimens leaf-bases of the _Kalymma_ type were found attached to the stem. The occurrence of tracheids in the axial region is a distinguishing feature and suggests a comparison with _Heterangium_, while _C. Saturni_ agrees more closely with _Lyginopteris_; the species _C. annularis_ would appear, from the recent observations of Scott and Jeffrey, to occupy an intermediate position.

_Calamopitys_, as the generic designation is here employed, is confined to central Germany and Kentucky and occurs in Upper Devonian and Lower Carboniferous strata. There is, however, some doubt as to the exact geological horizon of the rocks in both countries though in neither case is there any question of an horizon higher than Lower Carboniferous. Certain specimens from the Lower Carboniferous of Scotland described by Scott[496] as species of _Calamopitys_ have been made by Zalessky the type of a new genus, _Eristophyton_, and are dealt with under that name.

_Kalymma._ Unger.

_Kalymma grandis_ (petiole of _Calamopitys_). Under the generic name _Kalymma_ Unger described specimens from Thuringia of Upper Devonian age which he assigned to two species, _K. grandis_ and _K. striatum_. Solms-Laubach has shown that _Kalymma_ is not an independent stem as Unger believed but a petiole of _Calamopitys_, and this has been confirmed by Scott and Jeffrey who found a leaf-base with the _Kalymma_ type of structure in connexion with a piece of _Calamopitys_ stem, probably _C. americana_. An examination of a section (2·3 cm. broad) of Unger’s _K. grandis_ in the collection of the Geological Survey enables me to confirm the conclusions recently published by Scott and Jeffrey. The best specimens of _Kalymma_, which appear to be identical in essential features with Unger’s type-species, are from Kentucky, some from the Genessee shales of Upper Devonian age and others from beds (Waverley shales) believed to be Lower Carboniferous. Through the kindness of Prof. Bower I have had an opportunity of examining sections from the older horizon in his possession. The transverse section reproduced in fig. 456 has a diameter of 3·8 × 2·2 cm.[497]: on one side the radially placed plates of stereome are clearly shown, and in the outer portion of the ground-tissue is a ring of vascular bundles varying in size and shape but with a general tendency to a radially elongated form. The ground-tissue consists of homogeneous parenchyma: in one place I noticed what appeared to be a large secretory canal, but secretory tissue, generally at least, is unrepresented. The xylem is composed of imperfectly preserved elements, which appear to have scalariform pits; spiral protoxylem strands, embedded in the metaxylem as two or four groups, occur near the ends of the long axis of the bundle and in some cases also near the centre. The phloem probably surrounded the xylem, though it is not certain whether the arrangement was collateral or concentric: there are no secondary-xylem tracheids, though in some places I noticed a tendency to a radial disposition of cells at the periphery of the vascular tissue simulating an early stage of secondary growth. Unger’s second species _Kalymma striata_ is characterised by an arrangement of the bundles similar to that in a petiole described by Scott and Jeffrey as _Calamopteris Hippocrepis_ which differs from _Kalymma_ in the partial substitution of bands of vascular tissue for separate bundles and to some extent in the disposition of the bundles. The two types of petiole _Kalymma_ and _Calamopteris_, as Scott and Jeffrey state, are very closely allied. Dawson and Penhallow[498] have also described _Kalymma grandis_ from Kentucky but they, like Unger, mistook the hypodermal stereome for an outer zone of vascular bundles. The petioles from Germany and North America included under the name _Kalymma grandis_, though too similar to be referred to different species, no doubt represent petioles of stems which are unquestionably distinct types: as in the case of _Myeloxylon_ in its relation to the genus _Medullosa_, _Kalymma_ stands for several closely allied forms of petioles belonging to several species of _Calamopitys_.

[Illustration: Fig. 456. _Kalymma grandis._ (From a section in the possession of Prof. Bower; × 4.)]

=Eristophyton.= Zalessky.

_Eristophyton fasciculare_ (Scott). The generic name _Eristophyton_[499] was proposed by Zalessky[500] for two incomplete stems of Lower Carboniferous age provisionally[501] referred by Scott to _Araucarioxylon_ and subsequently to _Calamopitys_[502]. The species _E. fasciculare_ was founded on material obtained by Dr Kidston from the Lower Carboniferous of Dumbarton, Scotland, and on a specimen in the Williamson collection from Northumberland. There is a small pith of parenchyma, 2–3 mm. in diameter, with eight strands of primary xylem of varying diameter (fig. 457) and in each a single protoxylem-group. The primary xylem elements are considerably larger than the secondary tracheids. These xylem-bundles are leaf-traces and their disposition points to a phyllotaxis of ⅖. The traces attain their maximum size when about to pass out through the secondary xylem. The tracheids are reticulate and scalariform while some have an intermediate type of pitting. A leaf-trace on reaching the pith gradually moves further from the xylem-cylinder and may be separated from it by 2–6 layers of parenchyma: as it passes down the pith the protoxylem strand assumes an almost endarch position consequent on the reduction of the centripetal xylem. In Scott’s words, ‘each circummedullary strand branches at regular intervals; the one branch, that on the anodic side [turned towards the course of the genetic spiral], becomes the leaf-trace and passes out, while the other continues its course up the stem as a reparatory strand, until the next leaf of the orthostichy has to be supplied[503].’ The secondary xylem consists almost entirely of tracheids with 3–4 rows of pits on the radial walls and medullary rays usually one cell broad varying in depth from 1 or 2 to 16 or more cells. A characteristic feature of the secondary xylem is the occurrence on its inner face of numerous short and broad tracheae similar to the still larger tracheae in the primary stele of _Megaloxylon_[504]. Nothing is known as to the behaviour of the leaf-trace in the extrastelar region, but the fact that an outgoing trace was found to have two protoxylems points to a subdivision similar to that of the foliar bundles of _Calamopitys Saturni_. A well-marked difference between _Eristophyton fasciculare_ and _Calamopitys_ and _Lyginopteris_ is the more compact structure of the secondary wood; it is pycnoxylic and not manoxylic. Prof. Zalessky in criticising the use of the generic title _Calamopitys_ puts forward several arguments in support of his institution of a new designation: (i) the primary xylem strands of _Eristophyton_ are not confined to the periphery of the pith as is the case in _Calamopitys Saturni_, though he speaks of one leaf-trace in the latter species separated by several layers of cells from the xylem-cylinder; (ii) some of the pith-cells have thick walls and dark contents in distinction to the homogeneous parenchyma of _Calamopitys_, a feature of little importance; (iii) the difference in the structure of the secondary wood already alluded to, though this loses some of its significance by the occurrence of narrower rays, more like those of _Eristophyton_, in _C. annularis_; (iv) the more elliptical and broader pits in the secondary tracheids in place of the more regular hexagonal form in _Calamopitys_. While admitting a certain degree of relationship between the two types, Zalessky asserts that as yet we have insufficient evidence to justify their generic union. Scott[505] maintains that Zalessky does not attach sufficient weight to the form and mesarch structure of the primary xylem bundles as a feature common to both genera.

[Illustration: Fig. 457. _Eristophyton fasciculare._ Transverse section showing the relation of primary (black) to secondary xylem. (After Scott.)]

_Eristophyton Beinertianum_ (Goeppert).

1850. _Araucarites Beinertianus_ Goeppert, Mon. Foss. Conif. p. 233, Pls. 42, 43. 1872. _Araucarioxylon Beinertianum_ Kraus, in Schimper’s Trait. Pal. Vol. +ii.+ p. 381. 1888. _Araucarites Beinertianus_ Goeppert and Stenzel, Abh. K. Preuss. Akad. Wiss. p. 30, Pl. +iv.+ 1902. _Calamopitys Beinertiana_ Scott, Trans. R. Soc. Edinb. Vol. 40, p. 341, Pls. +i.+, +iv.+, +v.+ 1909[506]. _Ullmannites Beinertianus_ Tuzson, Result. Wiss. Erforsch. Balatonsees, Bd +i.+ Teil +i.+ p. 24. 1911. _Eristophyton Beinertianum_ Zalessky, Com. Geol. St Pétersb. p. 24.

The pith, 13–15 mm. in diameter, is rather larger than in _E. fasciculare_ and is characterised by the occurrence of dark sclerotic nests surrounded by radially disposed rows of parenchyma. The primary xylem strands are more numerous and smaller than in _E. fasciculare_ and these increase in diameter as they approach the secondary wood. In places the primary xylem elements form a more or less continuous band as in _Calamopitys annularis_. The largest leaf-trace bundles at the periphery of the pith are mesarch (fig. 458), but as each trace passes down the pith the reduction in the centripetal xylem is carried further than in _E. fasciculare_ until the xylem-strand becomes endarch in the lower part of its course. The secondary tracheids have usually two contiguous rows of pits and the medullary rays are one cell broad.

[Illustration: Fig. 458. _Eristophyton Beinertianum._ A strand of primary xylem, showing the protoxylem, _px_, abutting on the secondary xylem. (× 35. After Zalessky.)]

There can be no doubt as to the generic identity of the two species referred to _Eristophyton_, but the question as to the degree of affinity to _Calamopitys_ is more difficult to settle. There is force in Zalessky’s contention that these two stems should not be retained in _Calamopitys_: the recently described American species, _C. americana_ Scott and Jeffrey, gives emphasis to the view that the restriction of _Calamopitys_ to the German (and American) types is the safer course. While _Calamopitys_ as thus restricted is almost certainly a Pteridosperm, the inclusion of the types referred to _Eristophyton_ in the same category rests on a more slender basis.

=CLADOXYLEAE.=

This order was founded by Unger[507] for some imperfectly preserved stems from Palaeozoic strata in Thuringia and in it he included the two genera _Cladoxylon_ and _Schizoxylon_. There is some doubt as to the precise age of the Thuringian beds; they were assigned by Richter to the Devonian system and subsequently placed in the Culm: Solms-Laubach in his later reference to Unger’s plants favours a Devonian horizon[508]. Unger included the Cladoxyleae in the Lycopodiales, and though this conclusion is not accepted the position of the order is still uncertain. His genus _Schizoxylon_ has no claim to generic separation from _Cladoxylon_. An inspection of the illustrations in the memoir by Richter and Unger reveals a striking resemblance in the main anatomical features between several types assigned to different genera and distributed among the Cladoxyleae and Rhachiopterideae (a term first used by Corda for petrified rachises or petioles of ferns) and other orders. Solms-Laubach[509], to whom our more accurate information as to Unger’s plants is chiefly due, is inclined to regard the specimens referred by Unger to the genus _Arctopodium_ as young stems of _Cladoxylon_, and he draws attention to a close similarity between _Hierogramma_, another of Unger’s genera, and _Cladoxylon_. Paul Bertrand[510] goes further in considering that the following genera represent one generic type, namely _Syncardia_ (fig. 459, F), _Hierogramma_, _Arctopodium_, _Cladoxylon_, and _Schizoxylon_. The same author interprets the fossils so named by Unger as stems and does not agree with the inclusion of any of them in the Rhachiopterideae. Without losing sight of the fact that Bertrand’s conclusion is not based on proof but is the expression of a view suggested by a close agreement in general anatomical plan, I venture to adopt the designation _Cladoxylon_ in a wide sense primarily on the ground that Bertrand’s view is probably correct and in part for the sake of convenience of description. As Unger’s species of _Cladoxylon_ differ from one another in features which may fairly be regarded as of minor importance, they are included under one specific name.

=Cladoxylon=. Unger.

_Cladoxylon mirabile_ Unger[511]. The following are regarded as specifically identical with or closely allied to _Cladoxylon mirabile_: _C. dubium_, _Schizoxylon taeniatum_, _Hierogramma mysticum_, _Syncardia pusilla_, _Arctopodium insigne_ and _A. radiatum_[512].

_i. Stems._ The stems assigned to _Cladoxylon_ are characterised by a complex system of steles, either simple or branched and occasionally anastomosing, presenting in transverse section the form of oval or cylindrical strands or narrow, straight or curved bands arranged on a more or less clearly marked radial plan (fig. 459, A, B, D). In some stems the primary vascular tissue is enclosed by secondary xylem and phloem (fig. 460, B), while in others (Unger’s _Arctopodium_, _Hierogramma_[513], _Syncardia_) there is no evidence of secondary thickening. The diagrammatic drawing represented in fig. 459, F, shows a section of a small axis, regarded by Unger and Solms as a petiole (3 mm. in diameter), containing four vascular strands composed exclusively of primary xylem, each with one or, in the case of a double strand, two protoxylem groups. This type may be a slender stem or branch or possibly a petiole. The other extreme, as regards complexity of vascular structure, is represented by such stems as those shown in fig. 459, A, B, D. In Unger’s _Cladoxylon mirabile_ (fig. 459, A; fig. 460, B) the stem reaches a diameter of 3 cm. and consists of several radially disposed plates of vascular tissue with an occasional smaller oval or cylindrical stele embedded in a ground-tissue composed of thick-walled cells. The plates are curved like a UU or sinuous and not infrequently anastomosing. In a section of this type figured by Unger the vascular plates appear to form a complex anastomosing system, but Solms[514] states that the drawing exaggerates the amount of fusion between the strands, and an examination of a section in the collection of the English Geological Survey cut from Unger’s specimen enables me to confirm this statement. Each vascular plate consists of a narrow median region composed of primary tracheids with a scalariform type of pitting surrounded by secondary tracheids with interspersed medullary rays one cell broad. The thickness of the secondary xylem varies considerably in the same specimen and in places this tissue is hardly represented, a fact of importance in view of the very striking resemblance between _Arctopodium_ and _Cladoxylon_, the sections referred by Unger to the former genus having steles without any secondary xylem. The occurrence of one or two elongated spaces (shown in black in fig. 459, A) near the distal end of each plate mark the position of the protoxylem tracheids. Fig. 459, C, represents a stele of a stem referred by Unger and by Solms to _Cladoxylon dubium_ which shows the typical _Cladoxylon_ structure, namely the central primary xylem with distally placed protoxylem and the enclosing sheath of secondary xylem. In the stem shown in fig. 459, D (_C. dubium_) there are 12 steles, each constructed on the plan already described, differing in their relatively broader and shorter form and in the greater breadth of the secondary xylem from those seen in fig. 459, A (_C. mirabile_). The black areas in fig. 459, A, show the primary xylem, and the protoxylem is seen in fig. 459, C. A stem described by Dawson[515] as _Asteropteris noveboracensis_ from Devonian beds is compared by him with Unger’s _Cladoxylon mirabile_ and regarded as possibly allied to it. The radial plates of xylem in Dawson’s plant meet in the centre like those of _Asterochlaena_ and the leaf-traces are of the Clepsydropsoid type.

[Illustration: Fig. 459. A, _Cladoxylon mirabile_, section of stem; B, _Cladoxylon taeniatum_, section of stem; C, _Cladoxylon dubium_, section of stele; D, _C. dubium_, section of stem; E, _Cladoxylon (?) mirabile_, section of petiole; F, _Syncardia pusilla_; G, development of leaf-trace in _Cladoxylon taeniatum_. (A–E, after Solms-Laubach; F, after Unger; G. after P. Bertrand.)]

The type of stem for which Unger founded his genus _Schizoxylon_ is represented in fig. 459, B; there are five small steles in the centre and external to these eleven radially arranged plates, with oval steles between them, in the peripheral region of the stem. Each stele consists of primary (black in the figure) and secondary xylem and agrees with the steles in the other stems.

From the type of stem illustrated by _Cladoxylon mirabile_ to that on which the genera _Arctopodium_[516] and _Hierogramma_[517] were founded is a very small step: the vascular tissue has the same characters both as regards gross and minute anatomy, but there is no evidence of cambial activity in the stems referred to the two latter genera, a difference in itself hardly worthy of generic recognition.

ii. _Leaves._ Before describing a second type of stem referred to _Cladoxylon_ it is important to consider briefly such evidence as we have as to the vascular supply of the leaves. Nothing is known of the reproductive organs and there is no satisfactory information with regard to the form of the fronds. Solms-Laubach has described the only known example of a lateral branch of a _Cladoxylon_ stem (fig. 459, E): this has a single concentric vascular strand of plate-like form with two blunt projections and there are four protoxylem-groups, two in the angle of the plate and two at the base of the projections. The structure is essentially fern-like; the xylem is wholly primary. This type of vascular strand agrees fairly closely with that of a petiole described by Unger as _Megalorhachis elliptica_, a section of which is in the Museum of the Geological Survey[518]. The petiole is oval in section and laterally winged, and the meristele is tangentially elongated and has two blunt projections almost identical with those in fig. 459, F. There is no evidence as to the nature of the supporting stem, but there can be little doubt as to the close connexion with _Megalorhachis_ and the section shown in fig. 459, F. In a note published in 1908 P. Bertrand stated that he had identified several of Unger’s genera as stems which bore leaf-traces having the form and structure of _Clepsydropsis_, one of the types referred by Unger to the Rhachiopterideae and described in the second volume of this work[519] as a Coenopteridean petiole. Bertrand points out that in the oval or plate-like steles of _Cladoxylon_, _Arctopodium_, _Hierogramma_, etc., there is a single protoxylem group near the distal end of the primary xylem, and he adds that the leaf-traces were formed of strands cut off from the distal portions of the vascular plates. Similarly the hour-glass-like leaf-trace in the primary rachis of _Clepsydropsis_ gives off from each end a ring of xylem to supply a secondary rachis. These laterally detached annular strands are, he believes, similar to the leaf-trace cut off from the steles in a _Cladoxylon_ stem. The conclusion is that _Cladoxylon_ is a fern stem and its leaf-trace represents the simplest form of the _Clepsydropsis_ type, namely an oval bundle of xylem with a central protoxylem, which is also the form of the trace given off from the stem of _Asterochlaena_. Solms[520], while admitting that Bertrand may be correct in uniting under one genus _Cladoxylon_ and such types as _Syncardia_, _Hierogramma_, and _Arctopodium_, disagrees with the view that they are _Clepsydropsis_ stems. A Clepsydropsoid leaf-trace has never been found in direct association with any of the stems of the _Cladoxylon_ type and such evidence as there is indicates a leaf-trace of an entirely different form (fig. 459, E). In his more recent memoir on _Asterochlaena_ Bertrand[521] draws attention to Solms’ figures of a stele of _Cladoxylon_ (fig. 459, C) in which the distal portion is on the point of being separated as a small annular strand. This, Bertrand considers, would gradually become converted into a _Clepsydropsis_ form of stele as it passed to the petiole. Bertrand’s drawings made from a section of _Cladoxylon taeniatum_ (Ung.) (fig. 459, G) illustrate successive stages in the departure of a leaf-trace from one of the plate-like steles of the stem (fig. 459, B). In fig. 459, G, 1, a piece of the stele is detached and near its extremity is a group of thin-walled cells with protoxylem: a later stage is seen in fig. 2, and in fig. 3 a small ring of xylem is being detached which, Bertrand assumes, would later in its course be converted into the Clepsydropsoid strand (fig. 4), which consists of primary tissue. The weak point in Bertrand’s contention[522] is the absence of any proof of a true Clepsydropsoid trace in connexion with a _Cladoxylon_ stem, and there is a strong probability that the leaf-trace of _Cladoxylon_ has the form shown in fig. 459, E.

_Cladoxylon Kidstoni_ Solms-Laubach[523].

This species, founded on imperfectly preserved material in Dr Kidston’s collection from Lower Carboniferous rocks in Berwickshire, is referred to _Cladoxylon_ on evidence that cannot be regarded as convincing. The type-specimen consists of a small piece of stem about 3 cm. in breadth showing three complete oval steles and portions of two others which seem to be in their original position and probably formed part of a series of peripheral steles such as those shown in fig. 459, D. Each stele consists mainly of secondary xylem (fig. 460, A) with some crushed tissue, presumably phloem, on its outer face. The secondary xylem is narrower on the inner side of each stele where a wedge-shaped piece is partially detached. In the centre there is a narrow area parallel to the long axis of the stele containing crushed tissue which probably consists of parenchyma and primary xylem, but the preservation is very imperfect. The secondary xylem has a fairly compact structure and the rays are narrow, 1–10 cells in depth. The pits of the tracheids are described by Solms as scalariform with occasionally two rows of elliptical pits on the radial walls. A careful examination of the type-specimen leads me to describe the pits as uniseriate and transversely elongated, very like those of _Protopitys_, or biseriate and almost circular like those of Conifers, the pits of the two rows being alternate or sometimes opposite (fig. 460, C): in places three rows of bordered pits are present. There is a certain degree of resemblance between the steles of this species and those of the South African stem _Rhexoxylon_[524], but the data are inadequate for a satisfactory comparison.

[Illustration: Fig. 460. A, C. A single stele and tracheids of _Cladoxylon Kidstoni_. (Kidston Coll. 630 B, 630 C.) B. _Cladoxylon mirabile_, part of a stele. (Museum of Practical Geology, 15872.)]

There is a close similarity between the vascular systems of _Cladoxylon_ and _Medullosa_, but an obvious difference is the substitution of the oval, transversely elongated, pits on the xylem elements for the multiseriate pitting of _Medullosa_. In _Cladoxylon Kidstoni_ the pitting shows transitional forms between a narrow scalariform uniseriate type and a biseriate or triseriate arrangement similar to that in the Araucarineae and Cordaitales. In _Cladoxylon_, as limited by Unger, the presence of secondary wood is a generic feature, but by the inclusion of _Arctopodium_ and other forms this character no longer holds good. The inclusion of these more fern-like stems without secondary xylem brings _Cladoxylon_ (in the wider sense) into closer contact with _Asterochlaena_, a comparison previously suggested by more than one author. In _Medullosa_ the development of secondary xylem is on a larger scale than in _Cladoxylon_, and the vascular system of the former genus assumes a more complex form. Moreover the _Myeloxylon_ type of petiole, which is a distinctive feature of _Medullosa_, differs widely from any form of leaf-trace associated with _Cladoxylon_.

=Völkelia.= Solms-Laubach.

_Völkelia refracta_ (Goeppert). The generic name _Völkelia_[525] was proposed by Solms-Laubach[526] as a substitute for _Sphenopteris_[527] in the case of some petrified stems or petioles associated with fragmentary impressions of fronds from Lower Carboniferous rocks in Silesia. Both leaf-impressions and petrifactions were included in the genus _Sphenopteris_: Solms, while retaining Goeppert’s designation for the leaf fragments, proposed a new generic name for the petrifactions on the ground that there is insufficient evidence of their connexion with the leaves. The short account of Goeppert’s petrified specimen given by Graf Solms-Laubach in his ‘Fossil Botany[528]’ is supplemented by a fuller description in a later paper. The fragments of highly compound fronds are characterised by very small filiform ultimate segments, but the specimens are too imperfect to afford a clear idea of the habit of the leaf. The ‘stem’ bears a close superficial resemblance to that described by Unger as _Cladoxylon dubium_ (fig. 459, C, D) and was regarded by him as an example of that species: it contains several radially placed steles represented by fairly well-preserved xylem, but no phloem has been recognised. The steles vary in size and shape: five reach almost to the centre (fig. 461, A) and smaller xylem groups occupy a peripheral position. Each stele is excentric in structure and consists of (i) an outer zone of secondary tracheids of horse-shoe form in transverse section, but the apparent gap in the secondary xylem on the outer edge of each stele is due to the crushing of the tracheal tissue and to its smaller breadth in the distal part of each group; this is shown in fig. 461, B, where the apparent gap is seen to be occupied by distorted and crushed tracheids, _a_, identical with those which form the rest of the outer zone (fig. 461, B, _b_); (ii) a zone of tracheal tissue continuous with and originally identical in appearance—except that the elements are rather narrower—with the outer secondary xylem; (iii) an excentrically situated island composed of tracheids enclosing a small central area occupied by thin-walled parenchyma. This third region, represented by black patches in fig. 461, A, in all probability represents the primary part of each stele to which the rest of the tissue has been added by the cambium. A striking feature of the secondary xylem is the absence of medullary rays: the tracheids resemble those of _Cladoxylon_ and _Protopitys_ in the transverse elongation of the pits (fig. 461, D) which form either a single row or several irregularly distributed rows. The primary xylem consists in the peripheral region of tracheids with very narrow scalariform pitting which at first sight suggest close spiral bands (fig. 461, C), while the inner tracheids are either annular or reticulate and associated with elongated parenchyma. The imperfectly preserved ground-tissue appears to consist of homogeneous parenchyma with radially disposed bands of stereome in the outer cortex.

[Illustration: Fig. 461. _Völkelia refracta._ A. Transverse section of a specimen in the Breslau Museum. B. Portion of a stele; _a_, the crushed xylem on the outer side; _b_, the inner side. C. Longitudinal section showing the median protoxylem. D. Tracheid from the secondary xylem. (After Solms-Laubach.)]

Our knowledge of _Völkelia_, though far from complete, justifies its generic separation from _Cladoxylon_ from which it differs in the lack of medullary rays and in the structure of the primary portion of each stele. In the form and arrangement of the pits in the secondary tracheids _Völkelia_ differs from _Medullosa_ and resembles _Cladoxylon_. The opinion expressed by P. Bertrand[529] that _Völkelia_ is probably the stem of one of the Zygopterideae is based on the older accounts of the genus and not on the fuller description of 1910.

=PROTOPITYEAE.=

=Protopitys.= Goeppert.

The only species so far described is that for which Goeppert founded the genus in 1850, substituting _Protopitys_[530] for the name _Araucarites_, adopted in an earlier paper, on the ground that the structure of the xylem denoted a distinct generic type. The type-species is from the Upper Devonian rocks of Falkenberg in Silesia.

_Protopitys Buchiana_ Goeppert.

1845. _Araucarites Buchianus_ Goeppert, in Wimmer’s Flor. Schlesien (edit. +ii.+) p. 218. 1847. _Pinites Goepperti_ Unger, Chlor. Protog. p. 31. 1847. _Dadoxylon Buchianum_ Endlicher, Syn. Conif. p. 300. 1850. _Protopitys Buchiana_ Goeppert, Foss. Conif. p. 229, Pl. +xxxvii.+ figs. 4–7; Pl. +xxxviii.+ figs. 1, 2.

In his Monograph of Fossil Conifers Goeppert figured a large piece of stem consisting mainly of secondary wood and described the more important anatomical features. He recognised the narrow, transversely elongated, pits on the radial walls of the tracheids as a feature of special interest indicating a type of pitting transitional between that of Ferns and Conifers. A further description was given by Kraus[531] who included under Goeppert’s name both the Falkenberg stem and a second specimen from Basel though the latter is Triassic in age and a distinct plant; he suggested a comparison of _Protopitys_ with _Sigillaria_ and _Stigmaria_ rather than with Conifers. It is, however, to Graf Solms-Laubach[532] that we owe the most thorough account of this species. Nothing is known of the leaves or reproductive organs. The largest piece of stem is nearly 1 ft in diameter and consists mainly of secondary xylem resembling that of Conifers and _Cordaites_ except in the form of the bordered pits on the radial walls of the tracheids (fig. 462, D). The centre of the stem is occupied by a parenchymatous pith, elliptical in transverse section, enclosed by a band of primary xylem composed of large polygonal tracheids (fig. 462, C, _x_¹) characterised by a delicate scalariform pitting on all their walls (fig. 462, E). The primary xylem forms a narrow layer on the sides of the ellipse, 1–3 elements broad (fig. 462, A, B), but it increases in breadth at the ends of the long axis where the tracheids are intermixed with parenchyma. The primary xylem and pith-tissue at the ends of the major axis of the central region assume different forms at different levels, owing to the detachment of leaf-traces and the consequent formation of foliar gaps as portions of the primary xylem pass obliquely outwards into the secondary xylem on the way to the distichously arranged alternate leaves. The diagram, fig. 462, A, shows the inner part of the secondary xylem (see also fig. 462, C, x²) which at one end, _lt_, has formed an oval group about to pass out as a leaf-trace: at the opposite end the strand is detached and divided into two equal branches. The two swellings of the primary xylem ellipse shown at _a_ in figs. 462, A and B, are a characteristic feature: these are clearly seen after the leaf-trace has become detached; at the inner edge of each of them there appears to be a protoxylem strand. After the formation of a foliar gap these swellings of the xylem gradually meet and so re-establish continuity below the outgoing leaf-trace. No protoxylem has been detected in the actual trace, which is believed to be concentric. The formation of the leaf-gap and the shoulders bordering it constitute interesting filicinean features, recalling corresponding characters in solenostelic Ferns. At the upper end of the diagram, fig. 462, B, the outgoing leaf-trace is undergoing dichotomy while at the opposite end the trace has passed out of view. The secondary xylem shows incomplete rings or arcs of narrower elements, which at first sight give the impression of annual rings: the occurrence of similar incomplete or pseudo-rings is a common feature in _Lepidodendron_ and other Palaeozoic stems. The secondary tracheids (54·4μ in tangential diameter, 68·5μ in radial diameter) have usually a single series of broadly oval bordered pits on the radial walls with here and there two rows (fig. 462, D). In one case only were the pits of the medullary rays recognised (fig. 462, F). The rays are uniseriate, generally 1–2 cells deep, but occasionally 3 cells in depth and very rarely deeper. The cambium is of the normal type, and in some specimens secondary phloem was found consisting of bands, 4–5 layers broad, of stone-cells alternating with tubular thin-walled elements, presumably sieve-tubes.

[Illustration: Fig. 462. _Protopitys Buchiana_. A, B. Central region of the stem showing the pith, the primary xylem (black in fig. A) and (A) the inner part of the secondary xylem; _a_, shoulders of xylem at the leaf-gaps; _lt_ leaf-trace; _px_ protoxylem. C. Secondary, _x_², and primary xylem, _x_¹. D. Radial longitudinal section of the secondary xylem. E. Scalariform tracheid of the primary xylem. F. Pits on the medullary-ray cells. (After Solms-Laubach.)]

As Solms-Laubach says, it is highly probable that each leaf-trace, which forks close to its exit from the primary xylem, became further subdivided before reaching the leaf. Morphologically, _Protopitys_ is of special importance as a type possessing characters that indicate a connexion with Conifers or Cordaitean genera, notably the structure of the secondary wood, while the presence of foliar gaps is a feature reminiscent of Ferns. The primary xylem resembles that of some of the Palaeozoic arborescent Lycopodiales, but in _Protopitys_ the interruptions in this tissue are due to the emission of leaf-bundles, whereas in the discontinuous primary xylem of some Sigillarias[533] the gaps have no connexion with leaf-traces. Moreover the distichous leaves of _Protopitys_ and the larger, branched, leaf-traces are other distinguishing features. The pitting of the primary xylem is like that in the Lycopodiales and Filicales, while that of the secondary wood shows a closer approach to the coniferous type. A comparison may also be made with the transversely elongated pits of _Cladoxylon Kidstoni_[534].

A piece of wood agreeing anatomically with the Silesian species of _Protopitys_ has been found in the Yoredale rocks of England[535].

The peculiarities of the genus have been emphasised by Solms-Laubach by the institution of a family-name Protopityeae: the genus is essentially a generalised type exhibiting in the structure of its stem both Filicean and Coniferous features. The bordered pits differ from those in recent Conifers in their flatter form, but in this respect they exhibit a closer agreement with the transversely stretched pits of _Xenoxylon phyllocladoides_ Goth.[536], a Mesozoic species.