CHAPTER XXXIII.

CORDAITALES.

A. =POROXYLEAE=.

=Poroxylon=. Renault.

In 1879 Renault[537] briefly summarised the anatomical features of some silicified vegetative shoots from the Permian of Autun for which he instituted a new family, the Poroxyleae. The more complete account contains a description of two species, _Poroxylon Boysseti_ and _P. Duchartrei_: the latter was afterwards recognised as a stem of _Heterangium_. Renault considered this new genus to be closely allied to _Sigillaria_ and _Sigillariopsis_ and pointed out its resemblance to _Cordaites_. Additional species have since been described but as yet the genus has not been found outside France in Permo-Carboniferous strata of Autun and the St Étienne district. The results of a more detailed investigation of the anatomy of the genus were published by Bertrand and Renault in 1882 and since then[538] Bertrand, Renault, and Scott have added to our knowledge of this interesting type. In several respects _Poroxylon_ stems present a striking resemblance to _Lyginopteris_, but the recent discovery of the genus _Mesoxylon_ has given greater significance to the characters in which _Poroxylon_ agrees with representatives of the Cordaitales. Our knowledge of the genus, though exceptionally full with regard to the anatomy of vegetative shoots, does not include any precise information as to the reproductive organs.

The slender cylindrical stems, not exceeding 2–3 cm. in diameter in specimens so far recorded, bore large broadly linear leaves similar in form and venation to those of some species of _Cordaites_ which were attached singly to slightly swollen nodes separated from one another by internodes several centimetres long. The base of the rather fleshy lamina passes imperceptibly from the narrow lower portion into a tangentially expanded petiole which forms a decurrent ridge on the stem. Axillary buds frequently occur. Little is known of the leaf-impressions, but if Grand’Eury[539] is correct in his identification of certain specimens from French Stephanian beds as the leaves of _Poroxylon_, the lamina reached a length of 1 met. and a breadth of 15–20 cm. In habit the stems probably resembled some of the larger-leaved Bamboos. The only evidence bearing on the nature of the reproductive organs is furnished by Grand’Eury who believes that some _Rhabdocarpus_ seeds and bractless inflorescences associated with the leaves assigned to _Poroxylon_ belong to that genus.

The single cylindrical stele has a relatively large solid pith, the perimedullary region being characterised by the occurrence of a row of primary crescentic strands of centripetal xylem of exarch type, though not improbably in some cases slightly mesarch, varying in size and shape and forming single or paired bundles. These strands represent the xylem of collateral leaf-traces similar to those of _Lyginopteris_ but differing in the absence of well-defined centrifugal elements: the curved form of some of the xylem strands gives them an appearance similar to that of the leaf-traces of _Lyginopteris_. The leaf-traces, except in the lower part of their course through the pith, are double and pass through several internodes before the centripetal tracheids die out. The secondary xylem (fig. 463) is manoxylic and very similar to that of _Lyginopteris_ though rather less parenchymatous. The secondary phloem and cambium are often very well preserved. No endodermis and no distinct pericycle has been recognised. The cortex is parenchymatous and, like the pith and to some extent the phloem, contains numerous secretory sacs; in the outer cortex the presence of hypodermal strands is a prominent feature. At an early stage in the growth of the stem a deep-seated phellogen forms secondary tissue both externally and internally and decortication ensues.

[Illustration: Fig. 463. _Poroxylon Edwardsii_. A. Transverse section of stem. B. The central region of another stem of the same species. (A, × 9; University College Collection; B, from a photograph supplied by Prof. Bertrand of a specimen in the Renault Collection, Paris.)]

The bundle of each leaf-trace is accompanied by an arc of secondary centrifugal xylem as it passes through the secondary wood and this is retained in the leaf except in the finer veins. After entering the petiole the leaf-trace branches and an arc of bundles is produced, the concave side facing the upper surface of the thick lamina (fig. 464, A). Further reference is made to the structure of the leaves in the description of _Poroxylon stephanense_. The specimens of roots so far described are characterised by a diarch plate of primary xylem and two masses of secondary vascular tissue separated by two medullary rays opposite the protoxylems. Bertrand mentions the occurrence of roots of _P. stephanense_ with more than two protoxylem strands. The phellogen was produced in the pericycle as in the roots of recent Gymnosperms. It is suggested by Lignier[540] that some silicified rootlets from Grand’ Croix (Loire) described by him as _Radiculites reticulatus_ and at first compared with roots of _Sequoia_ may belong to some Cordaitalean plant, possibly _Poroxylon_.

_Poroxylon Edwardsii_ Renault.

This species[541] affords a good illustration of the generic characters already summarised. The strap-like leaves are fleshy and the occasionally forked, parallel or slightly divergent, veins are embedded in a homogeneous mesophyll with hypodermal strands of mechanical tissue. The pith consists of parenchyma in vertical series with scattered secretory sacs and differs from that of _Cordaites_ and _Mesoxylon_ in the absence of transverse discs. There are 13 primary-xylem strands close to the inner edge of the secondary wood: the centripetal tracheids are scalariform or have multiseriate pitting like that in the secondary xylem. The structure of the leaf-traces is clearly shown in fig. 464: the double trace seen in fig. 464, C, has two protoxylem-strands accompanied by some parenchyma, and these are almost enveloped by the metaxylem tracheids which abut on the secondary wood. At this stage in its course, that is just before bending outwards, the centripetal xylem reaches its maximum development and the trace forms a prominent and broad twin-strand in striking contrast to the two narrower and tangentially extended strands shown in fig. 464, E, D. Each of these strands with a single protoxylem-group would at a higher level assume the broader and more compact form and contain two protoxylems as in fig. 464, C. The tracheids of the secondary xylem have 4–7 alternate rows of contiguous alternate pits on the radial walls: the medullary rays are 2–3 cells broad and may be 60 cells deep. According to Renault[542] several small oblique pits occur on the radial walls of the ray cells. The secondary phloem, separated by a normal cambium from the xylem, forms a broad band of sieve-tubes with lateral sieve-plates like those in _Medullosa anglica_ alternating with tangential rows of parenchyma. The cortex is relatively narrow and in older stems is chiefly occupied by secondary tissue formed from deep-seated phellogens.

[Illustration: Fig. 464. _Poroxylon_. A, B, _Poroxylon Boysseti_. A, transverse section of leaf in the region of lateral expansion of the petiole; B, portion of vascular tissue of A. C–E, _Poroxylon Edwardsii_. C, leaf-trace showing recent separation of the two protoxylems; D, leaf-trace at a lower level with more widely separated protoxylems, _px_; E, leaf-trace intermediate between C and D. (From photographs supplied by Prof. Bertrand.)]

_Poroxylon Boysseti_ Renault.

The stems of this species agree closely with those of _P. Edwardsii_, the chief difference being in the structure of the secondary phloem which does not show the regular concentric alternation of sieve-tubes and parenchyma.

_Poroxylon stephanense_ Bertrand and Renault.

This the oldest species, from Stephanian beds at Grand’ Croix, differs in no essential features from the other representatives of the genus. It is from a study of the leaves of this type that Bertrand and Renault have obtained most of the facts with regard to the anatomy of _Poroxylon_ foliage. In the median region of the fleshy leaf the bundles are characterised by a comparatively large amount of centripetal xylem accompanied by a considerable development of secondary centrifugal tracheids: the bundles are connected laterally by both centripetal and centrifugal xylem and thus at certain levels in the lamina the vascular tissue has the form of a continuous plate (fig. 464, A, B). The veins become independent on branching and near the edge of the lamina they consist only of primary elements. Secretory sacs of elongated form are scattered in the homogeneous mesophyll, and thick stereome-strands underlie the epidermis. The epidermal cells are rectangular and rows of stomata occur on both surfaces.

B. =CORDAITEAE=.

=Cordaites=. Unger.

A preliminary statement with regard to nomenclature may serve to remove possible misconceptions in connexion with the application of the generic name _Cordaites_. It has been the general practice to apply this name to certain forms of linear leaves which are particularly abundant in Carboniferous and Permian strata in Europe and North America, and in recent years a few palaeobotanists have substituted _Cordaites_ for _Noeggerathiopsis_ as the more suitable designation for Permo-Carboniferous specimens abundant in the rocks of Gondwana Land. It has been customary to assign to _Cordaites_ certain reproductive shoots, seeds, and stems described under the generic names _Cordaianthus_, _Cordaicarpus_, _Cordaicladus_, _Cordaioxylon_, etc. Stems agreeing anatomically in their main features with those of recent Araucarineae have long been attributed to _Cordaites_, but a few years ago a new type of stem was discovered which, though almost identical with that of _Cordaites_, is distinguished by the character of the primary xylem. For this new type the name _Mesoxylon_[543] was proposed. Nothing is known as to the reproductive organs borne on _Mesoxylon_ stems, but the leaves are externally at least indistinguishable from those referred to _Cordaites_. It is therefore obvious that when we apply the name _Cordaites_ to leaves or other plant-organs, under that designation are undoubtedly included specimens belonging both to _Mesoxylon_ stems and to stems with the characters of _Cordaites_ (_Cordaioxylon_). Further research may enable us to subdivide _Cordaites_ into more precisely defined types distinguished by well-marked morphological characters, but at present the only course would seem to be to restrict the term _Mesoxylon_ to petrified stems exhibiting the features of that genus and to retain _Cordaites_ as a comprehensive designation in accordance with the general account of the genus given in the following pages. This widely distributed and mainly Palaeozoic genus is especially well represented in the coalfields of France where in some localities it contributed largely to the formation of seams of coal[544], and it is chiefly from the researches of French Palaeobotanists that our knowledge of its morphology is derived. _Cordaites_ has shared the fate of most other abundant fossil plants in the distribution of its _disjuncta membra_ among several genera and classes, but on the whole the information that is now available enables us to reconstruct the complete plant with a greater degree of confidence than is usually attainable.

_Cordaites_ may be described as a forest-tree closely resembling in habit and probably in size the recent Conifer _Agathis_, more especially such species as _A. macrophyllus_, _A. vitiensis_ and others with leaves considerably longer than those of the Kauri Pine (_A. australis_)[545]. The main stem reached a considerable height before giving off scattered branches bearing spirally disposed, sessile, and often crowded leaves[546] like the foliage of _Agathis_. The absence of any evidence of a two-ranked arrangement of leaves on lateral branches suggests a general tendency towards a vertical rather than a horizontal direction of growth. The sessile and closely set leaves for the most part of leathery texture vary considerably in length and breadth in different types (figs. 466–472): in some the broadly linear lamina with its parallel veins and perfectly constructed I-shaped girders (fig. 465) reached a length of nearly 100 cm., in shape like the blade of a straight broad-sword or the leaves of a _Yucca_, torn by the wind into strips; in other forms the lamina is shorter and more obovate, while in some the leafy shoots must have looked like slender stems of the smaller-leaved Bamboos. There is no proof that young vegetative branches with their spirally rolled leaves[547] were protected by bud-scales, but some oval triangular scales (fig. 468, C), occasionally found in association with larger foliage-leaves, may have served that purpose. The branches from which leaves had recently fallen at the time of fossilisation are characterised by transversely elongated oval scars, occasionally showing a slightly curved row of pits like the marks of leaf-traces on the scars of a Horse Chestnut, sometimes terminating a feebly projecting decurrent leaf-cushion (fig. 466, C). The leaves persisted for a comparatively long period as in _Araucaria imbricata_, and on older leafless branches the scars are transversely stretched; the leaf-cushion loses its individuality and eventually the development of secondary cortical tissue causes the exfoliation of the superficial bark.

In the form and structure of the fertile shoots _Cordaites_ parts company with _Agathis_; the trees bore no cones in the ordinary sense, but unisexual inflorescences—whether on one plant or on different individuals is uncertain—were produced in the axils or from a supra-axillary position as compound spikes or compact racemes. Both the longer female shoots and the shorter and more compact male branches are constructed on a similar plan. The ovulate inflorescence may exceed 30 cm. in length (fig. 479); a stout axis bears two-ranked linear bracts subtending short lateral bud-like shoots with one or several sessile or stalked ovules (fig. 480) between the sterile scales. The seeds are platyspermic and agree much more closely with those of Cycads and _Gingko_ than with the seeds of Conifers. The male inflorescence is on a smaller scale, in habit not unlike the elongated male shoot of _Cephalotaxus pedunculata_ and some other Conifers; each bract subtends a small oval bud composed of imbricate scales and highly modified microsporophylls borne singly or in clusters (figs. 481, F; 482). A microsporophyll consists of a comparatively long pedicel bearing at its apex a few long microsporangia. The term microsporophyll implies a morphological interpretation which is not accepted by all palaeobotanists, some of whom prefer to regard the microsporangia as stamens or microsporophylls reduced to their simplest terms and sessile on an elongated flower-stalk.

The stem agrees very closely in its more important features with that of an _Araucaria_ or an _Agathis_: the primary xylem forms the inner surface of the thick cylinder of secondary wood, merging gradually into it as in recent Conifers; there are no separate bundles of primary centripetal xylem. The medullary rays are narrow: in other words the secondary xylem is of the pycnoxylic type. The pitting of the tracheids is Araucarian and, as in _Agathis_, the leaf-traces arise as twin-bundles. The pith is larger than in the Araucarineae and more homogeneous in structure; it shares with the pith of _Juglans_ and some other recent plants an almost constant tendency to assume a discoid structure. Anatomically the leaves agree more closely in the structure of the vascular bundles with Cycads than with Conifers though there are points of contact with both of these classes. The roots branch freely and their horizontally extended arms (figs. 468, A; 478) suggest growth in swampy ground; anatomically they conform to the recent Gymnospermous type and there is good evidence that in some cases fungal mycelia lived symbiotically in the cortex of coralline rootlets.

Sternberg[548] figured some leaves of _Cordaites_ from Carboniferous rocks in Bohemia under the generic name _Flabellaria_ in the belief that they belonged to a Palm. Brongniart substituted a new name _Pycnophyllum_[549] on the ground that Corda had disproved the supposed relationship with Monocotyledons. The name _Cordaites_ was instituted by Unger[550], his definition being based on leaf-form as well as on stem-anatomy. It has recently been proposed to revive the forgotten designation _Pycnophyllum_[551], but the reasons given are hardly likely to induce botanists to discard the familiar generic name which perpetuates the memory of Corda. As already pointed out, the name _Cordaites_, even though employed in what has always been regarded a legitimate sense, is no doubt often given to specimens of some other allied member of the Cordaitales which can only be recognised as such in the case of more completely preserved material. The naming of wood of the Cordaitean type, but which may equally well belong to another genus, raises a difficult question: if there is satisfactory evidence from collateral sources that the wood is that of a _Cordaites_ Grand’Eury’s name _Cordaixylon_[552] or Schenk’s form _Cordaioxylon_[553] may be used, though there seems to be no adequate reason against the use of the name _Cordaites_. If there is no confirmatory evidence available and it is impossible to say whether the wood is that of a Conifer or a _Cordaites_, or some other plant with the same type of secondary xylem, Endlicher’s term _Dadoxylon_[554] is most conveniently employed. The confusion liable to follow from the use of the two generic names _Dadoxylon_ and _Araucarioxylon_ for wood of the same type differing only in geological age is an argument in favour of extending _Dadoxylon_ to all specimens having certain anatomical characters, which cannot be certainly assigned either to the Araucarineae or the Cordaitales, irrespective of geological age. The term _Cordaicladus_ sometimes applied to branches is hardly necessary, but the subgeneric names _Eu-Cordaites_, _Dory-Cordaites_, and _Poa-Cordaites_, instituted by Grand’Eury for different forms of leaf, are frequently employed and serve a useful purpose as descriptive terms though the characters which they connote are of small importance and by no means always well defined or constant. For inflorescences it is customary to adopt the name _Cordaianthus_ suggested by Grand’Eury as a substitute for _Antholithus_ and some other terms. The same author uses _Rhizo-Cordaites_ for roots.

The nomenclature of seeds is more difficult: in a few instances seeds occur in organic connexion with Cordaitean shoots, but there is no doubt that many platyspermic Palaeozoic seeds preserved as detached fossils belong to _Cordaites_ or some other member of the group. The difficulty is that in the present state of knowledge we cannot definitely determine in many cases whether a seed is Cordaitean or whether it belonged to a genus of Pteridosperms. For this reason the account of several seeds that were probably borne on _Cordaites_ or some allied genus is given in a later chapter devoted to Gymnospermous seeds. There is no doubt that under the generic names _Cardiocarpus_, _Cordaicarpus_, and _Samaropsis_ are included true Cordaitean seeds, though it would be incorrect to say that all the seeds so named belong to members of the Cordaitales.

_Cordaites_ reached its maximum development in the Carboniferous and Permian periods; the genus or some closely allied types persisted into the Triassic and Rhaetic periods, and there is reason to believe that the group was represented in some post-Rhaetic floras. The genus is one of many remarkable examples of the high degree of specialisation attained by Palaeozoic plants. The complex mechanisms represented by _Cordaites_ and similar types give force to the conviction that we cannot hope to penetrate below the higher branches of the genealogical tree which had its roots in a period of the earth’s history inaccessible to botanical investigation. The plants of the present age are to a large extent the result of evolutionary tendencies more correctly described as the result of degeneration or simplification than as the latest phase in a series composed of a succession of types gradually growing in complexity. _Cordaites_ is essentially a generalised type, a composite product of an age characterised by an activity in the elaboration of the complex from the simple. Botanical records furnished by the geological series available for investigation furnish evidence of the sorting of characters among gradually diverging races and of changes in plant-organisation tending towards simplification and increased efficiency.

_Cordaites_, using the generic designation in a wide sense, occurs in Carboniferous and Permian strata in Europe, North America, and China; it is recorded from several localities in Russia and Siberia for the most part from Permian rocks, from Permo-Carboniferous (Lower Gondwana) beds in India, Australia, South Africa, and South America. Wood agreeing generally in the structure of its secondary tracheids with that of _Cordaites_ is represented in Devonian rocks, and there can be no doubt as to the existence of Cordaitalean plants in pre-Carboniferous floras. It is represented in the Rhaetic flora of Tonkin and has recently been discovered in strata probably of Rhaetic age in Mexico.

+Leaves.+

It is important to recognise the fact that leaves included under the generic name _Cordaites_ were in many cases not borne on stems or branches with the anatomical characters of _Cordaites_. Scott in his account of the genus _Mesoxylon_ says, ‘I feel no doubt that most of the British specimens of Cordaitean leaves really belong to _Mesoxylon_, which is a much commoner type of stem in the Coal Measure petrifications than that of _Cordaites_ itself[555].’ Some of the Cordaitean leaves were probably attached to stems of the _Poroxylon_ type[556] and it is not improbable that, as investigations are extended, additional genera of vegetative shoots will be discovered provided with leaves similar at least in external characters to those which it is customary to refer to _Cordaites_. In the present state of our knowledge we cannot make use of anatomical characters as criteria by which to distribute the foliage of the _Cordaites_ form among the genera _Cordaites_, _Mesoxylon_, and _Poroxylon_, using these names as designations of certain types of anatomical structure. The specimen reproduced in fig. 465 is in all probability a piece of a leaf of _Cordaites principalis_, but on anatomical grounds Miss Benson[557] has made it the type of a new species, _C. Felicis_, and more recently Scott[558] has brought forward evidence supporting the view that it is a leaf of _Mesoxylon_. As, therefore, neither impressions nor petrifications of Cordaitean leaves can in the great majority of cases be referred with confidence to their respective genera of stems, pending fuller information the only course would seem to be to use the name _Cordaites_ in a comprehensive sense indicating in special cases where evidence is available the more precise systematic position of the specimen. The classification of Cordaitean leaves proposed by Grand’Eury[559] is based partly on the form of the lamina and in part on the equality or inequality of the ‘veins.’ The actual veins, which are embedded in the fairly thick mesophyll, do not directly affect the superficial ribbing on the carbonised impression of the leaves and, as seen in fig. 465, the most prominent hypodermal strands of supporting tissue which would appear as the main veins or primary ribs on an impression do not correspond in position with the vascular bundles. Although in some cases the largest stereome-strands coincide with the veins, forming the upper and lower parts of I-shaped girders the centre of which is occupied by the veins, this is by no means always the case. Grand’Eury has drawn attention to the difference between the upper and lower surface of some carbonised leaves: in _C. crassifolius_ (fig. 468, D)[560] there are five to seven finer ribs between each pair of primary ribs on one face while the other shows ridges and grooves with a rib corresponding to each. Attention is called on a later page to the variable character of the ribbing even on different parts of the same lamina. The lower surface of the leaf, seen in section in fig. 465, would show a number of approximately equal ribs, or possibly primary ribs (midway between the veins) separated by two interstitial ribs, while on the upper face there would be three rather smaller secondary ribs. In a section of a leaf called by Renault _C. crassus_[561], a specific name used also by Lesquereux[562] for an impression of a leaf originally described by Goeppert as _Noeggerathia crassa_, there are deep stereome-strands between the veins next the lower epidermis alternating with single smaller strands, while on the upper surface the hypodermal strands occur only immediately above the veins. In a section figured by Felix[563] from North Germany as _C. robustus_, the hypodermal stereome forms continuous bands; on the upper face the bands are uniform in thickness but next the lower epidermis they form a series of ribs.

Grand’Eury’s subgeneric terms _Cordaites_, _Dory-Cordaites_, and _Poa-Cordaites_ have therefore very little value as regards differences in the ribbing of leaf-impressions: the large size of leaves included in _Dory-Cordaites_ and the more acute apex of the lamina as compared with the obtuse apex of smaller leaves of _Cordaites_ are features of limited application and of minor importance as diagnostic characters. The name _Poa-Cordaites_ is, however, usefully employed for the narrower linear leaves with an obtuse apex.

The structure of a _Cordaites_ leaf is clearly shown in fig. 465; the lamina is approximately 1 mm. thick and there are about 30 veins in a breadth of 2 cm. Strong I-shaped girders with the webbing composed of thick-walled cells divide the mesophyll into rectangular compartments: the intervening hypodermal strands differ in number and size on the two faces. The epidermis is not preserved: specimens of other leaves show that the stomata[564] occur in rows on the lower surface. The mesophyll shows no differentiation into palisade and spongy parenchyma, and in this respect the leaf agrees with many other forms; but in some leaves the palisade-tissue is well developed, as in _C. lingulatus_ Ren.[565] The central region of the lamina consisted of lacunar tissue, portions of which are preserved, with a more compact sheath of parenchyma enclosing each vein. In some leaves there is a narrower sheath of thick-walled cells more sharply contrasted with the mesophyll. The vascular bundles agree in structure with those in the rachis of a Cycadean frond more closely than with the veins of an Araucarian or other Coniferous leaf. The xylem consists mainly of centripetal elements which form a deltoid strand with the protoxylem at the apex, and in close association with this is a larger or smaller amount of narrower centrifugal tracheids: in the section shown in fig. 465 the centrifugal xylem may extend all round the centripetal tracheids, but it usually forms an irregular arch with its base attached to the sides of the larger tracheal strand, _cp_, separated, except at the base of the arch, by a small amount of conjunctive parenchyma from the centripetal xylem. The phloem is not preserved and is represented only by a few patches, _ph_, below the centrifugal tracheids. Dr Benson[566] in her account of this type of leaf gives additional details and compares the anatomical features with those in other species. The dual nature of the xylem like that characteristic of recent Cycads has usually been regarded as a definite feature of Cordaites leaves; Dr Stopes, on the other hand, interprets the narrower tracheids (occupying a position similar to those in fig. 465) in some sections of a leaf identified with _C. principalis_ from Grand’ Croix, as an inner sheath of transfusion elements (‘primitive transfusion tissue’) possibly derived from the centripetal xylem with which it is clearly connected at the sides precisely as in fig. 465; but in the Grand’ Croix leaf the phloem is enclosed within the sheath of narrower tracheids and not external to it as it is in the section shown in fig. 465 and in a section of _C. lingulatus_ figured by Dr Stopes. It is, however, difficult to recognise any fundamental difference between the ‘inner transfusion tissue’ and centripetal xylem. The cells of the outer sheath in Dr Stopes’s specimens of _C. principalis_ have bordered pits on their walls and this character is mentioned also by Renault in other specimens.

[Illustration: Fig. 465. _Cordaites_ leaf (_Cordaites Felicis_ Bens.), probably borne on a _Mesoxylon_ stem. _cp_, centripetal xylem; _ph_, phloem; _px_, protoxylem. (Kidston Coll. No. 2194.)]

Prof. Lignier[567] has described the structure of fragments of adult leaves from the Stephanian of Grand’ Croix (Loire) which he refers to _Cordaites lingulatus_, and the same author gives an interesting account of the anatomical features of a bud of the same species. The bud, which resembles in general appearance that of _Dolerophyllum_ (fig. 430, p. 133) is 3 cm. long, oval in transverse section—as the result of compression—and consists of four convolute leaves and a piece of a fifth. The outer leaves have 75 to 80 veins: the inner laminae are sinistral in their curvature while the three outer leaves are dextral. In the second, the first in which the tissues are recognisable, the small desmogen-strands afford some evidence that the phloem preceded the xylem in the order of differentiation as is often the case in recent plants. The first tracheids occur almost in the centre of the desmogen-strand and to these are added the other tracheids of the centripetal xylem, the oldest elements being spiral, the next scalariform and the later tracheids reticulate. The centrifugal xylem is formed at a later stage, and at about the same time are differentiated the elements called by Dr Stopes the inner sheath and by Lignier the ‘bois diaphragmatique.’ Lignier also describes the development and structural features of the other tissues of the young leaves and compares the anatomical features of the French leaves with those of _Cordaites Felicis_ described by Prof. Benson.

The main features of _Cordaites_ leaves are (i) the presence of two kinds of xylem in the veins, the larger centripetal tracheids, or chief water-conducting elements, and the narrower tracheids, in some cases attached to the sides of the centripetal xylem, in others forming free groups, usually between the protoxylem and the phloem, but sometimes enclosing the phloem; (ii) the frequent presence of a well-defined sheath of cells round each vein composed of comparatively thick-walled elements comparable with the transfusion-tracheids in Conifers; (iii) the presence of lacunar tissue in the centre of the mesophyll and in some cases of transversely extended tracheids similar to those in some Podocarp leaves; (iv) a well-developed system of stereome-strands and I-shaped girders. The structural features on the whole suggest a xerophilous type, and the frequent absence or feeble development of palisade tissue points to diffused rather than to brilliant sunlight.

The considerable range in size and form among Cordaitean leaves as well as the obvious dependence on conditions of preservation or growth of such a relatively unimportant feature as the presence or absence of the so-called false or interstitial veins—the variability of which has been demonstrated in several instances—renders specific determination exceedingly difficult. The following species are briefly described rather with a view to illustrate the nature of the characters employed by authors than as implying the existence of so many well-defined types.

_Cordaites principalis_ (Germar).

This species was founded[568] on a large specimen from the Coal Measures of Wettin showing a fan-like cluster of longitudinally torn and partially overlapping leaves spread out in the position that would be assumed on the compression of a shoot with a close spiral phyllotaxis. This form of _Cordaites_ is the most abundant in the British Coal Measures. The broadly linear lamina is characterised by an obtuse apex (fig. 466, A), a tendency to split into strips, close-set parallel ribs, the stouter ribs or veins separated from one another by 2–3 or it may be as many as 5 finer ribs or interstitial ‘veins.’ A statement by Weiss that in Germar’s type-specimen the longitudinal ribbing of the lamina is very imperfectly preserved confirms the scepticism that is justly felt as to the validity of this character as a satisfactory specific criterion.

[Illustration: Fig. 466. A. _Cordaites principalis_, part of leaf. _B. Artisia_ _transversa_, pith-cast. C. _Cordaites principalis_, branch with leaf-scars. (A, ⅖ nat. size; B, C, nat. size; Kidston Coll.)]

The incomplete example shown in fig. 466, A, is 19·5 cm. long and has a maximum breadth of 3·5 cm., but the complete leaf was much larger and tapered gradually to the comparatively broad and slightly concave or amplexicaul base. The narrow elliptical proximal end of a specimen figured by Kidston[569] from the Middle Coal Measures of Yorkshire is 2·8 cm. broad indicating that the tangentially expanded leaf-scars on a branch recently deprived of its foliage must have been a conspicuous feature. In his synonymy of this species Kidston[570] includes _Knorria taxina_, a species founded by Lindley and Hutton[571] on a piece of stem from the Coal Measures of Newcastle. The type-specimen, as Mr Howse[572] states, is much larger than the published drawing and closely resembles in the decurrent leaf-bases with broad apices the piece of stem represented in fig. 466, C, which Kidston identifies as _C. principalis_. Geinitz[573] refers to this species the seeds named _Cordaicarpus Cordai_ (Gein.), but there is no evidence of connexion. Kidston[574] points out that this seed is rare in Britain: he believes that _Cordaianthus Pitcairniae_ (Lind. and Hutt.) is probably the inflorescence of _C. principalis_.

_Cordaites principalis_ occurs in both Carboniferous and Permian strata. The leaves described by Lesquereux[575] from Pennsylvania as _C. Mansfieldi_ agree closely with _C. principalis_. Another similar or possibly identical form is represented by _C. Ottonis_ Gein.[576]

_Cordaites borassifolius_ (Sternberg).

The leaves of this species, originally referred to _Flabellaria_[577], resemble those of _C. principalis_ but differ in the ovate-lanceolate and less obtuse apex and in the presence of only one or rarely two finer striations between the stronger ribs. Corda’s drawing[578] affords a good illustration of the crowded spiral disposition of the foliage comparable with that on an _Agathis_ shoot. The lamina is usually 4–8 cm. broad but in exceptional cases may reach a breadth of 12 cm. The species occurs in the Coal Measures, especially in the Westphalian series and in Permian rocks. Feistmantel[579] unites with this type _Cordaianthus Pitcairniae_ (fig. 480, A), but as in other cases there may be a confusion between _C. borassifolius_ and _C. principalis_. Leaves described by Lesquereux as _C. communis_[580] are, as White says, not distinguished by any well-marked characters from this species. White[581] figures some good examples of _C. borassifolius_ from Missouri, reaching in one case a length of 40 cm., showing on the lamina the fructifications of a fungus, _Hysterites cordaitis_[582] Grand’Eury. The leaves described from Canada and the United States as _C. Robbii_[583] Daws. are closely allied to if not identical with Sternberg’s type. Among other species differing in no definite character from _C. borassifolius_ is _C. lancifolius_ described by Schmalhausen[584] from the Permian of Russia.

_Cordaites lingulatus_ Grand’Eury.

The leaves of this species[585] are characterised by the obovate lamina and bluntly rounded or almost truncate apex; it affords a good illustration of the uncertainty of the ribbing as a diagnostic character. The lamina of a well-preserved specimen from the Blanzy coalfield described by Zeiller[586] reaches a length of 35 cm. and a breadth of 10–11 cm. decreasing to 4 cm. at the base (fig. 467). In the lower part of the lamina Zeiller describes the ribs as unequal in prominence, the stronger ones being separated by 1–3 finer ribs, while in the middle and upper portions the ribs appear to be of equal size. Some of the finer ribs are due to folding of the lamina and are not represented, as are the ribs due to the presence of stereome-strands, by dark streaks in the detached cuticle.

[Illustration: Fig. 467. _Cordaites lingulatus._ (After Zeiller; ⅚ nat. size.)]

Reference has already been made to the anatomical features of leaves of this species described by Lignier[587] and other authors.

_Cordaites grandifolius_ Lesquereux.

The leaves so named by Lesquereux[588], from the Coal Measures of Pennsylvania, are distinguished by the elongate cuneate lamina, which reaches a length of 38 cm. with a narrow base and a slightly rounded truncate distal end, 16 cm. broad, characterised by a few broad and shallow crenulations. By contrast with some American specimens in Dr Kidston’s collection Lesquereux’s figures convey an imperfect idea of the size of the leaf. A large leaf from the Coal Measures of Belgium described by Cambier and Renier as a new species of _Psygmophyllum_, _P. Delvali_[589], is perhaps identical with the American type; the lamina of sub-triangular form has approximately the same dimensions; the veins are numerous and repeatedly forked. The leaf is much longer than any known _Psygmophyllum_ and the veins are much more numerous than in _P. majus_ Arb.[590], the largest representative of that genus. Palaeobotanists who have seen the type-specimen inform me that they have no doubt as to the Cordaitean nature of the Belgian specimens, which may be designated _Cordaites Delvali_. It is, however, not impossible that _Psygmophyllum_ and _Cordaites_ are allied genera: our knowledge of the former is limited to unimportant characters.

_Cordaites_ (_Dory-Cordaites_) _palmaeformis_ (Goeppert).

This Permian type, originally described by Goeppert[591] as _Noeggerathia palmaeformis_, is characterised by numerous slender veins: according to Weiss[592] there may be as many as 3–5 in 1 mm. The leaf is broadly lanceolate; it tapers gradually to an acute apex reaching a length of 80 cm. and a breadth of 10 cm. In habit the young foliage-shoots[593] resemble those of _C. principalis_ and _C. borassifolius_. Grand’Eury records the frequent association of _Samaropsis_ seeds with this species; it occurs in Upper Carboniferous and in Permian strata and is recorded from a few British localities.

_Cordaites_ (_Poa-Cordaites_) _microstachys_ Goldenberg.

Weiss[594] first figured this species from drawings supplied by Goldenberg at whose suggestion the name _C. microstachys_ was adopted. The type-specimen consists of a slender axis bearing numerous narrow linear leaves and a few imperfect fertile axillary shoots. A specimen is figured by Kidston[595] from the Upper Coal Measures of Radstock: it is a rare type in Britain. The species is readily distinguished from _C. principalis_ and similar forms by the narrow lamina which varies considerably in length, rarely as long as 30 cm. and not exceeding 1 cm. in breadth. The apex is obtuse and the ribs are either equal in strength or 1–2 finer striae may alternate with the stronger ribs. The base of the lamina is 3–4 mm. wide and the leaf-scars have a slightly arched upper margin and an almost straight lower edge[596]. The foliage of this species, generally regarded as identical with _C. linearis_ Grand’Eury, bears a close resemblance to that of the Mesozoic genus _Phoenicopsis_ from which it is distinguished by the occurrence of the leaves in bunches.

_C. gracilis_ Lesq.[597] is a similar type. The shoot on which Lesquereux founded his genus _Desmiophyllum_[598] may perhaps be an example of _Poa-Cordaites_. _Poa-Cordaites tenuifolius_ Schmal.[599] from the Permian of Russia may be identical with _C. microstachys_.

As examples of other forms of leaf referred to _Cordaites_, though as in other cases without any proof of connexion with branches having the anatomical features of the genus, reference may be made to _Cordaites circularis_ Grand’Eury[600] from Gard (fig. 468, B) and a smaller leaf from the same locality compared with _C. Lacoei_ (fig. 468, C) Lesq. _Cordaites circularis_ is characterised by the almost orbicular lamina traversed by slightly spreading veins; it recalls some of the larger _Cyclopteris_ pinnules of Pteridosperm fronds and is indistinguishable from some leaves assigned to the genus _Dolerophyllum_[601].

[Illustration: Fig. 468. A, _Cordaites_ root-system (_Rhizo-Cordaites_); B, _Cordaites circularis_ leaf; C, _Cordaites_ sp., cf. _Cordaites Lacoei_; D, _Cordaites crassifolius_, upper and lower surface of leaf. (After Grand’Eury.)]

The species _C. Lacoei_ was founded by Lesquereux[602] on some detached specimens 3–12 cm. long and 1·5–5 cm. broad; it is by no means certain that a specimen referred by Grand’Eury[603] with some hesitation to this species is Cordaitean.

The generic name _Scuto-Cordaites_ was proposed by Renault[604] for a specimen from Commentry consisting of a flattened branch bearing a few imperfectly preserved leaves. The surface of the branch shows semicircular leaf-scars on decurrent, spirally disposed leaf-cushions and bears a certain resemblance to a slender stem of a Clathrarian _Sigillaria_. The leaves of the type-specimen of _Scuto-Cordaites Grand’Euryi_ appear to be broadly linear, 13 cm. long, the breadth gradually increasing from the base: a short distance from the proximal end the lamina is broken up into narrow segments; the veins are ·5 mm. apart with finer striations between them.

Some specimens from Pennsylvania made by Dawson[605] the type of a new sub-genus and named _Dictyo-Cordaites Lecoi_ agree in shape and arrangement with some species of _Cordaites_, but differ in an occasional anastomosis of the veins as in _Psygmophyllum flabellatum_. It is, however, impossible to determine the true nature of the fossils from the published figures.

+Cordaitean leaves from India, the Southern Hemisphere, and Siberia. _Noeggerathiopsis_, Feistmantel; _Rhiptozamites_, Schmalhausen; _Euryphyllum_, Feistmantel.+

{_Cordaites aequalis_ Goeppert. {_Cordaites_ (_Noeggerathiopsis_) _Hislopi_ (Bunbury).

In 1845 Goeppert[606] instituted the species _Noeggerathia aequalis_ (fig. 469) and _N. distans_ for incomplete broadly linear and obovate leaves, from Siberian Permian strata, having a contracted base and equal parallel veins. The specimens so named are no doubt specifically identical. Goeppert’s species _N. aequalis_ has recently been carefully investigated by Zalessky[607] who agrees with Kosmovsky[608] in identifying it with _Noeggerathiopsis Hislopi_ (Bunb.) and _Rhiptozamites Goepperti_ Schmal. Schmalhausen[609] had previously pointed out the probable identity of his species with _Noeggerathia palmaeformis_ Goepp. (= _Cordaites_). The question of specific identity of these leaves from different localities and of other hardly distinguishable forms is of secondary importance; the main point is that they are all examples of Cordaitean leaves, _Cordaites_ or some allied genus, and point to the existence of this group of Gymnosperms during Permo-Carboniferous times in Siberia, China, India, Australia, South Africa, and S. America, also in the Rhaetic floras of Tonkin[610] and Mexico[611]. The fragments from Devonian strata at Iguana Creek, Australia, named by McCoy[612] _Cordaites australis_ are probably pieces of the rachis of some large frond.

[Illustration: Fig. 469. A–C, E, _Cordaites aequalis_; D, _Cordaites Clerci_. (After Zalessky; ⅚ nat. size.)]

Wieland[613] recently discovered Cordaitean leaves exhibiting a wide range in size and shape in the Mixteca flora of Mexico in the lower members of a series which extends from the ‘upper borders of the Rhaetic’ through the Liassic to the lower beds of the Inferior Oolite. These leaves are referred to _Noeggerathiopsis Hislopi_, and it is clear from an examination of photographs received from Dr Wieland, one of which is reproduced in fig. 470, that the Mexican _Cordaites_ cannot be specifically distinguished from Bunbury’s type as represented by specimens described from India, South Africa, Siberia, Tonkin, and elsewhere.

The occurrence of _Noeggerathiopsis_ is also recorded by Newberry from the Rhaetic series of Honduras[614].

_Noeggerathiopsis._ This genus was founded by Feistmantel[615] for some leaves from Lower Gondwana rocks in India originally described by Bunbury[616] as _Noeggerathia (Cyclopteris?) Hislopi_ (figs. 470–472) and regarded by him as probably Cycadean. Several authors have added to our knowledge of this widely spread southern type and in many localities the leaves occur in association with platyspermic seeds of the _Samaropsis_ or _Cordaicarpus_ type, pieces of stems with Cordaitean leaf-scars, and petrified wood agreeing in the structure of the secondary xylem with that of European species of _Cordaites_. In some Permo-Carboniferous sandstones at Vereeniging, South Africa, stumps and spreading roots (fig. 478) resembling those described from France by Grand’Eury (cf. fig. 468, A) have also been discovered. A remarkable occurrence of roots and prostrate stems of some forest-tree was recorded some years ago in the bed of the Vaal river near Vereeniging where the surface of a seam of coal was exposed over an area of more than two acres[617]. Large branched roots (fig. 478) spreading over the coal for a distance of several feet and thick stems 40–50 ft in length with very few branches and but little decrease in diameter afford a striking picture of a forest-floor. The frequent occurrence of _Cordaites (Noeggerathiopsis) Hislopi_ in the associated strata suggests a reference of the stems and roots to that species. Moreover the structure of the secondary xylem of some petrified pieces of stem sent to me by Mr Leslie from Vereeniging agrees closely with that of a European Cordaitean stem.

[Illustration: Fig. 470. _Cordaites (Noeggerathiopsis) Hislopi_. From Mexico. (After Wieland; ⅚ nat. size.)]

[Illustration: Fig. 471. _Cordaites (Noeggerathiopsis) Hislopi_. From Vereeniging. (A, _ca_, ⅕ nat. size; B, _ca_, ⅙ nat. size.)]

[Illustration: Fig. 472. _Cordaites (Noeggerathiopsis) Hislopi_. From India. (Calcutta Museum; ⅔ nat. size.)]

The leaves of _Cordaites (Noeggerathiopsis) Hislopi_ vary considerably in size, in some cases reaching a length of 80 cm. (fig. 471); the lamina tapers gradually from a short distance behind the obtuse apex to a relatively narrow base: in venation and form the leaves are very similar to those of _C. principalis_ and other European and North American species. The specimen from India represented in fig. 472 shows several spathulate leaves attached in a close spiral to a branch. As White[618] and Zalessky have shown, the stronger ribs are separated by less prominent striations indicating the presence of two sizes of hypodermal strands. The obvious resemblance between _Noeggerathiopsis Hislopi_ and species of _Cordaites_ has long been recognised and many authors have included Feistmantel’s genus in the Cordaitales[619]. Prof. Zeiller[620] preferred to retain the name _Noeggerathiopsis_ as a precautionary measure, chiefly on the ground that the stomata appeared to be less definitely arranged in rows and more scattered than in the European leaves of _Cordaites_, and because of the absence of interstitial veins. We have as yet little information as to the arrangement of the stomata, but in view of the irregularity in stomatal grouping in recent leaves this feature is, perhaps, of minor importance. The presence of interstitial ‘veins’ has now been established in Indian[621] and South American[622] leaves. In a paper published in 1908[623] the name _Cordaites_ was substituted for _Noeggerathiopsis_ and Zalessky’s recent work supports this step. The description by Zalessky of the ribbing in Goeppert’s species _Cordaites aequalis_ from Siberia shows how uncertain and variable a character the venation is even in different parts of the same leaf.

_Cordaites Clerci_ Zalessky.

This species (fig. 469, D) was instituted for some small lanceolate or spathulate leaves from the Petschora basin (Adzva River)[624] reaching a length of 6 cm. and a breadth of 1 cm. It is separated from _Cordaites aequalis_ on the ground that the veins are more numerous, as many as 44 in a breadth of 1 cm.

A recent investigation by Miss Holden[625] of the carbonised cuticles of some Indian specimens, sent to Cambridge by the Director of the Indian Geological Survey, and a comparison of them with preparations made from European _Cordaites_ leaves, have revealed certain distinguishing features which support Zeiller’s view that the Gondwana-Land leaves, though similar superficially to those of _Cordaites_, are probably distinct. It is, however, impossible in many cases to obtain any information with regard to epidermal characters, and though it would seem probable that had we a fuller knowledge of the Indian and southern hemisphere plants represented for the most part by leaf-impressions well-defined distinguishing features would be recognised, the comprehensive name _Cordaites_ may conveniently be retained on the ground that in the absence of well-preserved cuticles no satisfactory distinguishing features are exhibited by the impressions of _Noeggerathiopsis_.

=Phylladoderma.= Zalessky.

_Phylladoderma Arberi_ Zalessky.

Zalessky[626] founded this genus on some Permian leaves from the Petschora basin (Adzva River) which closely resemble those of _Cordaites_ but are characterised by a coarser venation. The lanceolate lamina reaches a length of 18 cm. and a breadth of 4·2 cm.; the veins are 2 mm. apart and occasionally forked near the base of the leaf. The epidermal cells have straight walls and stomata are abundant on the lower surface. As Zalessky says, the systematic position of the leaves is uncertain though they are probably Cordaitean. The coarseness of the venation is a feature of minor importance and hardly worthy of generic recognition.

_Rhiptozamites_ Schmalhausen.

This genus was instituted by Schmalhausen[627] for leaves from beds in the Kusnezk basin regarded by him as Jurassic. These strata are now recognised as Permian[628] and homotaxial with those from which Schmalhausen[629] subsequently recorded the same species. The leaves, though smaller than many of the Indian and South African specimens of _Cordaites_ (_Noeggerathiopsis_) _Hislopi_, may belong to that species. Zeiller and others definitely assigned the Russian leaves to _Cordaites_.

_Euryphyllum._ The Indian leaves for which Feistmantel[630] proposed this name are, as several writers have pointed out, in all probability referable to _Cordaites_.

The general conclusion to be drawn from this imperfect summary of an extensive literature is that the employment of the generic names _Noeggerathiopsis_, _Rhiptozamites_, _Euryphyllum_, and others has tended to exaggerate the difference between the European and Southern botanical provinces during the Permo-Carboniferous period.

_Scale-leaves, seeds, and stems._

The occurrence of small scale-like leaves of the type represented in fig. 468, C, in association with _Cordaites_ (_Noeggerathiopsis_) _Hislopi_[631] in India, Brazil, Siberia, and elsewhere may mean that these organs are scales of large foliar buds. The occurrence of several forms of platyspermic seeds, in some cases apparently identical with European forms and sometimes distinct types, in close association with _Cordaites_ (_Noeggerathiopsis_) _Hislopi_ has already been mentioned. Examples of such seeds are described in Chapter +xxxv.+ under the genus _Samaropsis_.

There are very few satisfactory examples of Cordaitean branches from the southern hemisphere. Schmalhausen[632] figures good specimens from Siberian rocks from which his _Rhiptozamites_ leaves were obtained. Branches with spirally disposed leaf-scars figured by Zeiller[633] from the Rhaetic of Tonkin closely resemble _Cordaicladus_. Feistmantel’s drawing of a fossil from the Karharbari series, compared by him with a Fern rhizome[634], may be a Cordaitean branch, and the same author describes a stem[635] from New South Wales as _Caulopteris Adamsi_ which bears a close resemblance to a branch of _Cordaites_. Similarly a leafy shoot described from India by Zeiller as _Araucarites Oldhami_[636] may be compared with branches of the _Poa-Cordaites_ type.

[Illustration: Fig. 473. _Cordaites_ (or _Mesoxylon_?) stem showing the discoid pith partially enclosed by wood. (½ nat. size.) M. S.]

+Stems.+ i. _Pith-casts._

=Artisia.= Sternberg.

A character to which authors tend to attach excessive importance as a diagnostic feature is the almost invariable tendency of the parenchymatous pith of _Cordaites_ to break up on contraction into transverse diaphragms, thus producing what is known as a discoid pith. In the stem shown in fig. 473 the pith is represented by a more or less cylindrical cast characterised by fairly regular transverse ribs and narrow grooves; in the upper part of the fossil the peripheral tissue of the pith is preserved in the form of narrow plates projecting from the inner face of the wood. As Renault[637] pointed out, this type of pith is the expression of certain conditions of growth and is not a satisfactory distinguishing feature of any particular genus or family. The same tendency to form a discoid pith is characteristic of _Mesoxylon_, and it occurs also in some other Palaeozoic genera. Corda long ago figured a stem attributed by him to _Lomatofloyos_ with a typical discoid pith, and a similar pith is recorded in a stem of _Dicranophyllum_[638]. Among recent plants _Juglans regia_ affords perhaps the most familiar instance of an identical form of pith: the same type occurs in the white Jasmine, in _Ceropegia peltata_, and some other flowering plants. An interesting case is that of the tree Groundsel, _Senecio praecox_ D.C.[639], of Mexico: in this plant, which grows in arid districts, the pith serves as a water-store and as the water is drawn off the thick turgescent discs contract and form thin transverse diaphragms separated by wide spaces, as is also the case on drying in some succulent _Euphorbia_ stems. It may be that in _Cordaites_ the medullary region also served as a water-reservoir and the depth of the medullary discs would vary according to the state of their contents.

The earlier writers regarded the pith-casts as stems with scars of amplexicaul leaves: Artis[640] described specimens from the English Coal Measures as _Sternbergia_, one of which he stated to be 6 ft long; a few years later Sternberg[641] proposed the name _Artisia_ and this has been generally used on the ground that _Sternbergia_ is the name of a recent flowering plant. A specimen of _Artisia transversa_ (Art.) from the Coal Measures of Yorkshire is shown in fig. 466, B, and similar specimens varying considerably in diameter up to about 10 cm. are abundant in European and American Coal Measures. The prominence and depth of the transverse ridges, the presence or absence of anastomoses between adjacent discs are, as Zeiller[642] says, of very doubtful value as specific characters. Dawson in 1846[643] spoke of _Artisia_ as probably the pith of a tree, a view suggested to him by Mr Dawes. In 1851 Williamson[644] published a description of some specimens in which a pith-cast, _Artisia approximata_ Lind. and Hutt., was enclosed by wood showing very clearly Cordaitean characters. Further demonstration of the true nature of _Artisia_ was supplied by Grand’Eury from St Étienne material. If the generic name _Artisia_ is applied to all pith-casts showing the transverse ridges and grooves like those seen in fig. 466, B, it must be remembered that it is not safe to assume a connexion with _Cordaites_ or _Mesoxylon_. A Liassic species described by Lignier[645] from France as _Artisia alternans_ is quoted by authors as evidence of the persistence of _Cordaites_ into the Jurassic period; but in view of the fact that the discoid type of pith is not by any means confined to _Cordaites_ or even to the Cordaitales the occurrence of _Artisia_ is in itself of no great botanical significance.

It is also true that a discoid pith is not an invariable attribute of stems closely allied to the genus _Cordaites_; but if these reservations are made the use of the generic term _Artisia_ serves a useful purpose.

ii. _Petrified stems._

=Dadoxylon.= Endlicher.

Palaeobotanical literature contains numerous descriptions of Palaeozoic petrified wood occasionally enclosing an _Artisia_ pith-cast described under such names as _Dadoxylon_, _Cordaioxylon_, _Araucarioxylon_, etc., and regarded as portions of Cordaitean stems. It is, however, certain that much of this material belonged to stems other than those of _Cordaites_. Recent research has demonstrated the insufficiency of the secondary xylem alone, however well preserved, as a safe guide to generic position: stems identical in the structure of the secondary xylem differ in that of the primary portion of the stele, and it is on the characters of the latter tissues that several genera have recently been founded. _Mesoxylon_ affords a striking example of the importance of the primary xylem as a distinctive feature. As Gothan[646] points out, the species of _Calamopitys_ recently made the type of a new genus _Eristophyton_[647] would, in the absence of the primary xylem, probably be regarded as Cordaitean. It is important to recognise the limitations imposed by the imperfection of the material; we cannot in most cases determine whether a specimen should be referred to _Cordaites_ or _Mesoxylon_, and while it may be described as probably Cordaitean in affinity there remains the possibility that some of the Palaeozoic plants with secondary wood like that of _Cordaites_, if their reproductive organs were known, would not be included in the Cordaitales. Goeppert’s species _Araucarites Tchihatcheffi_, which Renault[648] quotes as _Cordaites_, has recently been assigned to a new genus _Mesopitys_[649] because of certain distinctive features of the primary xylem. Additional examples might be quoted pointing to the tendency of recent and more thorough investigation to establish the fact that the occurrence of Permo-Carboniferous wood of the Araucarian type does not necessarily denote the existence of _Cordaites_. The question of nomenclature is necessarily raised in this connexion.

In recent years it has been customary to assign Palaeozoic wood with Araucarian pitting to the genus _Dadoxylon_, while wood of the same general type from more recent strata is by many authors referred to _Araucarioxylon_[650]. This arbitrary distinction based on a difference in age is open to serious objection. Fossil wood of the Araucarian type is widely scattered in strata ranging from Carboniferous to Jurassic periods; it also occurs in later formations. The fact that on the one hand Araucarian plants, as recognised by cones and foliage-shoots, are especially characteristic of Jurassic floras and occur more rarely in Rhaetic and Triassic floras, and on the other hand that _Cordaites_ and its allies reached their greatest development in Permo-Carboniferous times, renders it probable that in the majority of cases a distinctive name based on geological age would be in accordance with botanical differences. But we have no satisfactory data as to the upper limits of the Cordaiteae or the lower limits of the Araucarineae: in all probability the two families overlapped and co-existed for more than one geological period. It is, moreover, the plants from formations where overlapping occurred that are the most critical from a botanical standpoint. The age-distinction is therefore at best an artificial one and may be seriously misleading. Potonié[651] and Gothan[652] have emphasised the desirability of adopting the name _Dadoxylon_ for all wood of the Araucarian type irrespective of age. If a particular specimen can be correlated definitely with _Cordaites_ or some other genus it should be so designated, but the fragmentary nature of the records usually precludes this simple course. The most logical plan is to use the name _Dadoxylon_ for all woods with Araucarian characters if there is no sufficient reason for employing a less provisional term. If the evidence clearly points to the Araucarineae the generic name _Araucarioxylon_ should be added in parentheses after _Dadoxylon_, but whether or not this is done, a statement as to the geological age of the fossil will in itself be some assistance in enabling the student to form an opinion on the balance of probability in favour of a Cordaitean or an Araucarian affinity. The course suggested by Gothan[653], namely to add _Cordaites_ after _Dadoxylon_ if an _Artisia_ pith is present, is rendered inoperative now that we know that a discoid pith occurs in more than one genus. In this chapter we are concerned primarily with _Cordaites_ and with such stems as may fairly be regarded as Cordaitean: examples of fossil wood from later formations are dealt with in another place. A distinction between _Araucarioxylon_ and _Cordaioxylon_ stems has been based by Felix on the nature of the pith-casts; those of the _Artisia_ type he refers to _Cordaioxylon_, while Palaeozoic stems with _Tylodendron_ pith-casts are assigned to _Araucarioxylon_[654]. This distinction can, however, only be made in the comparatively few cases in which the pith-cast is preserved. Its validity is, moreover, open to question. A _Tylodendron_ (= _Schizodendron_) cast shows on its surface the characters of the inner face of the secondary xylem, projecting spindle-shaped areas representing the inner ends of medullary rays and a reticulum of grooves formed by the more resistant and prominent inner edges of the rows of tracheids (fig. 746). A pith-cast of a stem in which the destruction by decay of the medullary parenchyma had not extended to the edge of the xylem-cylinder might show transverse diaphragms. The occurrence of _Tylodendron_ casts means that decay had extended to the surface of the wood. But in view of the occurrence of _Tylodendron_ casts in stems that are not those of _Cordaites_ a short account of the genus is given on another page[655].

The main features of the stem of _Cordaites_ have already been enumerated. The stele agrees with that of _Araucaria_ and _Agathis_ and especially with _Agathis_ in the double nature of the leaf-trace. Williamson[656] in 1877 described pieces of wood from the English Coal Measures and the Lower Carboniferous of Scotland which he referred to _Dadoxylon_ but without any specific name. These include the Coalbrookdale stem in which he had previously demonstrated the connexion between _Artisia_ and _Dadoxylon_. The structure of the xylem is like that in _D. Brandlingii_ and the specimens may belong to that species. The most interesting fact recorded by Williamson is the occurrence of double leaf-traces, a feature which led him to suspect a remote generic affinity to _Ginkgo_. This double trace may be an important diagnostic feature but unfortunately the majority of descriptions of species of _Dadoxylon_ throw no light on the character of the foliar bundles.

Thomson and Allin[657] have recently pointed out that a double leaf-trace occurs in a stem from the Permian of Kansas described by Penhallow[658] as _Pityoxylon chasense_ and referred to that genus because of the supposed occurrence of resin-canals in some of the medullary rays: the canals are apparently leaf-traces traversing broad rays in the secondary wood.

The primary xylem of _Cordaites_ is in direct continuity with the secondary tracheids and does not form mesarch strands as in _Mesoxylon_. The pith is usually discoid. The pitting on the tracheids is a character of special importance: while it is true to say that as a rule the number of pits on the radial walls of a single tracheid is larger than in the Araucarineae, this is not always the case. In _Araucaria_ there are occasionally as many as five rows of alternate polygonal pits (fig. 691, A) and in some Palaeozoic Dadoxylons there are only one[659] or two rows. The very broad zone of transitional elements at the inner edge of the xylem-cylinder is a characteristic feature shared by the Araucarineae[660]; the spiral protoxylem-tracheids are succeeded by scalariform elements and these, by the gradual anastomosing of the transverse bars, pass into tracheids with multiseriate pitting. In this broad zone we probably have a primitive feature, an epitome in a single stem of the course of development of multiseriate from scalariform pitting. In some Palaeozoic species with wood of the pycnoxylic type and agreeing generally with typical _Cordaites_ the bordered pits are sometimes separate and circular, and opposite pits occasionally replace the usual alternate arrangement. Another feature on which stress has been laid is that in _Cordaites_ the pits occupy the whole breadth of the tracheal wall; but this, though frequently the case, is by no means a constant feature. In _Dadoxylon Newberryi_[661] the pits tend to form groups, leaving unpitted areas, as in the genus _Coenoxylon_[662]. In the stem of _Dadoxylon materiarum_ Daws. represented in fig. 475 the pits do not always cover the whole of the tracheid-walls: this stem is also instructive as an example of the different appearance presented by pitted tracheids according to the state of preservation. In some places an oblique pore is well shown while in others only the outer border of the pit is seen. Gothan[663] has described a specimen in which some of the pits are circular and occupy only the central area of the xylem elements: separate circular pits occur also in _D. Pedroi_ Zeill.[664] (fig. 476). Similar departures from the normal are illustrated by recent species of Araucarineae. The absence of a torus is another feature shared by _Dadoxylon_ and true Araucarian wood. Annual rings other than incomplete and spasmodically formed rows of narrower tracheids are not as a rule present, and in this respect also _Araucaria_ affords a close analogy. Thomson[665] has figured a transverse section of a root from English Coal Measures in which rings of growth are well defined; and other instances are recorded. In an Australian species named by Arber _D. australe_[666], there are well-marked rings of growth, and this is equally the case in some Indian wood[667] of Permo-Carboniferous age, more nearly allied to _Mesoxylon_ than to _Cordaites_, and in a _Dadoxylon_ of similar age from South Africa. On the other hand the statement that annual rings occur in Palaeozoic wood is often incorrect, partial rings having been confused with regular concentric cylinders of summer elements. Dawson and Matthew[668] described rings in _D. ouangondianum_, and Goeppert and Stenzel[669], who examined the Canadian material, refer to circles like annual rings; but Penhallow[670] states that there is no evidence of true growth-rings.

The medullary rays are uniseriate and consist of thin parenchymatous cells with unpitted walls; they vary considerably in depth, usually comparatively shallow but in some cases 40 or 50 cells deep. In recent Araucarineae the rays are generally shallower. The absence of special receptacles, other than occasional resiniferous tracheids, for products of secretion is a feature common to _Dadoxylon_ and the Araucarineae. The phloem presents no features of special interest, but our knowledge of this tissue is comparatively meagre.

Among other examples of large _Dadoxylon_ stems some of which no doubt bore Cordaitean foliage—though as a rule we have insufficient information as regards anatomical characters to enable a decision to be made between _Cordaites_ and _Mesoxylon_—reference should be made to the imposing array of silicified trunks in the grounds of the Chemnitz Museum[671]. These were obtained from Lower Permian strata at Hilbersdorf near Chemnitz from beds overlain by porphyry tuff and resting on quartz porphyry, the volcanic material which furnished the siliceous solutions. Several large pieces of wood were found in association with stems of _Medullosa_ and _Psaronius_, leaves of _Cordaites_, _Artisia_ pith-casts, and _Cardiocarpus_ seeds with specimens of _Walchia_, _Gomphostrobus_ and other plants. Sterzel describes a stem 16·5 met. long and 1·5 met. in diameter; on the main trunk the branch-scars are scattered but on some branches there is a tendency to a whorled arrangement. This and many other stems are referred to _Araucarioxylon_ (or _Dadoxylon_) _saxonicum_, a species first described by Reichenbach as _Megadendron saxonicum_. In one specimen Sterzel states that the bordered pits are generally in 1–2 rows, though rarely in 3–4 rows, on the radial walls of the tracheids which they do not completely cover: the medullary rays reach a depth of between 20 and 30 cells. It is noteworthy that the stem 16·5 met. long has a pith-cast of the _Tylodendron_ type.

_Dadoxylon_ (_Cordaites_) _Brandlingii_ (Lindley and Hutton).

1831. _Pinites Brandlingii_ Lindley and Hutton, Foss. Flor. Vol. +i.+ Pl. +i.+ 1850. _Araucarites Brandlingii_ Goeppert, Foss. Conif. p. 232, Pls. +xxxix.–xli.+ 1890. _Cordaioxylon Brandlingii_ Schenk, in Schimper and Schenk, p. 853, fig. 408.

This species was founded on ‘a fossil giant of the vegetable kingdom’ discovered at Wideopen near Newcastle in Carboniferous strata on the estate of Mr Brandling. The stem, 72 ft long and far from complete, showed an irregular and not a whorled distribution of branch-scars. It is noteworthy that in _D. medullaris_ (Goepp.)[672], a Permian species from Saxony, the branch-scars, while for the most part irregularly scattered, in one case showed an approach to a whorled disposition as in recent Araucarias. Witham[673] gave a fuller account of the structure of the stem than is included in the original description, and the species has been described by many later authors from both Permian and Carboniferous localities. The pith is discoid and the broad transitional region at the inner edge of the wood is a characteristic feature[674]. Thomson[675] points out that there is a tendency to a retention of the scalariform type of pitting in the region of the medullary rays. There are 1–5 rows of pits on the radial walls of the tracheids. The rays may reach a depth of 40 cells; they are usually one cell broad. It has recently been shown that as many as six vascular strands[676] may form one leaf-trace instead of the customary pair, a feature suggesting comparison with _Metacordaites Rigolloti_ Ren. with its five foliar bundles. Other species agree very closely with _D. Brandlingii_ and it is impossible to determine with accuracy the precise specific limits of stems agreeing generally with this type; but for the sake of emphasising the variation in anatomical structure it is worth while to draw attention to a few more or less divergent forms from different geographical areas.

_Dadoxylon protopityoides_ Felix.

An interesting feature in this Westphalian type from Germany is the occurrence of transversely elongated pits on the tracheids[677] associated with those of normal form closely simulating the pits in the xylem elements of _Protopitys_.

_Dadoxylon nummularium_ White.

In this Brazilian wood[678] from Permo-Carboniferous beds the medullary rays are very numerous, mostly uniseriate and 1–30 cells in depth. The pits on the tracheids are in 1–2 rows and are often contiguous. In another type, _D. meridionale_, described by the same author[679], the pits are strictly uniseriate and generally contiguous. As White says, the absence of the pith and cortex and of any evidence as to the structure of the primary xylem renders impossible any definite expression of opinion as to the affinity of these and many other species.

_Dadoxylon Nicoli_ Seward.

Dr Arber[680] in naming this species, from the Newcastle (Permo-Carboniferous) Series of New South Wales, _Dadoxylon australe_, does not mention Crié’s earlier account of some wood from New Caledonia under the name _Araucarioxylon australe_[681]. The latter generic name according to the usage adopted in this volume should be superseded by _Dadoxylon_, and this necessitates a fresh specific name for Arber’s specimens. The name _Nicoli_ is suggested in place of _australe_, as the sections on which Arber founded his species form part of the Nicol collection in the British Museum.

The xylem shows distinct rings of growth, a feature also seen in Indian stems of approximately the same geological age and recorded by Shirley[682] in wood from Queensland which needs more careful examination. The bordered pits, usually multiseriate and contiguous, are not infrequently in 1–2 rows and separate. The uniseriate medullary rays are very numerous as in White’s Brazilian species _D. nummularium_, and as a rule 6–12 cells deep. Some well-preserved specimens from Permo-Carboniferous strata in Natal and Zululand have been described by Warren[683] as _Dadoxylon australe_ Arb., showing interesting anatomical features, but the material almost certainly includes more than one specific type and would repay more detailed investigation.

_Dadoxylon materiarum_ Dawson.

This species was described by Dawson[684] from Carboniferous strata in Nova Scotia and afterwards referred by Penhallow[685] to the genus _Cordaites_. In the transverse section reproduced in fig. 474, A, the tracheid-walls have been reduced in thickness by partial decay, but some of the bordered pits are clearly shown on the radial walls; the pits usually form 2–4 contiguous rows (fig. 475) in some cases with an oblique pore while others are represented either by the outer border of the pit or by the pore only. The narrow medullary rays are as a rule uniseriate and may be 60 cells deep (fig. 474, B). Dawson states that some specimens have large _Artisia_ pith-casts, a fact that formerly would have been regarded as proof of the _Cordaites_ nature of the wood, but in the absence of evidence with regard to the nature of the primary xylem it is impossible to say whether the stem is _Cordaites_ or _Mesoxylon_.

_Dadoxylon_ sp.

Some wood received from Mr Leslie, collected at Vereeniging, South Africa, in Permo-Carboniferous rocks, shows well-defined rings of growth. The pits form either a single row, a double, alternate and contiguous row, or rarely three series on the tracheid walls. The medullary rays are usually uniseriate and 1–30 cells deep.

_Dadoxylon Kayi_ Arber.

This species is represented by some large trunks, in some cases with a diameter of 40 cm., discovered by Mr Kay in the Coal Measures of Worcestershire[686]. The pith is very small and shows no indication of a discoid structure, but owing to its poor preservation no sections could be obtained of this region. The secondary wood is characterised by the large number of uniseriate medullary rays 1–27 cells in depth; the tracheids have usually two or sometimes three rows of alternate and contiguous bordered pits on the radial walls. Arber regards the absence of a discoid pith as a fatal objection to a reference of the stems to _Cordaites_ and speaks of them as affording further evidence of the occurrence of Coniferae in the higher Coal Measures of the Midlands. It is, however, impossible to determine the position of the species in the absence of any data with regard to the structure of the perimedullary region, and without such information we are hardly justified in regarding _Dadoxylon Kayi_ as a member of the Coniferales.

[Illustration: Fig. 474. _Dadoxylon materiarum._ Transverse (A) and tangential (B) sections of the secondary xylem. (Kidston Coll. 222, 224.)]

[Illustration: Fig. 475. _Dadoxylon materiarum._ A–C radial longitudinal sections of the secondary xylem. (Kidston Coll. 225.)]

_Dadoxylon Pedroi_ Zeiller.

This species from Upper Carboniferous or possibly Lower Permian strata in Brazil[687] has a pith 3·8 cm. in diameter composed of parenchyma with scattered secretory sacs and characterised by the occurrence of three equidistant bays projecting into the cylinder of wood (fig. 476, A) which extend through the length of the specimen (6 cm.): these, as Zeiller suggests, may be connected with the departure of leaf-traces or branches. The xylem is entirely composed of centrifugal elements and shows a broad transitional zone (fig. 476, B) including spiral, scalariform, and reticulate tracheids, but the bordered pits are less numerous and less crowded than in many species of _Dadoxylon_. The rays are 1–2 cells broad and reach a depth of 50 cells. The most striking features are the solid and not discoid pith with its three rounded bays and secretory canals, also the smaller number and frequently circular form of the pits on the tracheids. Zeiller considers that the stem is that of some Cordaitean plant though probably not a true _Cordaites_. White[688] questions the advisability of adopting the generic name _Dadoxylon_ and suggests the possibility, though without any satisfactory evidence, that it is the stem of a _Gangamopteris_. Failing further information, there would seem to be no sufficient reason for the institution of a distinctive generic name.

[Illustration: Fig. 476. _Dadoxylon Pedroi._ A, transverse section showing the pith and part of the secondary wood. B, longitudinal section of part of the secondary wood. (After Zeiller.)]

_Dadoxylon permiense_ (Renault).

This Permian species from Autun[689] differs from typical examples of the genus in the differentiation of the pith into a central thin-walled region contracted into transverse diaphragms surrounded by a cylinder of stouter tissue and in the greater breadth of the medullary rays. The tracheids have 3–4 rows of pits of the usual type. Spirally disposed, decurrent, leaf-bases occur on the surface of the stem, and the cortex includes secretory canals and strands of hypodermal stereome. A small number of veins pass up the median part of the lamina which in this respect and in its greater thickness differs from that of _Cordaites_ leaves. Renault speaks of the rays as a cycadean feature, but they are only two cells in width and shorter than in recent Cycads.

_Dadoxylon spetsbergense_ Gothan.

In this species[690] from Spitzbergen, of doubtful age though probably Palaeozoic, there is no xylem-parenchyma and the medullary rays are from 2 to 5 cells deep; the bordered pits occur in 1–2 or rarely 3 rows on the radial walls of the tracheids; they are alternate but not flattened and characterised by their small size (7μ high); they do not cover the whole face of the tracheids. It is pointed out that in many Palaeozoic and Mesozoic Dadoxylons the pits are larger than in recent species (16–17·5μ as compared with 9–12μ) while in _D. spetsbergense_ they are still smaller. The large size of the medullary-ray cells is another noteworthy feature, also the absence of annual rings, a character possibly connected with conditions of growth in northern regions. It is, however, pointed out by Nathorst[691] that the fossil was not found _in situ_ and, as he says, it may have been carried by currents from a more southern locality.

=Metacordaites.= Renault.

_Metacordaites Rigolloti_ Renault.

Renault founded this species[692] and genus on a stem from Autun which, like _D. Pedroi_, differs in certain respects from stems usually attributed to _Cordaites_. The pith is solid and contains secretory ducts and cells; the tracheids have often a single row of pits, and multiseriate pitting is much less common than in _Dadoxylon_. The medullary rays are generally 1–6 cells deep. A striking feature is the occurrence of groups of five vascular bundles penetrating the secondary wood in V-shaped groups, each group being regarded as a multiple leaf-trace, a type recently recognised by Thomson in _D. Brandlingii_. In one of Renault’s figures a larger scar, presumably a branch-scar, is shown immediately above a group of foliar bundles. The genus _Metacordaites_ is considered by its author to be intermediate between Conifers and the Cordaitales, but nearer to the former. This conclusion is, however, based on insufficient evidence, as nothing is known of the reproductive organs.

+Roots.+

In 1871 Williamson[693] gave an account of a petrified plant from the Lancashire Coal Measures which he named _Dictyoxylon radicans_, but he afterwards came to the conclusion that the specimens so named were portions of the subterranean axis of some other plant, possibly _Asterophyllites_, and proposed a new generic term _Amyelon_[694]. In 1874 he brought forward fresh evidence in support of connecting _Amyelon radicans_ with _Asterophyllites_ or _Sphenophyllum_, genera which Williamson believed to be very closely related. It has since been recognised that _Amyelon_ is the root of _Cordaites_ or of some closely allied member of the Cordaitales. Our knowledge of Cordaitean roots is based chiefly on the work of Williamson and Renault[695], and more recently Osborne[696] has added new facts of considerable interest. In the larger roots the primary xylem may be diarch or there may be as many as four or five protoxylem groups (fig. 477). The primary tracheids are spiral or scalariform and the space, _s_, separating them from the surrounding secondary xylem seen in fig. 477, B, was no doubt originally occupied by conjunctive parenchyma. The secondary wood is composed of tracheids, with contiguous bordered pits identical with those in the xylem of the stem, and narrow medullary rays. The section, 4 mm. in diameter, represented in fig. 477, A, shows a tetrarch primary xylem strand enclosed by secondary wood composed of rather thin-walled elements succeeded by a zone of phloem including some secretory sacs, and beyond this is a cylinder of periderm, _p_. In a section of a root figured by Renault from Autun the periderm is separated from the stele by a broad band of parenchyma which appears to be cortical, but in the British specimens the deep-seated origin of the periderm is clearly shown: Osborne states that it arises in a layer immediately outside the endodermis. In one of the specimens figured by Williamson[697] the secondary wood shows clearly marked irregular concentric lines simulating rings of growth, but there is no evidence of any regularly recurring variation in the diameter of the xylem-elements. From the descriptions of Williamson and Osborne it is evident that the roots of _Cordaites_ were profusely branched and, as the latter author has shown, the method of branching points to the formation of coralline roots like those of recent Cycads, some Conifers and Dicotyledons. Osborne found that the cortex of small rootlets is composed of two zones, an outer parenchyma without cell-contents and an inner parenchymatous tissue characterised by the occurrence in some of the cells of tangled masses of fungal hyphae almost always unseptate. In some cases the hyphae bear terminal vesicles similar to those observed on fungal hyphae in the cortex of _Podocarpus_ roots. Osborne makes out a good case for regarding the fungus as symbiotically related to the tissues of the lateral roots, a relationship identical with that in many existing trees, particularly _Myrica_ and _Alnus_. It is suggested that the formation of the coralline root-tubercles is a feature consistent with the view that _Cordaites_ lived in saline marshes, a physiologically dry habitat favourable to the occurrence of mycorhiza.

[Illustration: Fig. 477. A, root of _Cordaites_. B, centre of the root enlarged; _p_, periderm; _s_, space. (Kidston Coll. 1906.)]

[Illustration: Fig. 478. Root (_Cordaites_?) exposed in the bed of the Vaal river. (After Mellor and Leslie.)]

Reference has already been made to the habit of Cordaitean roots in the general account of the genus (figs. 468, A, 478). The specimen shown in fig. 478 may be a root of _Cordaites_ (_Noeggerathiopsis_) _Hislopi_, but nothing is known as to its structure[698].

+Reproductive Organs.+

=Cordaianthus.= Grand’Eury.

We have as yet no definite knowledge of the nature of the reproductive organs of _Mesoxylon_ and _Poroxylon_, but having regard to their close resemblance in other respects to _Cordaites_, particularly in the case of _Mesoxylon_, the presumption is that some of the seeds and fertile shoots attributed to _Cordaites_ may belong to other members of the Cordaitales. Despite the abundance of _Cordaites_, or at least of material assigned to that genus, and the comparative frequency of fertile shoots in actual connexion with foliage-shoots, the practical identity of _Mesoxylon_ and _Cordaites_ leaves precludes any confident use of the latter name in a strict sense.

In 1822 Brongniart[699] described a small bud-like fossil of Tertiary age as _Antholithes liliacea_, and this generic name in the form _Antholithus_ became widely used for fertile shoots or flowers from different geological horizons. As knowledge became more precise other names replaced _Antholithus_, though Renault[700] retained it for some inflorescences from Commentry which could not definitely be included in _Cordaites_. Lindley and Hutton[701] employed the genus for a specimen, now recognised as a Cordaitean fertile shoot, from the Coal Measures which they called _Antholithus Pitcairniae_, the specific name being chosen to indicate a possible affinity to the Bromeliaceous genus _Pitcairnia_. A few years later Morris[702] described a similar inflorescence as _A. anomalus_. In 1872 Carruthers[703] substituted Brongniart’s term _Cardiocarpon_ for _Antholithus_ and called _A. Pitcairniae Cardiocarpon Lindleyi_ and Morris’s species _C. anomalum_. The specific name _Lindleyi_ has been widely adopted, but there would seem to be no adequate reason for disregarding the priority-rule. It is, however, customary to use Grand’Eury’s term _Cordaianthus_ for all Cordaitean inflorescences. Goeppert[704] suggested the name _Botryoconus_ for an inflorescence similar to _C. Pitcairniae_ and for this C. E. Weiss[705] substituted _Noeggerathianthus_ on the ground that he considered Goeppert’s specimen to be the male inflorescence of _Noeggerathia_. Grand’Eury resuscitated _Botryoconus_[706] for some spikes from the Gard coalfield connected by him with _Dory-Cordaites_. The nature of the seeds borne by the inflorescences has largely influenced authors in the choice of a generic name: Carruthers used _Cardiocarpon_ while Zeiller[707] speaks of _Samaropsis Pitcairniae_. The genus _Cardiocarpon_ was founded by Brongniart for compressed cordiform seeds, but it was not until later that their gymnospermous nature was recognised. Further reference to the nomenclature of seeds of the _Cardiocarpon_ type will be found in Chapter +xxxv.+ The correlation by Grand’Eury and other authors of different species of inflorescences and species of _Cordaites_ is frequently based on association, and in the absence of more satisfactory evidence the safer course is to deal with Cordaitean fertile shoots in a general sense.

[Illustration: Fig. 479. _Cordaianthus._ From the Middle Coal Measures of Lancashire. (Manchester Museum; ⅓ nat. size.)]

(_a_) _Ovulate shoots._ These are represented by a considerable number of forms in both European and American localities. In rare cases the compound shoot reaches a length of 30 cm. (fig. 479), but it is usually much shorter; the lateral compact buds may be more or less widely separated: the seeds have long pedicels (fig. 480, A) or appear to be sessile (fig. 480, B) and there may be one or several seeds on a single lateral shoot. The seeds are platyspermic and, as seen in fig. 480, A, in some inflorescences they show very clearly the _Samaropsis_ features. It would, however, be unsafe to assume that all _Samaropsis_ seeds were borne on Cordaitean plants. Among other types of seed referred by authors to _Cordaites_ are _Cardiocarpus_, _Cordaicarpus_, _Sarcotaxus_, _Taxospermum_, _Diplotesta_, and _Leptocaryon_. But in most cases there is no evidence of actual connexion between seeds and vegetative organs, and while it is possible to state with confidence that many of the seeds represented by impressions described as species of _Samaropsis_ and _Cordaicarpus_ are undoubtedly Cordaitean, it is certain that not all seeds referable to these genera were borne by Cordaitalean plants. Cordaitean seeds are characterised by certain morphological features recalling those found in recent Cycads and in the seeds of _Ginkgo_ as illustrated by species of _Cardiocarpus_ and some allied types. As most of the Palaeozoic seeds known in a petrified state cannot be assigned to their parent-plants they are dealt with in a separate chapter[708].

_Cordaianthus Pitcairniae_ (Lindley and Hutton).

This type of inflorescence[709] is considered by Kidston to belong to the tree which bore leaves known as _Cordaites principalis_, but if this is the case it is probable that the stem possessed the anatomical characters of _Mesoxylon_.

[Illustration: Fig. 480. A, _Cordaianthus Pitcairniae_ with _Samaropsis_ seeds. B, _Cordaianthus Volkmanni_. B′, portion of B enlarged. (A, B, nat. size; Kidston Coll. 2374, 1174.)]

The portion of an inflorescence shown in fig. 480, A, from the Middle Coal Measures of Yorkshire, illustrates the occurrence of the bud-like fertile shoots and the stalked _Samaropsis_ seeds. A species described by Renault[710] from Commentry as _Cordaianthus acicularis_ may be identical with the British species.

_Cordaianthus Volkmanni_ Ettingshausen.

The example of this species[711] seen in fig. 480, B, shows the relatively small size of the lateral buds, presumably unexpanded, compared with the large subtending bracts.

=Petrified specimens of Cordaianthus.=

Our knowledge of the structure of _Cordaianthus_ is based on the researches of Renault[712], supplemented by those of Prof. Bertrand[713] to whose kindness I owe the photographs reproduced in fig. 481. The inflorescences described by Renault are referred by him to different species, but in the following brief account these are treated from a generic standpoint. The tangential section of _Cordaianthus Williamsoni_ Ren. shown in fig. 481, D, was originally figured by Renault and more recently by Bertrand; it shows the spirally disposed leaf-traces in the lower part of a stout axis, and at the sides some vascular bundles are seen passing up into the bracts. A very small proportion of the bracts subtend ovules; two are seen at _a_ and _b_, and at _c_ is the tangentially cut micropylar canal of a third borne near the apex and covered by the terminal cluster of bracts. The ovule _a_, separated by a narrow space from its short stalk, consists of a thick single integument—not two as stated by Renault—extended at the apex as a micropylar canal: the apical extension is more completely shown in the tangentially cut ovule _b_. The central body is much contracted and the two spaces, _s_, at the base are regarded by Bertrand as cavities in the integument separated from one another by a central strand of conducting tissue which gives off two bundles to the integument, one at each end of the long axis of the seed (fig. 481, A, _v_). The dark patch, _n_ (fig. D), is the upper and broader end of the shrunken nucellus the apex of which extends upwards as a slender beak, and this originally no doubt fitted into the micropyle. Fig. 481, C, shows a female inflorescence in transverse section; the stele consists of a ring of bundles separated by broad medullary rays and enclosing a comparatively large pith: the leaf-traces are seen in the cortex and one is cut through as it bends out into a bract which is not yet free from the axis. Two ovules, seen in section at _a_ and _b_, are represented by the bilaterally symmetrical and compressed integument enclosing small pieces of nucellar tissue. Fig. 481, E, is a transverse section of an inflorescence at a higher level and above the apex of the axis: there are four large ovules and one aborted ovule, _a_. Bertrand describes two vascular bundles in the integument of the ovule _a_, one at each end of the long axis.

[Illustration: Fig. 481. A, B, _Cordaianthus Grand’Euryi_, ovule showing apex of nucellus, B, with microspores, _p_; _pc_, pollen-chamber; _b_, beak of nucellus; _v_, vascular bundle. C, transverse section of flower-bud with ovules, _a_, _b_. D, _Cordaianthus Williamsoni_, longitudinal section; _a_–_c_ ovules; _s_, spaces; _n_, nucellus. E, _Cordaianthus Zeilleri_, transverse section showing four large ovules and one aborted ovule (_a_); _v_, vascular bundle. F, transverse section of male flower; _m_, microsporangia. (From photographs supplied by Prof. Bertrand.)]

Fig. 481, A, B, _Cordaianthus Grand’Euryi_ Ren., shows a longitudinal section of the nucellus, 1·5 × ·7 mm., and part of the integument of an ovule at the time of pollination, which was probably aided by the secretion of a drop of mucilage as in the ovules of recent Conifers. The integument, separated by a broad space from the nucellus, is cut in the plane of the two vascular strands, _v_. From the centre of its broad upper surface the nucellus projects upwards as a beak, _b_, and this originally engaged with the micropylar canal formed by the integument: the lighter patch below the beak is the pollen-chamber (fig. 481, B, _pc_) containing two microspores, and two more, _p_, are seen above the nucellar beak. In another species described by Renault, _C. Lacattii_, the nucellus fills the space bounded by the integument.

* * * * *

(_b_) _Staminate inflorescences._ The male inflorescence, though smaller, is similar in habit to the ovulate shoot: the secondary branch consists of a short axis bearing crowded, spirally disposed, bracts, and the actual flowers are represented by single stamens or groups of 2–3 highly specialised microsporophylls. Each microsporophyll consists of a long filament with a central vascular strand bearing at its apex 3–4 long microsporangia (fig. 481, F, _m_) which open longitudinally as seen in fig. 482, A. The microsporangia are 2·5–3 mm. long covered by dark palisade cells and thin-walled parenchyma, shown as indistinct patches in the photograph. Some of the elliptical and comparatively large microspores are seen in fig. 482, B; the exine is finely punctate and inside are the remains of a few thin cells in which presumably spermatozoids were developed. The microspores shown in fig. 482, B, have a maximum length of 0·1 mm.: Renault describes some as 0·9 mm. long while others are much smaller. Fig. 481, F, is a transverse section of a staminate inflorescence showing near the centre five groups of microsporangia, each sporangium having the form of a curved incomplete dark band indicating that dehiscence has occurred.

[Illustration: Fig. 482. _Cordaianthus._ Microsporangia, A, and Microspores, B. (University College Collection, London.)]

=Mesoxylon.= Scott and Maslen.

This generic name was instituted for stems obtained by Messrs Lomax from the Lower Coal Measures of Lancashire[714] previously referred by Scott[715] to _Cordaites_ and _Poroxylon_. Further investigation showed that while agreeing closely with those genera they possessed certain distinctive features demanding recognition. The name chosen suggests the intermediate nature of the stems. The more striking features may be summarised as follows: In the largest specimens so far described the stem, including leaf-bases, reaches a diameter of 5 or 6 cm.; the large pith consists in the central region of diaphragms of parenchyma separated by horizontal spaces produced by splitting and shrinkage consequent on the failure of the tissue to keep pace with the general growth of the stem. The secondary xylem is of the Araucarian type and has narrow medullary rays varying in depth from 1 to 25 cells. The leaf-traces are represented by twin-bundles which fuse in the downward direction, the level at which fusion occurs being regarded as a specific character. The presence of centripetal xylem is an essential feature of the traces: the occurrence of single or double traces consisting of centripetal elements and, externally, a large amount of centrifugal xylem is an important feature in which _Mesoxylon_ differs from _Cordaites_. The double leaf-traces divide after emerging from the secondary wood and each strap-like leaf receives several collateral bundles (fig. 483, C). An axillary bud may occur at the base of each leaf (fig. 483, A, _b_). The phloem, including sieve-tubes and secretory sacs, is succeeded by a broad pericycle, and the comparatively narrow cortex is traversed by successive bands of periderm. In the outer region of the cortex the presence of radial bands of fibres is a characteristic feature. The reproductive organs are unknown. The anatomical features are well illustrated by _M. Sutcliffii_ first described by Scott, who provisionally placed it in _Poroxylon_, and afterwards more fully investigated by Maslen[716].

_Mesoxylon Sutcliffii_ Scott.

The average diameter of the stem is 3 cm.: the section reproduced in fig. 483, A, has a maximum breadth of 3·5 cm.; the leaf-bases cut at different levels give an irregular contour to the surface like that of a _Lepidodendron_. An axillary bud, either reproductive or vegetative, is seen at _b_ consisting of a short axis bearing crowded bud-scales. The leaves are crowded and according to Maslen have a phyllotaxis of ⁸⁄₂₁: the lamina is linear like that of _Cordaites_ with 16 collateral bundles in the petiole. The presence of a meristematic band at the base of the lamina affords evidence of a deciduous habit. The large size of the pith is a striking feature with its central tissues in the form of transverse diaphragms and a narrower peripheral zone of solid parenchyma (fig. 483, A, _a_). The secondary wood of the stele is composed of tracheids with 2–3 contiguous alternate rows of bordered pits on the radial walls, but none on the tangential walls. In the stem shown in fig. 483, A, the secondary wood is preserved only in patches. Numerous blunt teeth varying in prominence project into the pith; these consist chiefly of serially disposed centrifugal tracheids distinguished by their spiral and scalariform structure and by the medullary rays which are broader than those in the more external xylem. Further reference is made to these perimedullary strands in the description of the leaf-traces. The medullary rays are uniseriate and usually 1–6 cells in depth: beyond the secondary wood is a cambium and a cylinder of secondary phloem (fig. 483, D, _ph_²) consisting of tubular elements, presumably sieve-tubes, and elongated secretory sacs. The pericycle is composed of several rows of rather large and short cells and has an ill-defined outer boundary. A succession of arcs of periderm-like tissue and phellogen, which may invade the pericycle and phloem, forms a prominent feature in the cortex; radially placed bands of fibres similar to those in _Lyginopteris_ and other genera occur in the outer cortex. At the edge of the pith the more prominent projections of xylem are arranged in pairs (fig. 483, B) and as each pair travels downwards the component strands gradually fuse[717]. Each bundle of a double trace consists internally of an arc of centripetal xylem, the elements of which are arranged in rows (fig. 483, B, _cp_), with a single protoxylem group in the middle of the inner face, _px_. It is not clear whether any primary centrifugal tracheids are present, but there are indications that such are occasionally represented. In most cases the primary xylem of the leaf-traces is exarch, but the existence of mesarch bundles is not improbable. The bulk of each foliar bundle is formed of a fan-shaped mass of secondary centrifugal xylem (fig. 483, B, _cf_) and an island of parenchyma occurs next the protoxylem. There is no clearly defined boundary between the outer or centrifugal xylem of the leaf-traces and the tracheids of the stem-wood; the latter may consist exclusively of tracheids with bordered pits or the inner rows of the xylem-cylinder may be of the scalariform or spiral type. Differences shown in transverse sections of the inner portion of the xylem are due to the circumstance that in certain parts of the inner face of the secondary wood leaf-traces are unrepresented, while in other places the dwindled remains of the outer, centrifugal, portions of a trace are still recognisable. As each double leaf-trace passes down the pith the bundles fuse and the single strand retains for a time some centripetal xylem; this gradually disappears and at a lower level the centripetal xylem also dies out. The space enclosing the obtuse apices of the bundles shown in fig. 483, B, was originally occupied by thin-walled tissue which accompanied the trace in its outward course. In _Mesoxylon Sutcliffii_ the leaf-strands pass almost horizontally through the secondary wood, bend outwards in the phloem and follow a steeply ascending course to the leaves. In fig. 483, B, a double leaf-trace is seen at the inner edge of the secondary wood with the centrifugal xylem, _cf_, continuous with that of the stele: fig. 483, D, _lt_, shows a leaf-trace in the pericycle where one of the bundles has divided and the other is tangentially extended and partially divided. The branching is carried further in the cortex, as seen in fig. 483, C, where the trace is represented by a curved row of six bundles, _lt_, and at a higher level further subdivision may occur. The leaf-bundles are collateral and in the leaf retain both centripetal and centrifugal tracheids. In the section shown in fig. 483, C, the oval stele of an axillary shoot is seen at _s_ subtended by the row of collateral bundles: the stele has a fairly large pith surrounded by a zone of secondary xylem with broad medullary rays.

[Illustration: Fig. 483. _Mesoxylon Sutcliffii_. Transverse sections of stem. A. _a_, outer pith; _b_, axillary bud; _x_² secondary xylem. B. _cp_, _cf_, centripetal and centrifugal xylem; _px_, protoxylem. C. _lt_, leaf-traces in the cortex; _s_, stele of axillary branch; _ph_², secondary phloem. D. Leaf-trace bundles, _lt_, external to the phloem, _ph_². (A, C, D, sections in the Manchester Collection, 717 _b_; B, Cambridge Botany School, 530.)]

* * * * *

Among other species of _Mesoxylon_ mention may be made of _M. Lomaxi_ and _M. poroxyloides_. _M. Lomaxi_ Scott and Maslen[718] generally resembles _M. Sutcliffii_ but shows the following distinctive features: the leaves are more scattered and less crowded; the twin-bundles of the leaf-traces fuse immediately on entering the pith, thus appearing for the most part as single and not double strands in the perimedullary zone; the centripetal xylem is well developed, the medullary rays are deeper and the outer cortex has shorter bands of mechanical tissue.

In _Mesoxylon poroxyloides_ Scott and Maslen[719], the twin-bundles of the traces unite soon after reaching the pith as in _M. Lomaxi_, the secondary tracheids have only two rows, or sometimes a single row, of bordered pits and the tracheids are rather smaller than in _M. Lomaxi_ (20–40μ as compared with 30–60μ) and the medullary rays are shallower. There is a particularly broad zone of spiral and reticulate transitional tracheids at the inner edge of the wood as in _Cordaites_ and in _Dadoxylon Pedroi_ (fig. 476). The leaves of this species are believed to be represented by the type described by Dr Benson as _Cordaites Felicis_ (fig. 465)[720], but, as already suggested, it is very probable that many or possibly nearly all the leaves from British Coal Measures described as _Cordaites_ may belong to _Mesoxylon_.

The chief interest of the genus _Mesoxylon_ is its close resemblance in certain characters to _Cordaites_ and _Poroxylon_: the presence of strands of centripetal xylem in the perimedullary region is an important feature in which _Mesoxylon_ differs from stems assigned (under the generic name _Dadoxylon_) to _Cordaites_. _Mesoxylon_ differs from _Poroxylon_ in having a discoid pith like that of _Cordaites_, but a more important difference is the absence in the leaf-trace xylem of _Mesoxylon_ of bordered pits of the Araucarian type, whereas in _Poroxylon_ Araucarian pits occur in both the centripetal and centrifugal tracheids. In _Poroxylon_ the secondary xylem is manoxylic; in _Mesoxylon_, as in _Cordaites_, it is pycnoxylic.

=The range of Cordaites and a consideration of other imperfectly known genera.=

An increased precision in knowledge derived from anatomical investigation often tends to demonstrate the untrustworthiness of criteria based on external features previously employed with confidence. This inevitable though, from the point of view of the systematist, inconvenient result of intensive study is well illustrated by the recent discovery of the stems named by Scott and Maslen _Mesoxylon_[721]. A separation of _Cordaites_ from _Mesoxylon_, which no doubt extended far beyond the British area, is possible only if well-preserved petrified material is available. The leaves of _Mesoxylon_, so far as our imperfect knowledge of them enables us to express an opinion, are constructed on a plan almost identical with those of _Cordaites_ and, as already stated, it is almost certain that many of the impressions referred to _Cordaites_ were borne on _Mesoxylon_ stems. An additional source of confusion is supplied by the _Cordaites_-like leaves of _Poroxylon_. It is evident, therefore, that even within the limits of the Carboniferous and Permian formations the recognition of true _Cordaites_ leaves must often be attended with considerable risk of error. Apart from the possible confusion between the foliage of _Cordaites_ and _Mesoxylon_ there are other difficulties as regards detached leaves which depart more or less widely from the typical Cordaitean form. Leaves such as _C. circularis_ (fig. 468, B) and _C. grandifolius_ emphasise the lack of any thoroughly satisfactory dividing line separating single pinnules of _Cardiopteris_ or _Cyclopteris_ on the one hand and leaves of _Psygmophyllum_ on the other from _Cordaites_. The petrified buds described as _Dolerophyllum_[722] have been quoted by several authors as examples of unexpanded shoots of _Cordaites_ though anatomical evidence warrants a generic separation. In the case of species founded on leaves described in this chapter as _Cordaites_ it should be remembered that further research may necessitate an alteration in nomenclature.

Among the species included in _Cordaites_ is _Noeggerathiopsis Hislopi_[723] (figs. 470–472), a type widely spread in India and in other parts of Gondwana-Land: if the change of generic name is accepted it involves the extension of the geographical range of _Cordaites_ from Northern Europe and North America to the southern botanical province. We have as yet no proof of the existence of _Cordaites_ in the Arctic regions. The range in time of _Cordaites_ or of the Cordaitales has generally been stated to be from the Upper Devonian to the Permian. It is, however, by no means certain that the genus flourished before the Carboniferous period, though it is clear that closely allied types must have lived in pre-Carboniferous floras. The strata in New Brunswick from which Dawson recorded his supposed Devonian _Cordaites_ have been shown to be Upper Carboniferous in age[724]. As regards the length of time during which the Cordaitales existed we have no decisive evidence. In recent years the tendency has been to extend their range into the Mesozoic era, and there are several pieces of evidence in favour of this. There is no doubt that considerations of age based on the arbitrary divisions of the geological scale sometimes insinuate themselves too thoroughly into questions connected with the duration of plant-types whether represented by families or genera. We have been accustomed to regard _Cordaites_ as a genus confined to the Palaeozoic period, a type which with many others carried on the tradition of Upper Carboniferous forests to the Permian floras and then made way for the precursors of Mesozoic types. There is, however, no valid reason for supposing that _Cordaites_ and other Palaeozoic genera did not survive as less prominent members in succeeding floras. It must be admitted that evidence in support of Mesozoic Cordaitales is not above suspicion, though the probability is that _Cordaites_ or some allied genera still flourished in the earlier stages of the Mesozoic era. The data on which this opinion is based cannot be fully discussed in a general treatise, but a few of the facts may be briefly considered. Zeiller[725] and other authors have expressed the view that the Cordaitales were not exclusively Palaeozoic. In addition to _Cordaites_ (_Noeggerathiopsis_) _Hislopi_ recorded from Rhaetic floras, other possible representatives of the group are illustrated by specimens included in such genera as _Yuccites_, _Bambusium_ and _Krammera_.

=Pelourdea= gen. nov.

The name _Yuccites_[726] was given to some detached, broad, linear leaves from the Bunter sandstone of the Vosges which were compared with the foliage of _Yucca_ and classed among Monocotyledons. The authors of the genus also described a cylindrical cast as a _Yuccites_ stem, including both stem and leaves in _Yuccites vogesiacus_. The supposed stem, as Fliche[727] has shown, is a pith-cast and is appropriately named by him _Endolepis vogesiacus_. The Vosges leaves are assigned by this author to the genus _Cordaites_, a change of name which may eventually be justified though as yet based on insufficient evidence. There are objections to the institution of a new name in place of _Yuccites_, but it is undesirable to retain a designation suggesting false ideas with regard to affinity. A new name _Pelourdea_ (after M. Pelourde of Paris, whose recent death deprives Palaeobotany of an able and promising investigator) is therefore proposed for leaves of the _Yuccites_ type which in form, venation, and spiral phyllotaxis agree with those of _Cordaites_ but cannot confidently be assigned to that genus or even to the Cordaitales. For linear leaves, especially from Jurassic strata, resembling those of _Phoenicopsis_ the name _Desmiophyllum_[728] is employed: these are very similar to those of _Pelourdea_; they are characterised by their fairly uniform breadth and afford no indication of their arrangement on the supporting axis.

_Pelourdea vogesiaca_ (Schimper and Mougeot).

The linear-lanceolate leaves described by Schimper and Mougeot as _Yuccites vogesiacus_ and transferred by Fliche to _Cordaites_ are probably specifically identical with specimens described by Mr Wills[729] from Lower Keuper rocks in Worcestershire. The English leaves were described by Arber[730] as _Zamites grandis_,—the name _Zamites vogesiacus_ having been previously used by Schimper and Mougeot,—on the ground that the supposed leaves were probably pinnae of a cycadean frond, a view in agreement with an opinion previously expressed with regard to similar leaves from Stonesfield[731]. A later discovery by Wills of specimens, on which the drawing reproduced in fig. 484 is based, of the same type of leaf showing the foliar nature of the fossils necessitated the abandonment of the pinna-hypothesis, and the original name _Yuccites vogesiacus_ was resuscitated[732]. The leaves reach a length of 50 cm. and a maximum breadth of 6·5 cm.; the lamina is entire, lanceolate or linear-lanceolate, the apex acuminate, and the lower part rather abruptly contracted and attached by a broad crescentic base; veins numerous, parallel, and occasionally forked. Fliche records the occurrence of a small _Artisia_-like pith-cast and pieces of stem with leaf-scars (4 × 3 mm.) in association with leaves of _Pelourdea vogesiaca_ in Triassic strata in Lorraine. An imperfectly preserved specimen described by Fliche as _Cordaianthus Minieri_[733] resembles an inflorescence of _Cordaites_. It consists of an axis 15 cm. long, the lower part forming a peduncle, and on the upper portion are linear bracts subtending oblong bodies which may be lateral fertile shoots.

[Illustration: Fig. 484. _Pelourdea vogesiaca_. Reconstruction of a foliage-shoot. (After Wills.)]

_Pelourdea hadroclada_ (Halle).

Dr Halle[734] recently published an account of some imperfect leaves and stem-fragments from the Rhaetic of Scania which he named _Phyllotenia_ (_?_) _hadroclada_, the generic name provisionally adopted having been proposed by Salfeld[735] for some rather obscure remains from the Corallian of Germany. It appears to have escaped the notice of both authors that Saporta[736] in 1894 had adopted the designation _Phyllotenia_ for some examples of broad parallel-veined leaves from Lower Cretaceous rocks in Portugal very similar to Velenovský’s _Krammera mirabilis_[737]. Some other generic name must therefore be used. For the Rhaetic species the name _Pelourdea_ would seem appropriate. The type-specimen consists of an axis 10–12 mm. in diameter with spirally disposed transversely elongated leaf-scars bearing sessile linear leaves similar to _Poa-Cordaites_; none of them are complete, the largest is 6 cm. long and 5–7 mm. broad with 8–12 parallel veins. An examination of the original specimens in the Stockholm Museum satisfied me that Dr Halle is justified in the opinion that they may be fragments of some Cordaitalean plant and that he was well advised to avoid the use of the name _Cordaites_. Salfeld’s species, _Phyllotenia longifolia_, may be an imperfectly preserved example of _Phoenicopsis_[738], but the material is too incomplete to be identified with any degree of confidence.

_Pelourdea Imhofi_ (Heer).

The Triassic leaves from Switzerland described by Heer[739], and more recently by Leuthardt[740], as _Bambusium Imhofi_, were referred by Fliche[741] to the genus _Cordaites_. The lamina is ensiform, 25 cm. long with a maximum breadth of 2·4 cm. Leuthardt’s photograph of aerial stems and rhizomes of this supposed Monocotyledon are far from convincing.

_Pelourdea keuperiana_ (Compter).

The leaves from the Lower Keuper of Thuringia assigned by Compter[742] to _Cordaites_ without adequate evidence resemble those of _P. vogesiaca_, but there is no evidence as to their manner of attachment; they are 30–40 cm. long and from 1·5 to 2 cm. broad.

_Pelourdea megaphylla_ (Phillips).

This species was first described by Phillips[743] from the Middle Jurassic Stonesfield Slate and afterwards referred to _Zamites_[744]: the leaves bear a striking resemblance to foliage of the type _Cordaites borassifolius_; the lamina is 30 cm. long and attains a breadth of 3 cm., the apex is acuminate and slightly contracted towards the broad concave base. My former comparison of these Stonesfield leaves with the long pinnae of _Ceratozamia mexicana_ seemed to be supported by Phillips’s type-specimen of _Palaeozamia longifolia_. It may be that the supposed pinnae in Phillips’s type are spirally disposed leaves: if this is the case the specimen may be a fragment of a _Podozamites_; its specific identity with the larger detached specimens, though probable, cannot be demonstrated. Some leaves figured by Zigno[745] from Jurassic rocks of Italy as _Yuccites Schimperianus_ may be identical with _P. megaphylla_.

_Pelourdea mirabilis_ (Velenovský ex Corda +MS.+).

The generic name _Krammera_, suggested by Corda, was employed by Velenovský[746] for large _Cordaites_-like leaves from the Lower Cretaceous of Bohemia, for casts of cones regarded by him as stems, and for fruit-like bodies. The leaves, previously described as _Flabellaria chamaeropifolia_ Goepp., _Dammara albens_ Presl, etc., bear a close resemblance to the large broadly linear leaves of _Cordaites_; the lamina reaches a length of 40 cm. and between the veins occur 1–4 finer striations. The fossils identified by Velenovský as stems bearing crowded imbricate scales, which he regarded as the persistent bases of _Krammera_ leaves, are probably cones; they agree very closely in size and shape, also in the form of the scales, with cones of _Agathis_ and some other recent Conifers. As the designation _Krammera_ was instituted primarily for cones and not leaves the name _Pelourdea_ is substituted for it.

=Niponophyllum.= Stopes and Fujii.

_Niponophyllum cordaitiforme_ Stopes and Fujii.

The generic name _Niponophyllum_[747] was proposed for some petrified specimens of leaves or possibly leaflets from Upper Cretaceous beds in Japan which, though not definitely assigned to a group or family, are considered by the authors of the genus ‘to lie [anatomically] somewhere between _Cordaites_ and _Cycadeoidea_’ ‘with a closer similarity to the former than to the latter if we compare the whole _Cordaites_ leaf with our blade.’ The data on which this conclusion is based are, however, insufficient to justify a reference of _Niponophyllum_ to the Cordaitales or indeed to lend any substantial support to the opinion that the Japanese specimens are anatomically more akin to _Cordaites_ than to other plants. The type-species is represented by two specimens of leaf-fragments about 0·4 mm. thick and from 6 to 9 mm. broad containing from 21 to 33 vascular bundles; the upper part of the mesophyll is composed of palisade tissue and the stomata appear to be confined to the lower epidermis. Each bundle is accompanied by an =I=-shaped girder, and small patches of sclerenchyma occur next the upper epidermis between the girders; there are no resin-canals: the vascular bundles are collateral, the xylem is said to be almost entirely centripetal and exarch, but in the absence of evidence afforded by longitudinal sections the details of structure cannot be definitely determined. A comparison is made with Cycadean leaves and with leaves of Araucarineae and Podocarpeae, also with _Cordaites_; another type with which _Niponophyllum_ may be compared is _Desmiophyllum Solmsi_[748].

The genus is interesting as an example of a petrified gymnospermous type of leaf characterised by the absence of resin-ducts and transfusion-tracheids, the possession of collateral, apparently exarch, bundles enclosed in a double sheath; but the data supplied are insufficient to enable us to allocate the specimens to a position within the class.

A specimen described by Schenk[749] as _Eolirion primigenium_ from Lower Cretaceous beds in the Carpathian mountains closely resembles in habit a foliage-shoot of _Poa-Cordaites_; the leaves are narrow and linear with obtuse apices and attached, apparently, in a close spiral. Schenk assigns the plant to the Monocotyledons, but its systematic position must be left unsettled.

* * * * *

The list of Mesozoic specimens resembling _Cordaites_ leaves might be extended. Apart from some Triassic and Rhaetic examples which may well be Cordaitalean, there are many others which, though similar in form and venation to _Cordaites_, are in all probability more closely related to _Agathis_ and other genera; the species _Dammarites Bayeri_ recently described by Zeiller[750] from the Upper Cretaceous of Bulgaria is a case in point. The Araucarian character of the wood of _Cordaites_ precludes any satisfactory discrimination between Mesozoic Araucarian stems and those of Cordaitalean species, at least in the case of such material as is usually available.

=Titanophyllum.= Renault.

_Titanophyllum Grand’Euryi_ Renault. The remarkable leaves on which this genus and species are founded[751] were discovered in the Commentry coalfield; they occur as detached specimens and cannot be correlated with any known stem. Renault suggests that the Autun stems referred to _Colpoxylon_ may have borne the _Titanophyllum_ leaves, but this correlation rests only on the dimensions of the stems and the occurrence of transversely elongated scars on the surface. The lamina is thick and coriaceous, 70–75 cm. long and 20–25 cm. in breadth; the veins are parallel but not branched; numerous longitudinal striations on the upper surface indicate the presence of hypodermal stereome-strands; stomata are abundant on the lower surface and the more or less rectangular cells in the neighbourhood of the stomata appear to be papillose (fig. 485, A, B). The distal region of the lamina is often torn into strips (fig. 485, A); the approximately rectangular leaf has a broad elliptical base 9–10 × 3–4 cm.

[Illustration: Fig. 485. _Titanophyllum Grand’Euryi._ A, leaves ¹⁄₃₀ nat. size; B, stomata on the lower surface of the lamina. (After Renault and Zeiller.)]

Dr White[752] describes a specimen from the Lower Coal Measures of Missouri as ? _Titanophyllum Brittsii_ which he speaks of as the thick base of a leaf similar to that described by Renault but, as White says, no formal diagnosis is possible without more satisfactory material. Such evidence as is available suggests that _Titanophyllum_ is a type of Cordaitalean leaf probably closely allied to _Cordaites_.