Chapter XXXIII: Cordaitales (1)

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.

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.

_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.

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.

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.

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].

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.

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.

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.

+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.