CHAPTER XXXIII.
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 [Symbol: 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 [Symbol: 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 [Symbol: 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 [Symbol: 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.