Chapter XXXIII: Cordaitales (2)

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

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

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

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

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

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

_Dadoxylon_ (_Cordaites_) _Brandlingii_ (Lindley and Hutton).

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

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

_Dadoxylon protopityoides_ Felix.

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

_Dadoxylon nummularium_ White.

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

_Dadoxylon Nicoli_ Seward.

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

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

_Dadoxylon materiarum_ Dawson.

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

_Dadoxylon_ sp.

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

_Dadoxylon Kayi_ Arber.

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

_Dadoxylon Pedroi_ Zeiller.

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

_Dadoxylon permiense_ (Renault).

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

_Dadoxylon spetsbergense_ Gothan.

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

=Metacordaites.= Renault.

_Metacordaites Rigolloti_ Renault.

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

+Roots.+

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

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

+Reproductive Organs.+

=Cordaianthus.= Grand’Eury.

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

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

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

_Cordaianthus Pitcairniae_ (Lindley and Hutton).

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

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

_Cordaianthus Volkmanni_ Ettingshausen.

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

=Petrified specimens of Cordaianthus.=

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

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

* * * * *

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

=Mesoxylon.= Scott and Maslen.

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

_Mesoxylon Sutcliffii_ Scott.

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

* * * * *

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

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

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

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

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

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

=Pelourdea= gen. nov.

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

_Pelourdea vogesiaca_ (Schimper and Mougeot).

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

_Pelourdea hadroclada_ (Halle).

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

_Pelourdea Imhofi_ (Heer).

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

_Pelourdea keuperiana_ (Compter).

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

_Pelourdea megaphylla_ (Phillips).

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

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

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

=Niponophyllum.= Stopes and Fujii.

_Niponophyllum cordaitiforme_ Stopes and Fujii.