Chapter XXIX: Pteridospermeae (2)

The name, _Codonotheca pusilla_, is given by Nathorst[192] to some doubtful specimens from the Culm of Spitzbergen representing short stalks bearing linear-lanceolate scale-like bodies, 9–10 mm. long by 1 mm. broad, coalescent at the base. Nathorst compares them with Sellard’s species, _Codonotheca caduca_, but adds that they may be cupules of some Pteridosperm and calls attention to their resemblance to some fossils figured by Carpentier as _Calymmatotheca acuta_. Both Nathorst’s species and the French specimens described by Carpentier[193] as cupules may be referred to Dr Stopes’ genus _Pterispermostrobus_ as their morphological nature cannot be determined.

+v.+ _Roots._

In 1876 Williamson[194] described some petrified vegetative organs from the Lower Coal Measures of Lancashire under the name _Kaloxylon Hookeri_ characterised by a division of the secondary xylem into cuneate masses (fig. 415, C) like those in some recent Bignoniaceous stems. Williamson at first believed _Kaloxylon_ to be a stem, but in a later memoir he expressed the opinion that ‘it is difficult to believe that these organs have been other than roots’[195]. Felix[196] had meanwhile described a specimen from the Coal Measures of Westphalia as _Kaloxylon cf. Hookeri_ and suggested that it might be a waterplant. In 1894 Williamson and Scott[197] demonstrated that _Kaloxylon Hookeri_ is the root of _Lyginopteris_, a conclusion independently reached by Hick[198].

The roots of _Lyginopteris_ arise on all sides of the radially symmetrical stem in the pericycle region; they are copiously branched as is shown by the abundance of roots of various sizes in close association. No roots have been discovered exceeding 1 cm. in diameter nor have any been recorded with secondary periderm-tissue. The absence of root-hairs and the more or less lacunar structure of the cortex are indicative of swampy ground. It is seldom that the palaeobotanist has an opportunity of investigating the growing-points of Palaeozoic plants, and for this reason some well-preserved apices of _Lyginopteris_ roots, attributed to that genus on the ground of constant association with fragments of stems in the calcareous nodules of Dulesgate, are of special interest. One of these specimens was figured by Dr Stopes and Mr Watson[199] in their account of plant-bearing nodules, and a description of that and other examples has since been published by Prof. Weiss[200]. A longitudinal section of a root-tip, ·21 mm. in diameter at its broadest part, shows a root-cap which suggests an origin from a single cell, but an examination of the plerome-cylinder in a slightly tangential section does not afford conclusive evidence of the occurrence of a single initial cell. Weiss on the whole inclines to the view that _Lyginopteris_ possessed a single apical cell like the roots of Leptosporangiate Ferns, though he prefers to leave the decision open. Attention is drawn to the fact that the plate of tracheidal tissue in emerging lateral rootlets is vertical as in Phanerogams and not horizontal as in recent Pteridophyta.

The vascular tissue of a _Lyginopteris_ root (fig. 410) consists of from three to eight alternate strands of centripetal xylem and phloem, and with the metaxylem is associated a small amount of conjunctive parenchyma which does not form a central pith. The pericycle, one to several layers broad, is succeeded by an endodermis which occasionally shows the characteristic thickenings on the radial walls. A broad cortex of thin-walled lacunar tissue with numerous secretory cells is bounded externally by a superficial cylinder of two or more layers of comparatively large and thin cells, the outermost of which are radially elongated. This superficial tissue forms a striking feature by which a _Lyginopteris_ root may often be recognised at a glance. The root represented in fig. 410, approximately 2 mm. in diameter, has a heptarch stele divided into seven xylem-groups by crushed bands of parenchyma and a protoxylem strand occupies the apex of each projecting angle (fig. 410, _px_). The superficial cylinder of clear cells is seen at _a_. A very small root is seen at _r_ in fig. 410.

Fig. 415, C, represents part of an older root in which the pentarch primary xylem is enclosed by broad wedge-like groups of secondary xylem and phloem separated by conspicuous medullary rays opposite the protoxylem strands (_px_). Crushed primary phloem arcs, _p_, are often clearly recognisable beyond the cambium. The secondary thickening, as Williamson and Scott state, ‘takes place exactly in the manner typical of roots of Dicotyledons, so that this fossil might very well be used for purposes of demonstration as illustrating the secondary growth of a root with diagrammatic clearness’[201]. The young roots of _Lyginopteris_ resemble in many respects those of Marattiaceous Ferns, though the presence of a single apical cell, if such occurs, is a distinguishing feature; but in the presence of secondary conducting tissue they agree with those of Phanerogams.

_Distribution of Lyginopteris._

The frequency with which petrified fragments of _Lyginopteris_ stems occur in the calcareous nodules of the English coal seams shows that the genus must have been plentifully represented in the Upper Carboniferous vegetation, and the occurrence in both North American[202] and European localities of fronds identical with or closely resembling _Sphenopteris Hoeninghausi_ affords evidence of wide geographical range. Petrified specimens were recorded by Felix[203] from Westphalia in 1886, and Zalessky[204] has recently discovered _Lyginopteris_ in the Donetz coal-basin of Russia. An investigation by Kubart[205] of the calcareous nodules, to which attention was first drawn by Stur, in the Ostrau Coal Measures led to the discovery of several examples of _Lyginopteris_ stems. The descriptions and figures so far published are hardly sufficient to enable us to estimate the degree of relationship to the English type, but some of the stems appear to be new species and Kubart considers them all to be specifically distinct from _Lyginopteris oldhamia_. _Lyginopteris heterangioides_ contains scattered tracheids in the pith and thus affords an interesting transitional type between _Lyginopteris_ and _Heterangium_. In _L. lacunosum_ the inner cortex is lacunar and the primary xylem bundles pursue an independent course in the stele in contrast to the anastomosing arrangement in _L. oldhamia_ and in another Hungarian species _L. tristichum_. The species recorded by Kubart occur in the Millstone grit and the Coal Measures.

The geological range of _Lyginopteris_ as represented by petrified stems does not extend beyond the limits of the Carboniferous system.

=HETERANGIUM.=

The generic name _Heterangium_ was first used by Corda[206] for a piece of stem from the Coal Measures of Radnitz, Bohemia, represented by part of the vascular axis of a stem consisting of strands of large reticulately pitted tracheids intermixed with parenchyma and exhibiting structural features differing apparently from those of any known type. Corda’s material has been re-examined by Kubart[207] who figures a section from it. _Heterangium_ is a genus closely allied to _Lyginopteris_ both in habit and in general anatomical characters. The stem is monostelic; the vascular cylinder prior to secondary thickening resembles the protostele of certain recent species of _Gleichenia_ and may be compared also with _Trichomanes scandens_[208]. It agrees with that of _Lyginopteris_ in the possession of primary mesarch bundles but differs in the substitution of a cauline axial mass of metaxylem for the pith of _Lyginopteris_. The secondary vascular tissue agrees closely with that of recent Cycads and _Lyginopteris_. A characteristic feature is the occurrence of numerous horizontal bands of sclerous cells in the cortex (fig. 412) of the stem and in the ground-tissue of the rachis and larger branches of the fronds. The stem was erect and rarely branched ‘giving off large foliar appendages at somewhat distant intervals and from its entire circumference’[209]. Our knowledge of the reproductive organs is less precise than in the case of _Lyginopteris_; but we are justified in asserting that _Heterangium_ is a Pteridosperm which in all probability bore fern-like microsporangia and seeds similar in general plan to _Lagenostoma_.

The association of some seeds included in Williamson’s genus _Conostoma_ with _Heterangium Grievii_ in the Pettycur beds and their resemblance to _Lagenostoma_, the seed of _Lyginopteris_, suggested the possibility of actual connexion: further evidence in support of this view has recently been brought forward by Dr Benson[210] in the case of a species of _Conostoma_ which she transfers to a new genus _Sphaerostoma_.

The two species _Heterangium Grievii_ and _H. tiliaeoides_ are described in illustration of the genus and reference is made to a few other types.

_Heterangium Grievii_ Williamson.

(_Stem_.)

1872. _Dictyoxylon Grievii_, Williamson, Brit. Ass. Rep. (Edinburgh
Meeting), p. 112.

1873. _Heterangium Grievii_, Williamson, Phil. Trans. R. Soc. Vol. 162,
p. 404.

(Leaf.)

[1720. _Fumaria officinalis_, Volkmann, Silesia subterranea, p. 111, Pl.
+xiv.+ fig. 2.]

1822. _Filicites_ (_Sphenopteris_) _elegans_, Brongniart, Class. Vég. p.
233.

1828. _Sphenopteris elegans_, Brongniart, Hist. Vég. Foss. p. 172.

1836. _Cheilanthites elegans_, Goeppert, Foss. Farnkr. p. 233.

1877. _Diplothmema elegans_, Stur, Culm Flora, +ii.+ p. 130.

i. _Stem_.

At the Edinburgh meeting of the British Association Williamson[211] gave a brief account of some petrified stems from the Lower Carboniferous strata of Burntisland on the Firth of Forth which he included in the genus _Dictyoxylon_. One of these was named _D. Grievii_ after Mr Grieve the discoverer of the specimens. In a later and more complete description Williamson adopted Corda’s generic name on the ground of the close resemblance of the Scotch stem to the Bohemian fragment _Heterangium paradoxum_. In 1873[212] Williamson added new facts in regard to _H. Grievii_ and in 1890[213] he described a very closely allied type from the Lower Coal Measures of Lancashire. Five years later his descriptions were considerably extended and modified in the joint memoir with Dr Scott[214].

The great difference in age between the English Upper Carboniferous stem and the Scotch specimens from the Lower Carboniferous beds of Burntisland suggests a probable specific difference. Dr Scott has recently adopted the name _Heterangium Lomaxi_, proposed but not published by Williamson, for the English type. Though in the following account the species _Heterangium Grievii_ is treated in the broader sense it should be recognised that the geologically younger stem is worthy of specific recognition; it is characterised, to quote Scott[215], by ‘the great distinctness of the primary xylem strands, by their nearly exarch structure, with little primary centrifugal wood, by the abundant secretory sacs of the stele, and by the rather scattered leaves.’

_Heterangium Grievii_ has a radially symmetrical stem bearing compound leaves with decurrent petioles which give to the otherwise cylindrical axis an angular outline as seen in transverse section (fig. 411, A). The phyllotaxis appears to be ⅜. The stem rarely exceeds 1·5 cm. in diameter: in the centre is a comparatively large stele consisting in young stems of primary xylem and phloem, but in older stems these are separated by a cylinder of secondary vascular tissue which in this species is always narrower than in _Lyginopteris oldhamia_ and, as Williamson pointed out, often of unequal thickness on different radii. The medullated stele of _Lyginopteris_ is replaced by a solid xylem-cylinder consisting mainly of groups of large tracheids, reaching ·3 mm. in diameter, with multiseriate bordered pits (fig. 411, D) embedded in an anastomosing parenchymatous tissue-system. In the stele reproduced in fig. 411, B, which with the exception of a very narrow zone of secondary xylem, _x_², consists entirely of primary xylem, _x_¹, the parenchyma is represented by a darker reticulum (_cf._ fig. 415, B) dividing the metaxylem into islands as in _Gleichenia_. In the peripheral portion of the xylem the tracheids are rather narrower and arranged in more definite groups in many of which is a single strand of narrow spiral elements (fig. 411, A′, _px_) close to the outer margin. These peripheral primary bundles in which protoxylem is recognisable may be described as leaf-traces of mesarch structure consisting of centripetal xylem and, to a much less extent, of smaller centrifugal elements for the most part with dense spiral bands in place of the multiseriate pits of the rest of the metaxylem. The structure of these leaf-traces is practically identical with that of the primary bundles of _Lyginopteris_. There is, however, a difference to which attention is drawn by Williamson and Scott. While in _Lyginopteris_ in any transverse section the primary bundles in the stele are equal in number to the leaf-traces in the pericycle and cortex, in _Heterangium_ the peripheral groups in the stele may be as many as twenty, a number considerably in excess of the leaf-traces beyond the limits of the primary xylem of the stele. It may be that the leaf-trace of each leaf, which joins the stele at a distance of 6–10 internodes below its entrance into the cortex from the leaf-stalk, may branch in its descent in the axial region, or some of the primary groups of xylem may be confined to the axial region and independent of the leaf-traces. Portions of the peripheral region of the stele may be occupied by metaxylem groups without protoxylem and identical with those which make up the bulk of the metaxylem.

Scott[216] has recently published a note in which he states that most of the British Coal Measures Heterangiums were polydesmic. Two bundles, and not a single strand as in the Scotch _H. Grievii_, leave the stele for each leaf, and these divide into four, in some cases at least, before entering the petiole.

The secondary xylem is continuous at its inner edge with the outermost primary tracheids (fig. 411, A′) and consists of rows of tracheids, 1–3 elements broad, alternating with numerous broad medullary rays of radially elongated parenchyma. Beyond a typical cambium-zone the secondary phloem consists of parenchyma and sieve-tubes bounded by crushed arcs of primary phloem. Abutting on the phloem is a pericycle composed of several layers of small parenchymatous cells (fig. 411, A, _p_) and in the outer layers of this tissue a phellogen (fig. 411, C, _p_) and some periderm are usually present though, as Williamson and Scott point out, the periderm is less regular and narrower than in _Lyginopteris_. The inner cortex, composed of short parenchymatous cells, is traversed by numerous narrow bands of dark, thick-walled cells similar in the structure of the elements, though peculiar in the horizontal elongation of the groups, to the sclerous nests in the pericycle and pith of _Lyginopteris_. These characteristic bands are chiefly seen in the oblique longitudinal section of a stem represented in fig. 412. In this section, 25 mm. in length, the lighter band, _p_, is the pericycle and in it a few obliquely cut leaf-traces are shown as dark patches. The horizontal bands are similar in structure and shape to the diaphragms of thick cells in the pith of _Abies magnifica_[217], and in both plants they probably serve as supports to the softer parenchyma. There may be as many as 46 bands in a vertical length of cortex of 1 inch (about 19 per centimetre). It was the occurrence of precisely similar transverse lines on the carbonised impressions of the rachis of _Sphenopteris elegans_ that led Kidston[218] to suggest a connexion between that species and the stem of _Heterangium Grievii_.

The outer cortex, consisting of alternate strands of parenchyma and stereome similar to that of _Lyginopteris_, is much narrower and a less conspicuous feature than in _Heterangium_; the stereome bands do not form so regular a hypodermal network and extend much further vertically without anastomosing. The epidermis has been described as a layer of fairly thick cells showing in one case an appearance of a depressed stoma[219]. There are no secretory canals like those of Cycads but, as in _Lyginopteris_, scattered cells with dark contents in the stem-tissues probably represent secretory sacs.

The leaf-traces on leaving the stele pursue a very gradually ascending course to the petiole; they retain their collateral structure in the pericycle and cortex and have no secondary xylem, but become concentric as they enter the base of a leaf.

Before passing to the description of the leaves, the more striking features in the stem may be summarised with reference to the diagrammatic sketches shown in fig. 411. Fig. 411, A, represents a section of _Heterangium Grievii_ approximately 2 cm. in its maximum diameter; at the periphery of the primary xylem, _x_¹, and close to its outer margin are several protoxylem groups, not shown in the drawing, each of which marks the position of a mesarch trace. The zone of secondary xylem, _x_², is interrupted by the exit of leaf-traces and one of these is seen at _a_ in fig. 415, A, separated from the central primary xylem by a foliar gap filled with parenchyma. The pericycle is shown at _p_ in fig. 412 and its outer boundary at _p_ in fig. 411, A. Beyond the pericycle is the broad parenchymatous cortex with leaf-traces, _lt_, and some sclerenchymatous patches, _s_. The vascular strand _a_ is passing into the base of a leaf-stalk. In the stem shown in fig. 411, B, 1·4 cm. × 7 mm., a decurrent petiole is seen at the upper end with its single vascular strand, _a_, and two sclerous nests; a similar though detached leaf-base occurs at the opposite end of the long diameter. Other leaf-traces are seen at _b_ and _c_. From the left-hand side of the stele a curved strand of tracheids is passing out to supply a root, _r_.

ii. _Root_.

Adventitious roots of endogenous origin are occasionally met with in _Heterangium_ stems, but we have less information as to their anatomy than in the case of _Lyginopteris_. In a specimen of _Heterangium Lomaxi_ figured by Williamson and Scott[220] three roots are seen in a vertical series growing outwards through the cortex of a stem. The roots agree generally with those of _Lyginopteris_ but the outermost cortical layers possess no special features.

iii. _Leaf_.

The large compound fronds long known as _Sphenopteris elegans_ were recognised by Kidston as the leaves of _Heterangium_ by the closely arranged transverse striae or narrow ribs on the rachis and pinnae which are the expression on the carbonised impressions of the horizontal plates of sclerous tissue in the petrified stems and petioles of _Heterangium_. The dichotomously branched fronds are included by Stur in his genus _Diplotmema_ and that author figures several typical examples in his ‘Culm Flora’[221]. Fig. 413, A, shows a forked axis with the bases of more slender branches and the characteristic transverse bands and in fig. 413, B, part of a pinna is reproduced. In general appearance, except in the bifurcating pinnae, the fronds resemble those of _Davallia tenuifolia_ with which Brongniart compared the Palaeozoic species. There is little doubt that _Sphenopteris dissecta_ and some other species were also borne on _Heterangium_ stems. The rachis and petioles differ from those of _Lyginopteris_ fronds in the absence of emergences (_cf._ fig. 404, E). The petioles (fig. 411, B, _a_) have a single concentric vascular bundle with internal protoxylem.

iv. _Reproductive organs_.

As yet no satisfactory evidence has been published with regard to the nature of the microsporangia but in all probability these were constructed on the same plan as those of _Lyginopteris_. There is a strong _prima facie_ case for assigning the seed _Sphaerostoma_ to _Heterangium_: absolute proof of organic connexion is still lacking though Dr Benson’s recent account of the seeds associated with _Heterangium Grievii_ almost amounts to demonstration of continuity between vegetative organs and seeds.

_Sphaerostoma ovale_ (Williamson).

In 1877 Williamson described some detached petrified seeds from the Lower Carboniferous rocks of Fifeshire, Scotland, as _Conostoma ovale_ and _C. intermedium_. Dr Benson’s investigation[222] of these two forms leads her to confirm Williamson’s doubts as to the validity of a specific separation and she assigns the single type to the new genus _Sphaerostoma_. The seeds are always associated with the vegetative organs of _Heterangium Grievii_. In 1909 Oliver[223] expressed the opinion that the Burntisland species of _Conostoma_ (= _Sphaerostoma_) is probably the seed of _Heterangium_. The seed consists of a central body representing the nucellus, an inner integument, and an enveloping cupule or outer integument: most specimens have lost the cupule and in this condition they are 3·5 mm. long with a maximum breadth of 2·2 mm. In the middle the seed is circular in transverse section and octagonal near the base and apex. The free apical part of the integument forms a frill (canopy) round the micropyle and extends beyond the nucellar apex which consists of a relatively flat plinth surmounted by a central dome or lagenostome (fig. 414). The lagenostome is surrounded by an annular pollen-chamber on to the lower surface of which abuts the large embryo-sac, and remains of archegonia were noticed below the pollen-chamber. The roof of the chamber in the young state consists of a layer of thin-walled cells extending across the flattened apex of the nucellus, _n_, but as the pollen-chamber becomes differentiated from the nucellar tissue by the disorganisation of the zone of cells its roof-cells thicken their vertical walls and assume the structure of a multiseriate annulus, which acts as a mechanism for opening the pollen-chamber by a circular dehiscence in such a way that the edge of the ruptured roof of the pollen-chamber slightly overlaps the periphery of the central column of nucellar tissue after it has returned to its original position subsequent to the entrance of the microspores. The micropylar region is surrounded by eight lobes of the integument and each is characterised by a crest of radially elongated cells, fig. 414, _f_, especially prominent on the outer side. External to this is the slightly longer cupular sheath (fig. 414, _c_) which may also have been lobed. The surface of the integument below the terminal crests consists of a layer of cells with small papillae which eventually ruptured and discharged mucilage. Both integuments have a vascular supply, that of the inner integument being represented by eight vascular bundles, some of which were found to have mesarch xylem, given off from the single strand in the pedicel. Fig. 414 shows the apical region of a seed of _Sphaerostoma_: the flat-topped nucellar cap, _n_, is surrounded by the annular pollen-chamber, _pc_, below which are indicated the archegonia: the wall of the megaspore (embryo-sac) is seen at _m_ and external to this vascular bundles, _v_, run up the inner portion of the integument accompanied by some large cells (aqueous tissue). The elongated epidermal cells at the apices of the lobes of the integument form the frill, _f_, and at a lower level the cells of the same layer are much smaller and papillate (_e_): the outer integument, _c_, forms the so-called cupule. The transverse section shown in fig. 414, B, is taken at the level of the roof of the pollen-chamber and of the nucellar cap; it illustrates the contrast between the ‘multiseriate annulus’ and the central column of small parenchyma.

_Sphaerostoma_ differs from _Lagenostoma_ in the whorl of crests around the micropyle, in the nearly hemispherical form of the lagenostome and in the relatively wider pollen-chamber with its peculiar form of dehiscence. Miss Benson, while regarding _Sphaerostoma_ as similar to _Lagenostoma_ in general plan, believes the distinguishing features of the former to be such as are consistent with a more primitive form.

An important argument in support of connecting this seed with _Heterangium_ is derived from the juxtaposition of some seeds and portions of _Heterangium_ petioles, a juxtaposition that is believed to demonstrate original continuity.

Grand’Eury[224] has recorded the association of two species of leaves, _Sphenopteris elegans_ and _S. dissecta_, with small seeds compared by him with _Lagenostoma_. In the absence of petrified specimens it would be practically impossible to distinguish between _Lagenostoma_ and _Conostoma_ or _Sphaerostoma_.

Carpentier[225] has described some impressions from French Westphalian beds as _Conostoma_ and he records cupules without seeds on fronds of _Sphenopteris obtusifolia_ which he speaks of as having transverse striations like those of _Heterangium_. Dr Kidston pointed out to me that the surface-features of the _Sphenopteris_ rachis are probably due to ramental scales and not to the presence of horizontal sclerous bands. Carpentier’s seeds may be compared with _Lagenospermum Sinclairi_.

_Heterangium tiliaeoides_ Williamson, Phil. Trans. R. Soc. Vol. 178, p. 289.

This species, founded by Williamson on material from the Lower Coal Measures of Halifax, Yorkshire, while agreeing in the structure of the primary stele and in the general features of the cortex with the older _Heterangium Grievii_, is clearly distinguished by certain well-marked characters. Sclerous groups occur in the inner cortex as in _H. Grievii_ but they are present also in the pericycle. The peripheral leaf-traces in the stele show the mesarch structure rather more distinctly than in _H. Grievii_, and the secondary xylem, which forms a much broader cylinder than in the Scotch type, is divided by broad medullary rays into characteristic cuneate masses each of which rests at its base on the centrifugal tracheids of a leaf-trace strand of xylem (fig. 415, B). The most striking distinctive feature is afforded by the secondary phloem, which is often preserved in wonderful perfection; this is unusually thick and owing to the tangential expansion of the principal medullary rays the secondary phloem is divided into separate masses which decrease in breadth towards the external arcs of primary phloem. The triangular form of the phloem rays, composed of tangentially stretched parenchyma, suggested the specific name _tiliaeoides_ on account of their striking resemblance to the rays of _Tilia_. The leaf-traces are nearly always in pairs as they pass out through the cortex; they subsequently divide and appear as four vascular strands in the petiole. The portion of stem reproduced in fig. 415, _B_, 8 mm. broad, shows clearly the separation of the secondary xylem and phloem into wedge-shaped groups: in each group there are several narrow medullary rays. The extrastelar tissues are represented by a few fragments only. Several layers of crushed periderm occur in the pericyclic region but the more external tissues have been almost completely exfoliated[226].

(A, B, Kidston Coll., 529, 294; C, Williamson Coll., 1631.)]

Reference has already been made to _Heterangium Lomaxi_, the English type originally included by Williamson in _Heterangium Grievii_. The provisional species _Heterangium cylindricum_ Williamson and Scott[227] differs, as Scott says, in no important respect from _H. Lomaxi_ and should not be retained. A new species, _H. minimum_ Scott[228], has been founded on a very small stem from the Coal Measures of Dulesgate in which the leaf-traces leave the stele as single bundles as in the Scotch _H. Grievii_.

The French species _Heterangium Duchartrei_[229] Ren. from Permian rocks was originally referred by Renault to the genus _Poroxylon_: it is represented by little more than the xylem of the stele and bears a close resemblance to _H. tiliaeoides_. _Heterangium punctatum_ Ren. and _H. Renaulti_[230] (Brongn.) also from the Permian of France were originally placed in the genus _Lycopodium_ and afterwards recognised as stems of _Heterangium_. A fourth French Permian species, _H. bibractense_[231], is peculiar in the possession of a very small primary stele encircled by deep wedges of secondary xylem, but without more information it is impossible to speak with confidence as to its systematic position. Kubart[232] has recently published brief descriptions of some stems from the Ostrauer coal-basin in Moravia all of which he regards as specifically distinct from the English types. In _Heterangium Sturi_ the primary xylem is almost exarch and the peripheral xylem groups are not very clearly defined: in _H. alatum_, so called from the presence of lateral wings on the petioles, the leaf-trace strands are more sharply differentiated from the rest of the stele. _H. polystichum_ is a similar type, and _H. Andrei_, with a relatively larger amount of parenchyma in the stele and thicker stems forms an additional link between _Heterangium_ and _Lyginopteris_[233]. Prof. Johnson[234] has described a species of _Heterangium_, _H. hibernicum_, from Upper Devonian and Lower Carboniferous beds in Co. Cork, Ireland, based on some impressions of frond fragments without any pinnules. The occurrence of numerous transverse striae on the rachis and lateral branches suggests comparison with _Heterangium_ fronds, but an examination of the specimens led me to suspect that some at least of the striae are cracks and not original features. The presence of spur-like appendages from the lower surface of the pinnae near their origin from the rachis is recorded as a peculiar character, and some obscure oval bodies, the nature of which is extremely doubtful, are considered to be seeds. The imperfection of the material hardly justifies the institution of a new species of _Heterangium_.

_Heterangium_ ranges from the Lower Carboniferous to the Permian strata and is thus older than _Lyginopteris_ which in the form of petrified stems is not recorded from the Lower beds of the Carboniferous system. _Heterangium_ has been described as having a ‘great preponderance of fern-like characters,’ but having regard to the resemblance of the primary xylem of the latter to that of the Osmundaceae it would seem doubtful whether in their relation to the Ferns there is any important difference. _Heterangium_ may safely be spoken of as the more primitive genus. The polydesmic character of the petioles of most species is particularly interesting as it brings the genus nearer to the Medulloseae and to _Rhetinangium_[235].