CHAPTER XVIII

.

=Bothrodendreae.=

_Bothrodendron._ Figs. 211–216.

Although in many respects the genus _Bothrodendron_ agrees very closely in habit and in its anatomical features with _Lepidodendron_, there are reasons for referring it to a distinct family of Palaeozoic Lycopods. As the following description shows, the external features do not differ in any essential points from those of certain types of the genus _Sigillaria_, particularly such a species as _S. rimosa_, Gold.[608], which has recently been refigured and described by Nathorst[609] from Goldenberg’s type-specimen in the Stockholm Museum. The small size of the leaf-scars is, however, a characteristic feature of _Bothrodendron_ (fig. 212, F); but a more important point is the fact that in a recently described[610] English example of a cone of _Bothrodendron_ (fig. 216), the sporangia are very like those of recent Lycopods, and differ from the radially elongated sporangia of _Lepidostrobus_. On the other hand, a French cone described by Zeiller[611] as _Lepidostrobus Olryi_, which is probably a strobilus of _Bothrodendron_, has the radially elongated type of sporangium (fig. 212, E). The comparative abundance of _Bothrodendron_ in Lower Carboniferous and Devonian rocks points to the greater antiquity of this member of the Lycopodiales as compared with _Lepidodendron_.

The name _Bothrodendron_ was instituted by Lindley and Hutton[612] for impressions of stems from the English Coal-Measures, characterised by two opposite rows of large depressions like those shown in fig. 211 and, in one of the specimens, by “a considerable number of minute dots, arranged in a quincuncial manner.” The minute dots were recognised as leaf-scars and the cup-like cavities were described as probably connected with the occurrence of large cones. On very slender evidence this Palaeozoic plant, which was named _Bothrodendron punctatum_, was considered by these authors as probably a member of the Coniferales. The large stem from the Coal-Measures in the neighbourhood of Mons, Belgium, shown in fig. 211, affords a good illustration of _Bothrodendron_ in a partially decorticated condition, exhibiting a row of depressions similar to those on the Ulodendron form of _Lepidodendron Veltheimianum_ (fig. 157), but distinguished by the eccentric position of the scar at the bottom of each cup-shaped cavity: in the Belgian specimen, which is partially decorticated and shows the leaf-traces as small dots, the depressions have a diameter of 9 cm. It is believed by some authors that these Ulodendron shoots of _Bothrodendron_ and _Lepidodendron_ owe their characteristic appearance to the pressure of large cones, but, as I have already stated, there are reasons for preferring the view that these crater-like hollows are the scars of deciduous branches. Our knowledge of the strobili borne by Bothrodendron stems is still meagre, but we have no reason to assume the existence of any cones large enough to produce by the pressure of their bases such depressions as those shown in fig. 211. In one species at least the strobili were borne terminally on slender shoots (fig. 213). The Ulodendron condition has so far been recognised in one species only, _B. punctatum_.

In his catalogue of Palaeozoic plants, Kidston[613] included _Bothrodendron punctatum_ as a synonym of _Sigillaria discophora_ König, a mistake which he afterwards rectified[614]: the generic name _Bothrodendron_ was generally ignored by authors in the belief that the specimens described by Lindley and Hutton were not generically distinct from the fossils originally figured by Rhode as _Ulodendron_. It was Prof. Zeiller who first demonstrated that the English authors were justified in their choice of a new designation for stems with large depressions in association with minute leaf-scars. In 1859 Haughton[615] proposed a new family name Cyclostigmaceae for some Upper Devonian plants from County Kilkenny, Ireland: he described three species of his new genus _Cyclostigma_, _Cyclostigma kiltorkense, C. minutum_, and _C. Griffithsi_; these are now generally recognised as a single species of _Bothrodendron_, though, as Nathorst suggests, the Irish plant should perhaps be separated as a sub-genus _Bothrodendron_ (_Cyclostigma_) by reason of certain minor differences which distinguish it from other species of the genus.

[Illustration: FIG. 211. _Bothrodendron punctatum._ Part of a specimen from near Mons (Hainaut), in the Brussels Museum. (Reduced.)]

Another generic name, _Rhytidodendron_, was instituted by Boulay in 1876 for stems characterised by a finely wrinkled bark and small spirally disposed leaf-scars. A short description of this type, which occurs in the Middle and Lower Coal-Measures, may serve to illustrate the external features of the commonest British example of the genus.

_a. Bothrodendron minutifolium_ (Boulay.) Figs. 212, A, C, D; 213.

1875. _Lycopodium carbonaceum_ (_Lycopodites carbonaceus_), Feistmantel, Palaeontographica XXXIII., Pl. XXX. figs. 1, 2; p. 183.

1876. _Rhytidodendron minutifolium_, Boulay, Terr. Houill. Nord France, p. 39, Pl. III. fig. 1.

1886. _Bothrodendron minutifolium_, Zeiller, Bull. Soc. Géol. France [iii] XIV. p. 176, Pl. IX. figs. 1, 2.

1888. _Lepidostrobus Olryi_, Zeiller, Flor. Valenciennes, p. 502, Pl. LXXVII. fig. 1.

1889. _Bothrodendron minutifolium_, Kidston, Trans. R. Soc. Edinburgh, Vol. XXXV. Pt. ii.

1893. _Sigillaria_ (_Bothrodendron_) _minutifolia_, Weiss and Sterzel, K. Preuss. Geol. Landesanstalt, Heft 2, p. 49, Pl. I. figs. 3 and 4; Pl. II. figs. 8 and 9.

1904. _Bothrodendron minutifolium_, Zalessky, Mém. Com. Géol. Russie, Pl. VI. fig. 6.

[Illustration: FIG. 212. _Bothrodendron._

A. _Bothrodendron minutifolium_, var. _rotundata_ Weiss. After Weiss and Sterzel. B. _B. punctatum._ After Zeiller. C. _B. minutifolium._ After Weiss and Sterzel. D. _B. minutifolium._ After Zeiller. E. _Lepidostrobus Olryi._ After Zeiller. F. _Bothrodendron punctatum._ After Zeiller. G, H. _B. kiltorkense._ G, after Nathorst; H, after Weiss and Sterzel.]

In habit a plant of _Bothrodendron_ recalls _Lepidodendron_ and recent species of _Lycopodium_; the slender dichotomously branched twigs bearing numerous leaves (fig. 212, D), have been mistaken for shoots of _Lycopodium_, and fragments of branches might well be identified as impressions of Mosses. The leaf-scars on the smaller shoots occur on elongated cushions (fig. 212, C, D) with a transversely wrinkled surface; on the older branches the leaf-scars are separated by fairly large areas of bark characterised by sinuous transverse grooves and narrow ridges bearing numerous small pits, as shown on an enlarged scale in fig. 212, A. The original surface-features are shown on the left of the drawing, and a slightly deeper level in the cortex is represented on the right-hand side. The absence of leaf-cushions on the older shoots is probably the result of secondary thickening, which also alters the size and shape of the leaf-scars. Each scar has three pits on its surface, as in _Lepidodendron_; a central leaf-trace scar and lateral parichnos scars. The circular pit above the leaf-scars, which occurs in most species, marks the position of the ligule. The relation of the short leaves, 5 mm. long, to the leaf-cushions is shown in fig. 212, D. The absence of leaves, except in impressions of slender twigs, may be interpreted as an indication that they were shed at an early stage and did not persist many years. The leaf-cushions of the smaller shoots of _Bothrodendron minutifolium_ closely resemble those figured by Weiss on a Devonian plant, _Lepidodendron Losseni_[616].

One of the few examples so far discovered of a Bothrodendron cone is shown in fig. 213; this specimen, at least 10 cm. long, was found by Mr Hemingway in the Middle Coal-Measures of Yorkshire and described by Dr Kidston. Numerous sporophylls are attached at right angles to the axis, the surface of which is protected by their upturned distal portions; the arrangement of the parts appears to be the same as in _Lepidostrobus_. A specimen figured by Zeiller as _Lepidostrobus Olryi_, which Kidston is probably correct in identifying with _Bothrodendron minutifolium_, shows that each sporophyll carries a horizontally elongated sporangium (fig. 212, E).

_b. Bothrodendron punctatum_ Lindley and Hutton[617]. Figs. 211, 212 B, F.

This species, which is less abundant than _B. minutifolium_, in British Coal-Measures, has been described by several authors as _Ulodendron_ on account of the occurrence of large depressions, like those shown in fig. 211, on certain branches of the plant. At the suggestion of Dr Kidston, Prof. Zeiller[618] figured an English specimen of this species, presented to the Paris Museum by Mr Hutton, in which the leaf-scars are preserved on the bark of a stem with Ulodendron scars. The surface of the bark is characterised by numerous small pits and discontinuous vertical lines in contrast to the transverse lines of _B. minutifolium_ (cf. fig. 212, A and F). The leaf-scars on the smaller shoots may have a diameter of only 0·3–0·5 mm., while on the larger branches they reach a breadth of 1 mm. The ligule-pit may be in contact with the upper edge (fig. 212, F) of the leaf-scar or separated from it by a short distance.

[Illustration: FIG. 213. _Bothrodendron minutifolium_ Cone. From a specimen in Dr Kidston’s Collection. (Slightly reduced. Kidston (02) Pl. LIX.)]

_c_. _Bothrodendron kiltorkense_ (Haughton). Fig. 212, G, H.

1859. _Cyclostigma kiltorkense_, Haughton, Journ. R. Soc. Dublin, Vol. II. p. 418, Pls. XIV.–XVII.

_C. minutum_, Haughton, Journ. R. Soc. Dublin, Vol. II. p. 418, Pls. XIV.–XVII.

_C. Griffithsi_, Haughton, Journ. R. Soc. Dublin, Vol. II. p. 418, Pls. XIV.–XVII.

1870. _Lepidodendron Veltheimianum_, Heer (_ex parte_), K. Svensk. Vet. Akad. Handl. Vol. IX. Pl. IX. figs. 2–4.

_Cyclostigma kiltorkense_, _ibid_. Pl. XI. figs. 1–5.

_Calamites radiatus_ (_ex parte_), _ibid_. Pl. III. fig. 2_a_: Pl. IX. fig. 2_b_.

_Stigmaria ficoides minuta_, _ibid_. Pl. IX. fig. 2_c_.

_Knorria imbricata_, _ibid_. Pl. X. fig. 4.

1889. _Bothrodendron kiltorkense_, Kidston, Ann. Mag. Nat. Hist. [VI.], Vol. IV. p. 66.

1894. _Bothrodendron kiltorkense_, Nathorst, K. Svensk. Vet. Akad. Handl. Vol. XXVI. No. 4, p. 65, Pls. XIV. XV.

1902. _Bothrodendron (Cyclostigma) kiltorkense_, _ibid_. Vol. XXXVI. No. 3, p. 31, Pls. X.–XIV.

The specimens from the Upper Devonian rocks of Co. Kilkenny on which Haughton founded this and two other species may be regarded as representing one specific type. He described the circular leaf-scars as arranged in alternating whorls. In habit the Irish species agrees with _Bothrodendron minutifolium_, but the leaf-scars are more elliptical (fig. 212, H) and the ligule-pit is usually absent. The leaf-scar shown in fig. H is 1·2 mm. broad and 1·4 mm. in height. The large collection obtained during the visit of a Swedish expedition to Bear Island in 1898 under the leadership of Dr Nathorst has materially increased our knowledge of this ancient type. The form of the leaf-scars varies according to the age of the branch and their disposition is far from constant even on the same specimen; in some cases the scars are in fairly regular whorls (fig. 212, G; an Irish specimen) while in others they are in regular spirals. This irregularity of arrangement, which is well illustrated by Nathorst’s figures of Bear Island and Irish specimens, finds its counterpart, though in a less marked form, in recent species of _Lycopodium_, _e.g. L. Selago_. Partially decorticated stems may present a superficial resemblance to _Calamites_, the fissured bark simulating the ribs of a Calamitean cast. Such stems, as Nathorst has pointed out, were mistaken by Heer for _Calamites radiatus_. The smaller branches are characterised by a smooth surface, and older shoots resemble _Bothrodendron minutifolium_ in the presence of fine vertical lines. The preservation of only one pit on the leaf-scars of many examples led authors to conclude that the species is peculiar in this respect, but Nathorst has shown that in more perfectly preserved specimens each leaf-scar bears three small dots. A specimen from Ireland in the British Museum[619] illustrates the dichotomous branching and the longitudinal wrinkling of the bark; the leaf-scars are 2 mm. broad and 2·5 mm. deep.

Nathorst[620] has described some examples in which the leaf-scars occur on the lower instead of on the upper end of the leaf-cushions; these and other specimens with obscure surface-features he suggests may be underground axes, comparable in habit with _Stigmaria_ though not identical as regards details. It is pointed out that the absence or scarcity of _Stigmaria_ in the Bear Island beds renders it unlikely that _Bothrodendron_ bore typical Stigmaria branches. F. E. Weiss[621] has recently described root-bearing organs possessing primary xylem identical with that of _Bothrodendron mundum_; while closely resembling _Stigmaria ficoides_ in certain anatomical characters, they clearly represent a distinct type. This discovery of a Stigmaria-like axis almost certainly belonging to _Bothrodendron_ is consistent with Nathorst’s views on some of the Bothrodendron impressions from Bear Island.

Information as to the cones of this species is restricted to a description by Schimper[622] of a specimen in the Dublin Museum as _Lepidostrobus Bailyanus_; this has sporophylls with a subtriangular base bearing several megaspores and terminating distally in a slender lamina 12 cm. in length.

An example of a _Bothrodendron_ with more prominent leaf-cushions than those already mentioned is afforded by a species from Bear Island described by Heer[623] as _Lepidodendron Wükianum_ and afterwards referred by Nathorst[624] to _Bothrodendron_. The same type is recorded also by Schmalhausen[625] from Lower Carboniferous or Devonian strata of Siberia. Certain Scotch specimens from the Calciferous Sandstone, which Kidston[626] referred to Heer’s species, are regarded by Nathorst and, in part at least, by Weiss[627] and Sterzel as representing a distinct species which these authors designate _Bothrodendron Kidstoni_[628].

Without attempting the hopeless task of discriminating between the various Carboniferous and Devonian specimens described under the names _Cyclostigma_ or _Bothrodendron_, reference may be made to the following records as illustrating the wide distribution of the genus. Schmalhausen[629] records _Cyclostigma kiltorkense_ from Siberian rocks assigned to the Ursa stage (Devonian or Lower Carboniferous). The fossil described by Dawson[630] from the Devonian of Gaspé as _Cyclostigma densifolium_ probably represents a badly preserved example of _Bothrodendron_: Weiss’s species _Cyclostigma hercynium_[631] from Lower Devonian rocks of the Hartz district may be identical with _Bothrodendron kiltorkense_. The supposed identity of the latter species with _Dechenia Roemeriana_ Goepp., as described by Potonié[632], appears to require confirmation[633], but if this author is correct the connexion demonstrates the continuity of Bothrodendron shoots and Stigmaria-like subterranean organs. The specimens described from South Africa, from strata which may be correlated with the Upper or possibly with the Lower Carboniferous series of Europe, as _Bothrodendron Leslei_[634] in all probability represents a species closely allied to the Irish and Bear Island type. _Bothrodendron Leslei_ named after Mr Leslie whose discoveries in the Carboniferous Sandstone of Vereeniging (Transvaal) have added considerably to our knowledge of the South African Palaeozoic types, is represented by imperfectly preserved casts characterised by more or less circular scars displaying the same irregularity of arrangement as in _Bothrodendron kiltorkense_. The leaf-scars appear to have only one small pit, but this may not be an original feature. The identification of this plant as _Bothrodendron_ receives support from the discovery of rather more satisfactory specimens at Witteberg sent to me for examination by Dr Schwarz[635]. These fossils bear a striking resemblance to _B. kiltorkense_. _Cydostigma australe_[636] Feist. described from the Lower Carboniferous rocks of New South Wales, though too imperfectly preserved to refer with confidence to _B. kiltorkense_, is no doubt a closely allied type.

[Illustration: FIG. 214. _Bothrodendron Leslei_ Seward.

_b._ Natural size. _a, c._ Slightly enlarged.]

[Illustration: FIG. 215. _Bothrodendron mundum_ (Will.).

A, B. From a specimen (No. 26) in the Cambridge Botany School. C. British Museum, Williamson Collection. (No. 416 b.) D, E. From a section in Dr Kidston’s Collection.]

Reference was made in Volume I. (p. 133) to the so-called paper coal of Carboniferous age from Central Russia, which consists of masses of thin strips of cuticle of Bothrodendron stems. The figures published by Zeiller[637] show that the plant possessed an epidermis consisting of polygonal cells interrupted by spirally disposed gaps marking the position of leaves; the gaps measure 0·5–1·5 mm. in breadth and agree, therefore, with the size of the leaf-scars of the smaller forms of _Bothrodendron_. The specimens from the Russian mines were first figured by Trautschold and Auerbach[638] as _Lepidodendron tenerrimum_ and afterwards referred by Zeiller to _Bothrodendron punctatum_[639]. Nathorst[640], however, states that an examination of the Russian material leads him to retain the name originally proposed; he records the same type from Upper Devonian rocks of Spitzbergen. The chief interest of these Russian specimens is their manner of preservation, which Renault has described as the result of bacterial action; he claims to have recognised the actual bacteria associated with the cuticular membranes[641].

_Anatomy of vegetative shoots_ of Bothrodendron.

In 1889 Williamson[642] described several specimens of petrified shoots from the Coal-Measures of Halifax which he named _Lepidodendron mundum_: these are now known to be branches of a _Bothrodendron_. The discovery was made by Mr Lomax[643] who found specimens showing the external characters of _Bothrodendron_ and the anatomical characters of _Lepidodendron mundum_. In some of the smaller twigs, the stele consists of a solid core of xylem with external protoxylem; but in the majority of specimens the centre of the xylem is replaced by parenchymatous tissue, either as a small axial strand or, as in the specimen shown in fig. 215, D, a wide pith, the elements of which are arranged in regular vertical series. A diagrammatic section of a small axis is represented in fig. 215, A: this branch, 2 mm. in diameter, is composed of a broad outer cortex consisting exclusively of primary tissue the outer cells of which are smaller and have thicker walls than the more internal elements. The leaf-traces, _lt_, are accompanied by a strand of delicate tissue, the parichnos. The stele is almost solid; the tissues in contact with the xylem have not been preserved but the inner cortex is represented by a few layers of small parenchymatous cells, _c_¹. The larger section shown in fig. 215, D, was cut from a specimen from Dulesgate of which the smooth surface exhibits the characteristic leaf-scars of _Bothrodendron_. The section measures 3 cm. in its longest diameter and the stele has a breadth of 3 mm. The outer cortex has a smooth surface and is composed of rather thick-walled cells succeeded by a zone of secondary elements. The middle cortex has disappeared and the space is partially occupied by Stigmarian rootlets, _s_, and crushed patches of cortical tissue. The position of a leaf-scar is seen at _a_; this is more clearly shown in the enlarged drawing fig. E.

In his account of _Lepidodendron mundum_, Williamson[644] described a section in which the primary wood is surrounded by a considerable thickness of secondary xylem; a diagram of this is shown in fig. 215, C. An examination of the section led me to compare the structure of the outer cortical cells, characterised by radial rows of tangentially elongated elements, with the outer cortex of _Stigmaria_. It has recently been shown by Weiss[645] that this and other similar sections present several points of agreement with _Stigmaria_, particularly with _Stigmaria Brardi_ as described by Renault. At _s_ in fig. 215, C, a vascular strand is seen passing through the outer cortex; this is almost certainly the bundle of a rootlet: in the sections described by Weiss rootlets are shown in a similar position. The chief anatomical features of the Stigmaria-like organs of _Bothrodendron_ are:—the considerable development of secondary xylem, the structure of the outer cortex, which is practically identical with that of _Stigmaria ficoides_, and the association of groups of short transfusion tracheids with the bundles of the rootlets. It is very probable that the absence of secondary xylem in the vegetative shoots of _Bothrodendron_ is merely an accident and not a real distinction between the aerial and subterranean branches of the plant; a supposition rendered probable by the occurrence of secondary xylem in the axis of the cone described by Watson. As Weiss points out, there are certain differences between the true _Stigmaria_ and the corresponding organ of _Bothrodendron_; the secondary xylem in _Bothrodendron_ is not broken up by broad medullary rays as in the common _Stigmaria_, and in _Bothrodendron_ the occurrence of a ring of primary xylem is another peculiarity.

In the vegetative shoots of _Bothrodendron mundum_ the stele differs from those of _Lepidodendron_ in the narrower primary xylem ring and in the large size of the metaxylem tracheae; from _Lepidodendron Harcourtii_ and _L. fuliginosum_ the xylem is distinguished by its smoother outer face which consists of numerous narrow xylem elements.

[Illustration: FIG. 216. _Bothrostrobus._ _l_, ligule. (After Watson.)]

_Cones_ of Bothrodendron (_Bothrostrobus_[646]).

The long and narrow cones referred to _Bothrodendron minutifolium_ from English and French Coal-Measures are known only as impressions and it is not possible to say whether they were heterosporous or homosporous; the drawing given by Zeiller (Fig. 212, E) shows that the sporangia were of the same form as those in _Lepidostrobus_, but we have no more exact information as to their morphology. A recently published description of a petrified strobilus by Mr Watson affords a welcome addition to our knowledge. There is little doubt that this cone was borne by a species of _Bothrodendron_; the evidence for this conclusion is supplied by the agreement of the anatomical characters of the stele with that of the vegetative shoots originally described by Williamson as _Lepidodendron mundum_ and by the constant association of the cones and vegetative shoots. In 1880 Williamson described a crushed cone containing both megaspores and microspores which he spoke of as “a diminutive organism, reminding us more of the dwarfed fruits of many living Selaginellas than of the large _Lepidostrobi_[647].” Watson’s specimens enable us to give a more complete account of this type. The axis of the strobilus bears short sporophylls bent upwards into a distal limb with a conspicuous ligule in a deep pit beyond the shortly stalked sporangium. The length of the strobilus is estimated at 10 mm.; the stele is of the same type as that of _Bothrodendron mundum_, but it differs from the specimens of the vegetative shoots so far found in having some secondary xylem. As shown in the sketch reproduced in fig. 216 each sporophyll is characterised by two tangentially placed grooves, _g_, on the lower face, and by numerous transfusion tracheids, _tr_, above the vascular bundle, _vb_, immediately below the ligule, _l_. Megasporangia and microsporangia occur on the same cone, the megasporangia being on the lower sporophylls and containing a single tetrad of megaspores. Fig. 219, E, shows a radial longitudinal section of a microsporophyll bearing a sporangium on the adaxial side of the ligule, _l_, below which is the single vascular bundle and a group of short tracheids at _t_. The sporangia closely resemble those of species of _Selaginella_ and _Lycopodium_ and, as pointed out by Watson[648], they also recall the sporangia of the Palaeozoic genus _Spencerites_. _Bothrostrobus_ is distinguished from _Spencerites_ by the presence of a ligule, by the structure of the axis, and by the different form of the sporophylls. The occurrence of four spores only in the megasporangia is another character in which the extinct type resembles recent Lycopods. It is impossible to decide whether Watson’s cone represents a more or a less primitive type than _Lepidostrobus_: if we accept Professor Bower’s views in regard to the evolution of vegetative organs by the sterilisation of sporogenous tissue, we should probably place _Lepidostrobus_ lower in the series than _Bothrostrobus_; but the greater resemblance between the fertile and vegetative shoots of _Bothrodendron_, as compared with the more pronounced difference in the case of _Lepidodendron_, may be regarded as an argument in favour of recognising _Bothrodendron_ as the more primitive type.

Another possible example of a _Bothrodendron_ cone has been described by Nathorst from Spitzbergen as _Lepidostrobus_ _Zeilleri_[649]; this appears to consist of an axis bearing spirally disposed sporangia without any indication of sporophylls. This strobilus may belong to _Bothrodendron tenerrimum_.

_Pinakodendron_.

The name _Pinakodendron_[650] was instituted by the late Prof. Weiss for a type of stem closely resembling _Bothrodendron_ but differing in the presence of a fine reticulation on the outer bark and in the form of the leaf-scars. Weiss’s genus has been recognised by Kidston in Dumfriesshire but our knowledge of the plant is as yet based solely on a few small specimens.

_Omphalophloios_ (_a genus of uncertain systematic position_).

Figs. 193, C, 217.

This generic name was instituted by White[651] for certain specimens of large stems originally described by Lesquereux from the Coal-Measures of North America as _Lepidodendron mammillatum_ and _L. cyclostigma_. The photograph reproduced in fig. 193, C, for which I am indebted to Dr Kidston[652], represents a specimen described by him from the Upper Coal-Measures of Somerset as _Omphalophloios anglicus_, and identified with _Lepidodendron anglicum_ of Sternberg.

The surface of the impression shown in fig. 193, C, is characterised by clearly defined rhomboidal areas or cushions (fig. 217, E) like those of _Lepidodendron_, except in the absence of a median keel, and similar to those on some forms of _Sigillaria Brardi_. A short distance above the centre of each cushion is an oval or subcordate region bounded by a rim-like margin and containing a small oval scar, presumably that of a vascular strand. A triangular elevation which also shows a small pit (Fig. 217, E, _a_) occurs below the oval area. The appearance of the surface-features varies considerably on different parts of a single specimen. Fig. 217, D, represents one of the numerous figures published by White in his detailed account of the American material. Each cushion bears a widely open =V=-shaped ridge, which is described as a leaf-scar; above this is an oval area (2·5 mm. × 1·75 mm.), the surface of which is bounded by a narrow rim. Within the rim is a smaller concave oval region with a small pit near its upper end.

[Illustration: FIG. 217. _Omphalophloios._

D. After White. E. After Kidston.]

We cannot, in the absence of petrified material, arrive at any satisfactory conclusion as to the meaning of these surface-features. White considers that _Omphalophloios_ is probably a rhizome of one of the arborescent Lycopods, but whether or not this is its true nature must be left for future discoveries. The fact that the rootlet bundles of some Stigmarian axes are accompanied by a parichnos strand, as Weiss has shown, may prepare us for the discovery of surface-features on _Stigmariae_ not unlike those of _Omphalophloios_. (Fig. 193, C.)

A possible comparison may be suggested also with _Sigillaria Brardi_ as figured by Germar (fig. 196, A) in which circular scars, which may be the scars of rootlets, occur below the leaf-base areas. It is not impossible that in the surface-features of _Omphalophloios_ we have both leaf and rootlet scars represented.

_General considerations._

The solid xylem core characteristic of the stele of some species of Palaeozoic Lycopodiales (e.g. _Lepidodendron esnostense_ and _L. rhodumnense_) may probably, as Tansley and Chick[653] point out, be regarded as the lineal descendant of a primitive axial strand of water-conducting elements. In the course of evolution the centre of the tracheal column became partially converted into parenchymatous tissue, as in _Lepidodendron vasculare_. The arrangement of the short cells in regular vertical series is reminiscent of an early stage in the development of tracheae: instead of forming tubular conducting elements the central part of the stelar meristem acquired the short-celled form; some of the cells became lignified as isodiametric storage tracheae while others persisted as thin-walled parenchyma.

The production of secondary xylem and an increase in the girth of the whole stem led to reduction in the amount of centripetally developed conducting channels. Some of these assumed a new rôle and a shape in harmony with their functions. A later stage is represented by a further encroachment of the central parenchyma on the cylinder of centripetal xylem, as seen in _Lepidodendron Harcourtii_ and other species. The next stage is afforded by ribless species of _Sigillaria_ in which the primary xylem is broken up into separate conducting strands. As Kidston[654] reminds us, it is in the geologically more recent species of _Sigillaria_, such as _S. Brardi_, which persist into the Permian era, that this more extreme case of reduction occurs. The older genus _Lepidodendron_ seems to have retained to the last the complete cylinder of primary xylem. In the stele of _Stigmaria_, the rhizome of _Sigillaria_ and of _Lepidodendron_, reduction of the centripetal xylem has passed beyond the stage represented by the broken cylinder of the ribless Sigillarias. With the exception of the examples described by Renault[655] and by Weiss[656], _Stigmaria_ is characterised by little or no centripetal primary xylem. It is, however, noteworthy that Renault’s _Stigmaria_, in which centripetal xylem forms a prominent feature, is attributed to _Sigillaria Brardi_, a species in which the vascular cylinder of the aerial stem illustrates a later and not an earlier phase in the replacement of centripetal by centrifugal wood.

It would seem, as Lady Isabel Browne[657] says, that most Stigmarian axes had reached a more advanced stage in specialisation than is shown in the stelar structure of the aerial shoots. The relatively greater and probably the more precocious development of secondary xylem in _Stigmaria_ than in _Lepidodendron_ or _Sigillaria_ may have some significance in relation to the smaller amount of “old wood[658]” (in a phylogenetic sense) in their steles.

As is pointed out in a later chapter, recent researches into the anatomy of extinct members of the Osmundaceae by Kidston and Gwynne-Vaughan have brought to light a striking parallelism in evolutionary sequence between the Lepidodendreae and the ancestors of _Osmunda_ and _Todea_, the two surviving genera of one of the most ancient families of ferns.

There can be little doubt as to a very close relationship between _Sigillaria_, _Lepidodendron_, and _Bothrodendron_. _Sigillaria_ seems to have outlived _Lepidodendron_ and _Bothrodendron_. The two latter genera are recorded from Upper Devonian rocks in several localities, _Bothrodendron_ being particularly abundant in the pre-Carboniferous floras of Bear Island and other parts of the world. A remarkable stem described by Dr White[659] as _Archaeosigillaria primaeva_ from Upper Devonian shales of New York is spoken of by him as “one of the most highly developed representatives of a fairly distinct archaic group foreshadowing the later genera _Bothrodendron_, _Sigillaria_, _Lepidodendron_ and _Lepidophloios_.” The type-specimen, when first discovered, consisted of an apparently unbranched stem reaching a length of 5 metres. From the swollen basal part Stigmaria-like rootlets spread into the surrounding shale. At a higher level the fissured bark shows indistinctly defined leaf-cushions which pass gradually upwards into cushions and scars arranged in closer order on regular vertical ribs. The surface-features in this region are practically those of a ribbed _Sigillaria_. Traced farther upwards the vertical ribs die out and cushions of the Lepidodendroid form cover the surface of the bark. The leaf-scars, with a supraposed ligular pit and two vertically elongated parichnos-scars, are said to bear a closer resemblance to those of _Sigillaria_ and _Bothrodendron_ than to the leaf-areas of _Lepidodendron_. Nothing is known as to the anatomy of this stem, nor have fertile shoots been discovered. In the absence of more trustworthy evidence than is available conclusions of a phylogenetic nature must be accepted at their true value. It is however legitimate to describe _Archaeosigillaria primaeva_ as one of the oldest examples of a lycopodiaceous plant which shows well-preserved external features, and these are of exceptional interest as indicating a combination of generic characters. This Devonian type lends support to the view that _Lepidodendron_ and _Sigillaria_ are offshoots, differing from one another in comparatively unimportant points, from a common ancestral type.

The generally accepted statement that arborescent Palaeozoic Lycopodiales bore their sporangia on specially modified leaves (sporophylls) grouped in cones which were usually produced at the tip of slender branches, has recently shared the fate of most rules. Prof. Bower in his _Origin of a Land Flora_ mentions a Belgian specimen of _Pinakodendron musivum_ Weiss from the Westphalian series (Middle Coal-Measures), to be described by Dr Kidston, which bore its sporangia “associated with the leaves of certain portions of the stem, without any cone-formation. The fertile and sterile portions are distinguished only by the presence or absence of sporangia[660].”

_Lepidodendron_ and _Sigillaria_ can hardly be claimed as the direct ancestors of any existing type of Lycopodiales, but while exhibiting points of contact with _Lycopodium_, _Selaginella_, and _Psilotum_ they are perhaps more closely allied to _Isoetes_.

Lady Isabel Browne[661], who has recently published an excellent summary of the evidence on the relation of the Lepidodendreae to _Isoetes_, concludes her examination of the arguments by expressing the opinion that there is a strong probability of the correctness of the view that _Isoetes_ may be derived “from the Lepidodendraceae in the widest sense of the word.” This decision seems to me to accord best with the facts.

The further question as to the relation of these Palaeozoic genera to plants higher in the scale must be reserved for fuller consideration in another volume. An attempt will also be made to consider how far anatomical structure may be used as a guide to the conditions under which _Lepidodendron_ and _Sigillaria_ as well as other members of the Permo-Carboniferous floras passed their lives. The secondary xylem of _Lepidodendron_ and _Sigillaria_ affords a striking example of water-conducting tissue of homogeneous structure comparable with the wood of Conifers rather than with that of Angiosperms. It was presumably formed, for the most part, under uniform climatic conditions: the absence of rings of growth points to uninterrupted supply to evergreen shoots exposed to no alternation of activity and arrested growth. Attention has already been called to the absence of any tissue corresponding to secondary phloem. Even in young shoots of _Lepidodendron_, no tissue has been found external to the meristematic zone agreeing in the form of its elements with the channels through which the elaborated food is conveyed from the leaves of recent plants to the regions of cell-building. That the ‘secretory zone’ may have served this purpose, at least in young stems, is not improbable. On the other hand, it is difficult to understand why older Lepidodendron stems show no indication of additions to the secretory zone. If this tissue served for the transport of proteids we should expect to find provision made for its constant renewal _pari passu_ with the secondary growth of the xylem. The conclusion seems to me inevitable that the supply of building-material was otherwise provided for than in recent vascular plants. The physiological division of labour may have been less complete in the tissue-systems of the Palaeozoic Lycopods than in the more highly specialised organs of such an extinct genus as _Lyginodendron_ or than in recent plants. Our knowledge of the anatomical structure of many extinct types has already reached a stage when we should take greater heed of the _modus operandi_ of the complex machinery revealed by a study of petrified stems. From the known we proceed to interpret the unknown; but there is a danger of neglecting the possibilities of evolution during the countless ages which separate the forests of the Coal period from those of the present era. We may easily allow preconceived ideas to warp our judgment in attempting to distribute the manifold activities which made up the life of a _Lepidodendron_ among the structural units of the plant-body.

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