Lepidodendron

“ I may remark, however, that in its scalariform vessels it agrees with all the fossil genera supposed to be Acotyledonous. In the structure of its sporangia and sporules it approaches most nearly, among recent tribes, to Lycopodiaceæ, and Ophioglosseæ; and among fossils, no doubt, to Lepidostrobus, and consequently to Lepidodendron.
The stem structure of Lepidodendron, known to me only in one species, Lepidodendron Harcourtii, offers no objection to this view, the vascular arrangement of the axis of its stem bearing a considerable resemblance to that of Triplosporite. ”

“ In the genus Lepidophloios the leaf-cushions are more prominent than in Lepidodendron, and their greatest diameter is in the transverse direction; on the older stems the leaf-scar lies towards the lower side of the cushion. The genus Bothrodendron, going back to the Upper Devonian, differs from Lepidodendron in its minute leaf-scars and the absence of leaf cushions, the scars being flush with the smooth surface of the stem. ”

“ In the ancient Lepidodendron, on the other hand, the more ancient the scale of the leaf-stalk, the more apparent it still remains. Portions of stems have been discovered which contain leaf-scars far larger than those referred to above, and we deduce from these fragments the fact that those individuals which have been found whole, are not by any means the largest of those which went to form so large a proportion of the ancient coal-forests. The lepidodendra bore linear one-nerved leaves, and the stems always branched dichotomously and possessed a central pith. ”

“ The leaf-cushions of Lepidophloios differ from those of the true Lepidodendron in their relatively greater lateral extension (cf. fig. 146, A and C) , in their imbricate arrangement and in bearing the leaf, or leaf-scar, at the summit. In some species referred to Lepidophloios the cushions are however vertically elongated and in this respect similar to those of Lepidodendron: an example of this type is afforded by Lepidophloios Dessorti a French species described by Zeiller [256] . In younger branches the cushions may be directed upwards having the leaf-scar at the top ”

“ In external form as in internal structure it is often impossible to distinguish between the leaves of Lepidodendron and Sigillaria. The distinguishing features enumerated by the late M. Renault cannot be employed, with any great degree of confidence, as diagnostic characters. In transverse section the lamina of a Lepidodendron leaf presents the same appearance as that of the Sigillarian leaves represented in fig. 142. Near the base the free part of the leaf is usually sub-rhomboidal in section with short lateral wings, a ventral keel and two stomatal grooves (fig. 142, A, B, g) . ”

“ Among the best known plants in the Palaeozoic floras are the genera Lepidodendron and Sigillaria, types which are often spoken of as Giant Club-Mosses or as ancestors of existing species of Lycopodium and Selaginella. Of these genera, but more particularly of Lepidodendron, we possess abundant records in a condition which have made it possible to obtain fairly complete information not only in regard to habit and external features but as to the anatomical characters of both vegetative and reproductive shoots. ”

“ Lepidodendron Sternbergii (Fig. 43) is represented as it is found beneath the shales in the collieries of Swina, in Bohemia. Fig. 46 represents a portion of a branch of L. elegans furnished with leaves. M. Eugene Deslongchamps has drawn the restoration of the Lepidodendron Sternbergii, represented in Fig. 47, which is shown entire in Fig. 44, with its stem, its branches, fronds, and organs of fructification. The Ferns composed a great part of the vegetation of the Coal-measure period. ”

“ As the stem of a Lepidodendron grew in girth the leaf-cushions became separated by intervening depressions composed of the secondary cortex formed external to the phellogen, but at a later stage the cushions were thrown off, leaving the outer edge of the phelloderm as the superficial tissue. ”

“ The tendency of the primary xylem strands to become united laterally, forming broader bands, was first described by Solms-Laubach [522] in a French specimen of Sigillaria spinulosa in the Williamson collection. The leaf-traces arise from the middle of the concave outer face of the primary xylem groups. The inner cortex is composed of small parenchymatous cells as in Lepidodendron,220 and it is noteworthy that traces of partially disorganised tissue, described as large canals, in the region external to the secondary wood, bear a resemblance [523] to the secretory tissue of Lepidodendron. ”

“ In a Lepidodendron stem embedded in sediment the more delicate portions of the leaf-cushions would tend to shrink away 128from the internal and more resistant tissues of the outer cortex, thus producing spaces between each cushion; further decay would cause rupture of the leaf-traces and the superficial tissues would thus be separated from the rest of the stem. The tendency of Lepidodendron stems to split along the line of phellogen in the outer cortex is seen in fig. ”

“ 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. ”

“ Specimens that are now recognised as casts of stems from which the cortex had been more or less completely removed before preservation, were originally described under distinct generic names, such as Bergeria, Knorria and others. These are now known to be imperfect examples of Sigillarian or Lepidodendroid plants. Grand’Eury [146] quotes the bark of Lepidodendron Veltheimianum Presl. as a fossil which has been described under twenty-eight specific names, and placed in several genera. ”

“ They often form the nucleus of concretionary balls of clay-ironstone, and are well preserved, exhibiting a conical axis, around which a great quantity of scales were compactly imbricated. The opinion of M. Brongniart that the Lepidostrobus is the fruit of Lepidodendron has been confirmed, for these strobili or fruits have been found terminating the tip of a branch of a well-characterised Lepidodendron in Coalbrook Dale and elsewhere.
Equisetaceæ.—To this family belong two fossil genera of the coal, Equisetites and Calamites. ”

“ 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. ”

“ In describing some Belgian impressions of Devonian age as Lepidodendron gaspianum Daws. Crépin [72] states that Carruthers has come to regard the specimens named by him Psilophyton Dechianum as branches of a Lepidodendron; he also quotes Carruthers as having expressed the opinion that the name Psilophyton had been employed by Dawson for two kinds of fossils, some being twigs of Lepidodendron while others, identified by Dawson as the reproductive branches of species of Psilophyton, represent the spore-cases of ferns comparable with Stur’s genus Rhodea [73] . ”

“ Within the compass of a text-book it is impossible, even if it were desirable, to include an account of the majority of the species of the widely distributed Palaeozoic genus Lepidodendron. In spite of the great number of known species of this common member of Carboniferous floras, our knowledge of the type as a whole is deficient in many points, and such information as we possess needs systematising and extending by comparative treatment based on a re-examination of available data. ”

“ These are Lepidodendra, gigantic [104] club-mosses, which were developed to still greater dimensions in the coal period. Near these we may see a still more curious tree, more erect in its growth, with rounded and somewhat rigid leaves and cones of different form, and with huge cable-like roots, penetrating the mud, and pitted with the marks of long rootlets. This is Cyclostigma, a plant near to the Lepidodendron, but distinct, and peculiar to the Devonian. Some of its species attain to the dimensions of considerable trees; others are small and shrubby. ”

“ 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. ”

“ The annexed figures represent a large fossil Lepidodendron, 49 feet long, found in Jarrow Colliery, near Newcastle, lying in shale parallel to the planes of stratification. Fragments of others, found in the same shale, indicate, by the size of the rhomboidal scars which cover them, a still greater magnitude. The living club-mosses, of which there are about 200 species, are abundant in tropical climates, where one species is sometimes met with attaining a height of 3 feet. ”

“ The species of Lepidophloios are readily distinguished from Lepidodendron by the form of the areoles, and by the round scars on the stem, which usually mark the insertion of the large strobiles, though in barren stems they may also have produced branches; still, the fact of my finding the strobiles in situ in one instance, the accurate resemblance which the scars bear to those left by the cones of the red pine when borne on thick branches, and the actual impressions of the radiating scales in some specimens, leave no doubt in my « 166 » mind that they are usually the marks of cones ”

“ They are usually ranked with Lepidodendron and Ulodendron, but sometimes, and probably with greater reason, are regarded as allied to tree-ferns. At the Joggins a very fine species (M. magnificum) has been found, and at Sydney a smaller species (M. humile) ; but both are rare and not well preserved. If the large scars bore cones and the smaller bore leaves, then, as Brongniart remarks, the plant would much resemble Lepidophloios, in which the cone-scars are thus sometimes distichous. But the scars are not round and marked with radiating scales as in Lepidophloios ”

“ In the case of specimens showing prominent tubercles, the superficial tissues are usually absent and, as in the fossil represented in fig. 161, the name Halonia does not necessarily imply the presence of leaf-cushions of a particular type.
vii. Anatomical characters of Vegetative Lepidodendron shoots (Lepidodendron and Lepidophloios) . ”

“ In specimens which have lost more of the cortical tissues the surface is characterised by spirally disposed, discontinuous vertical grooves representing portions of leaf-traces precisely as they appear in similar casts of Lepidodendron. There is no direct evidence of the existence of secondary wood in the stem, but, as Potonié has pointed out, the greater transverse elongation of the leaf-scars in the lower part of a cast (fig. 134, A) points to the production of some secondary tissue either in the vascular cylinder or cortex, or possibly in both regions. ”

“ The vessels are exclusively scalariform, very closely resembling those of the recent Ferns and Lycopodiaceæ; and among fossils, those of Psarolites, Lepidodendron, and its supposed fruit, Lepidostrobus, as well as several other fossil genera; namely, Sigillaria, Stigmaria, Ulodendron, Halonia? and Diploxylon.
The coat of the sporangium appears to be double; the outer layer being densely cellular and opake, the inner less dense, of a lighter colour, and formed of cells but slightly elongated. ”

“ It is, however, in their relationship with modern plants that they are principally interesting. In the moist woods of New England, and farther south along the summits of the Alleghany Mountains, there lingers a group of little plants, called Lycopods, ground-pines or club-mosses, that must be regarded as the nearest living relatives of Lepidodendron. The habit of growth is very much the same; the mode of fruiting is almost identical; the little spores are produced with the same extravagant copiousness, and, being resinous, are highly inflammable. ”

“ The lamina has a well-defined median keel on the lower surface and on either side a groove in which sections of petrified leaves have demonstrated the occurrence of stomata (cf. fig. 142) .
ii. Leaves and Leaf-cushions.
All Lepidodendron leaves, so far as we know, possessed a single median vein only. In some species, as for example in Lepidodendron longifolium Brongn., they have the form of long 98and slender acicular needles very similar to those of Pinus longifolium; in L. Sternbergii (fig. 141) they are much broader and shorter. ”

“ Nor does it seem improbable, that what occasionally takes place among the higher plants of the present time may have been common among some of the comparatively low plants of so ancient a period as that of the Middle Old Red Sandstone. This formation of the gray tilestones has furnished one vegetable organism apparently higher in the scale than those just described, in a well marked Lepidodendron, which exhibits, like the Araucarian of the Lower Old Red, though less distinctly, the internal structure. ”