ON THE STRUCTURE AND AFFINITIES OF LEPIDODENDRON AND CALAMITES.

BY WILLIAM CARRUTHERS, F.L.S., BOTANICAL DEPARTMENT, BRITISH MUSEUM.

(PLATES LV., LVI.)

The imperfect knowledge we have of fossil plants is the result of the fragmentary condition in which they occur. The deciduous leaves, ripe fruits, or broken branches that fell into streams, and were carried to sea or lake, had so many dangers to encounter, that only a very few of them ever reached the usual deposit where they would be preserved, and these few in such a decayed and fragmentary condition that it is often impossible to do more than make the most vague guesses at the nature of the vegetation to which they belonged. The occurrence of vegetable remains on the site where they grew, is extremely rare in all the formations which form the crust of the earth, except in the coal-measures. The plants of this period might therefore be expected to be well known, especially as the beds containing vegetable remains, of carboniferous age, have been more exposed, because of their economic value, than those of any, or indeed of all the other formations put together. The peculiar nature of the vegetation, and perhaps the extreme humidity of the atmosphere, and the swampy localities in which the plants grew, have made the superabundant mass of vegetable remains as great a mystery as the scanty and fragmentary fossil plants of other periods. Except in the thin films of charcoal which occur in most coals, traces of structure are scarcely to be found in the coal itself, so thoroughly has the vegetable matter been converted into amorphous pulp before mineralization took place, or so completely has it been metamorphosed subsequent to deposition. The plants themselves have all been so brittle, that when portions are preserved, as they are in immense quantity, especially in the roof shales, they are so fragmentary, that it is difficult to determine the various portions that belong to the same plant. The root is rarely connected with the stem, the stem with the branches, or the branches with the leaves or the fruit. As a result, all these parts have been often referred to different genera, and have received different names. With additional observations, the means are, however, occasionally turning up, which enable us to reduce some of these genera, the VOL. IV. [NOVEMBER 1, 1866.]

establishment of which was absolutely necessary in the earlier days of palæontological botany. Thus, to give an example:-the trees belonging to the same set as those which were found imbedded in the sandstones at Craigleith quarries have been constituted into the genus Dadoxylon; the pith forms the genus Sternbergia, and some fluted and constricted specimens have been referred to Calamites. The leaves were considered to be ferns, and named Cyclopteris; and the fruit was thought to belong to a Palm, and received the name of Trigonocarpon. We have not seen evidence sufficient to convince us that all these are correctly referred to the same plant; but this is the opinion of some distinguished palæontologists, and it serves as a good illustration of the present satisfactory tendency of palæontological botany.

A similar multiplication of generic names encumbers the synonymy of the two genera Lepidodendron and Calamites.

Lepidodendron was a branching tree of considerable size. It is separated from the other genera of coal plants by the form and arrangement of the leaf-scars upon its stem. More than forty species have been recorded; but as the scars present different appearances on different portions of the same plant, no doubt more species have been established than the materials fairly warrant. But that they were numerous in species, and very numerous in individuals, any one who has even cursorily examined a coal-pit, or the fossils in any public museum, must be convinced. They certainly contributed largely to the formation of coal.

The researches of Witham,* Lindley and Hutton,† Brongniart‡ and Binney, § have made us acquainted with the stem. These published *The Internal Structure of Fossil Vegetables,' 1833.

The Fossil Flora of Great Britain,' 1831-1837.

Observations sur la Structure intérieure du Sigillaria elegans, etc.'— Archives du Muséum, 1839.

§ Geological Society's Journal,' 1862, and 'Philosophical Transactions,' 1865. Mr. Binney, in these papers, gives most careful and elaborate drawings and descriptions of some fossils in his extensive collection. He refers them to the genus Sigillaria, because of their agreement in internal structure with Brongniart's S. elegans; but he cannot separate them by their external markings from Lepidodendron selaginoides, Lindl. and Hutt.; and as the only characters by which the two genera are distinguished are derived from the markings on the stem, we must consider Sigillaria vascularis as a true Lepidodendron. I am the more satisfied as to this, because I believe no essential difference exists, as has been hitherto maintained, between the stems of Sigillaria and Lepidodendron, or any of the other lepidodendroid plants of the coal period. I cannot enter into this question here, but I shall take an early opportunity of publishing my views, and the reasons for maintaining them.

observations, together with the examination of some beautiful specimens in the collections of Robert Brown, now in the botanical department of the British Museum, and of Mr. Alexander Bryson, enable me to give a somewhat complete description of its singular structure.

The axis of the stem cannot be considered as a true medulla or pith, inasmuch as it is composed not of simple cells, but of elongated utricles of various sizes, irregularly arranged, and having thin walls marked with scalariform bars (t. 56, f. 2). These utricles, indeed, differ from the vascular tissue of the woody cylinder which surrounds them only in their length. The tissue of the woody cylinder consists of long scalariform vessels, which increase in size from the inner margin to the outer, this increase being sufficient to meet the requirements of the enlarged circumference, with the help of only a few additional series of vessels. As there is no true medullary cellular tissue in the axis, so there are no medullary rays passing through this cylinder. In radial sections an appearance is seen singularly resembling, to the naked eye, the "silver grain" produced in dicotyledonous woods by the medullary rays; but this arises from a very different The diameter of the vessels is so great, that on a polished surface only the scalariform wall of the vessel, that lies on or near the surface, is exhibited; and when the upper wall of a vessel is cut away, the lower wall is often so deeply buried in the opaque substance, that the peculiar structure is obscured. In the case of sections prepared for microscopic examination, both surfaces of some vessels are often removed, and the scalariform markings on the lateral walls, or on any horizontal walls which by chance occupy a medial position between the polished surfaces, only are seen. This absence of the scalariform bars gives at first sight the appearance produced by medullary rays.

cause.

The continuous cylinder of scalariform vascular tissue appears to be penetrated by the vascular bundles which ultimately supply the leaves. These bundles apparently originate either in the scalariform tissue of the axis, or on the inner surface of the woody cylinder. They have been mistaken for, or misnamed, medullary rays.

The woody cylinder is surrounded by a great thickness of cellular tissue, which extends to the exterior of the stem, and is composed of three distinct and separable zones. The inner zone has never, as far as I know, been perfectly preserved in any specimen, yet traces of it sometimes may be seen; and it is rightly restored in Brongniart's

drawing of Lepidodendron Harcourtii, in the Archives du Muséum,' vol. i. plate 31. Its absence in fossils is owing to its extremely delicate structure. The cells of the middle zone have thicker walls, and they have consequently frequently resisted decomposition before fossilization made them permanent. In the outer zones the cells are very much lengthened, and have a smaller diameter. They nearly resemble true vascular tissue; but the progress of lengthening may easily be traced from the interior outwards, and no distinction can be drawn between the true cells, and the long and slender ones of the outer circumference. The cell-walls of all the three zones are without

markings of any kind.

These three cellular zones are traversed by the vascular bundles which rise from the outside of the interior woody cylinder, if they do not actually pass through it, and pass to the leaves and branches. These bundles separate from the woody cylinder a long way below the point where they pass off into the leaf. At first their direction is almost parallel with the cylinder, slightly inclining outwards; they then incline more outwards, and as they approach the circumference of the stem, they resume their nearly ascending direction for some distance, until they finally pass out to the leaves which they support. Each bundle consists of scalariform vessels, very much finer than those of the woody cylinder, surrounded by elongated cells like those of the outer zone, and probably still further enclosed by a delicate parenchyma, which disappeared before it could be fossilized. The only evidence I have of the existence of this cellular tissue is, that the bundles never fill the cavities in the parenchyma of the stem through which they pass. The bundles terminate in the points seen on the areoles of the stem, which are the scars of the leaves.

The woody cylinder is of different thicknesses in different stems, and appears to have increased with the growth of the tree. There is, however, no indication of interruption in the growth or of seasonal layers. Yet it cannot be conceived that the whole vascular cylinder arose and was developed at the same time. It is very probable that the zone of slender, and consequently rarely preserved cellular tissue which surrounded the woody cylinder, was analogous in its functions to the cambium layer of phanerogamous stems, like the similar layers in recent Lycopodiacea, described by Spring in his Monographie de la Famille des Lycopodiacées" (page 294).

If we separate the different structures we have described in the axis into two series, the one series axial, and the other epidermal, we shall have the axis composed of scalariform utricles, the woody cylinder and the vascular bundles passing to the leaves belonging to the first series, and the two external zones of the vascular tissue to the second. The inner zone of cellular tissue, like the cambium layer, was most probably common to both series, the cells of the outer circumference being developed into the parenchyma of the epidermal series, while the vessels of the woody axis were produced from the cells of the inner series.

Stigmarioid roots have been determined to belong to Lepidodendron as well as to Sigillaria, and their whole structure supports this determination. I have satisfied myself that there is nothing that can be truly called a medullary ray in the woody cylinder of Stigmaria, but into the proof of this I will not now stay to enter. The base of the trunk was divided into a few principal roots, and these again divided dichotomously, but the ultimate divisions were never much attenuated. Throughout their whole course, and from every portion of their circumference, they gave off rootlets of considerable length, which, with the exception of a slender vascular bundle, were entirely composed of delicate hexagonal cells. They were articulated to flagon-shaped bodies sunk in cavities, arranged in a quincuncial manner over the stem. The internal structure of the Stigmaria root corresponds to that of the trunk of Lepidodendron. The axis was composed of fusiform barred cells, and this was surrounded by a woody cylinder, which was certainly penetrated by the vascular bundles that supported the rootlets. Beyond the woody cylinder came a great thickness of cellular tissue, almost always destroyed, but probably agreeing in its structure with the three zones of the stem.

In speculating upon the conditions under which the forests of Lepidodendron flourished, it is most important to observe whatever is peculiar in those organs by which the plants were connected with the physical conditions around them. Geologists have too much overlooked such considerations in their deductions as to the physical phenomena of a period from the plants and animals that then existed. They have often taken for granted that the known conditions of the living species of a genus are true also of the fossil members of the same genus. In the want of other evidence, such an assumption may be cautiously employed; but unless its true value be accurately estimated,