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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 cause. 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.
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, the greatest errors may arise, as they have in the past. For example, the systematic position of Elephas primigenius having been clearly established, the inference was thought legitimate tliat, as the modern representatives of the genus were confined to tropical or subtropical countries, the boreal regions must have enjoyed a similar climate when they were inhabited by these ancient elephants. It was, however, discovered that their skin was clothed with wool and long hair, and that, consequently, they were adapted to endure a cold climate. In plants, the structure of the fruit would in most cases teach nothing as to the temperature and humidity of the atmosphere in which, or the kind of soil upon which, the organism grew, though it would be of the first importance in determining systematic position. On the other hand, the root, the leaves, and the tissues of the plant, would be of only secondary importance in regard to systematic position, but of the highest value in determining physical condition. In regard to Lepidodendron, its singular roots would seem to imply that it derived a large amount of moisture through them from a moist soil, and so far differed from most living cryptogamia, which obtain it mostly from the atmosphere. The roots of this genus presented in their crowded and long rootlets an immense surface for the absorption of moisture; and in their great abundance of lax cellular tissue possessed the means of containing this moisture, and transmitting it to the foliage.
The leaves of Lepidodendron were simple, lanceolate, acute, and sessile. They had a single medial nerve. The younger branches were densely covered with leaves ; and the scars left on the trunk after they perished, give the numerous beautiful markings by which the species have been distinguished. The leaves when found separated from the branches, are called Lepidophylla.
The fruit was a strobilus (t. 56, f. 3), formed from a shortened branch, the leaves of which are converted into scales, that support on their upper surface a single large sporangium (Lepidostrobus, t. 56, f. 4), or perhaps several small ones (Flemingites, t. 56, f. 6). There appear to be both macrospores (t. 56, f. 5) and microspores in the saine sporangium. I have examined at length the structure and affinities of these fruits, in a paper published in the Geological Magazine, vol. ii. p. 433, to which I must refer, without here dwelling further on the subject. Flemingites, although the sporangia are enormously abundant in some coals, have not yet been found connected with any fossil ; but specimens of Lepidostrobus, attached to branches of Lepidodendron, have been described by Dr. Paterson, Brongniart, and others; and I have noticed a fine specimen in the Museum of the Edinburgh Botanic Gardens, and others exist in the collections at the British Museum and elsewhere.
In tracing the affinities of Lepidodendron, we have the safest guide in the organs of fructification, and fortunately these have been satisfactorily determined. The sporiferous strobilus shows that it is a true cryptogam; and in general appearance and arrangement of parts, the strobilus can scarcely be distinguished from that of some living Lycopodia, except in the great difference of size; this affinity is strengthened by the character of the leaves, and the structure of the stem. But the possession of both kinds of spores in the same sporangium exhibits stronger affinity to Rhizocarpea than to Lycopodiacee.
The structure of the arboreal stem of Lepidodendron is much more complex than that of any known cryptogam. "The central axis of irregularly-arranged vascular tissue in Lycopodium is suited to the low stature of the plants of that genus ; but in the giant Lepidodendron there is a complexity, which approaches the structure of some dicotyledonous stems. The general arrangement of the tissues, resembling what exists in some Cycadeæ and Cactacea, has caused this fossil plant to be referred sometimes to the one, and sometimes to the other of these Orders; but the resemblance is only one of analogy, and not of affinity. The presence of scalariform tissue, of which the woody portion is entirely composed, and the absence of medullary rays, would, even if the fruit were unknown, be sufficient to establish the cryptogamic nature of the plant. A comparison with the Cycadean stem may help us, by the resemblances and differences which will appear, better to understand the stem of Lepidodendron. The Cycads have all a large medulla, composed of large-sized parenchyma ; in some genera traversed by numerous vascular bundles, as in Encephalartus, and in others entirely cellular, as in Cycas and Zamia. This is surrounded by a single woody cylinder, or several, everywhere penetrated with medullary rays. Beyond this there is a considerable thickness of parenchyma, composed, in their inner portion, of cells whose length exceeds only slightly their breadth ; these gradually lengthen, until they assume an appearance very like the external portion of Lepidodendron. This cortical parenchyma is traversed by the vascular