lated, layer over layer and growth over growth, until the mass Lewistown, Mifflin County, Pennsylvania, ART. IX. Remarks and Illustrations on the Decay of the Stems of succulent Plants. By FREDERICK C. LUKIS, Esq. THE fossil vegetables which are found in the secondary formation, designated as coeval with the consolidation of the sandstone and coal measures, present to the eye of the observer patterns of great beauty and regularity, and display figures which have been often compared with the eccentric ornaments of ancient architecture. Without entering into the general geological hypothesis, much may be done, in the study of these interesting remains, by close investigation and analogy; and, indeed, the nature of fossil vegetables must receive its principal elucidation from the careful examination of existing species analogous to them, when they can be found. The physiological botanist is well aware that plants in decay are disposed to shrivel up, each in a manner peculiar to its own class, and that this operation is governed by their internal as well as external organisation, and by the circumstances under which they may be placed. As the greater number of our fossil plants have been referred to the ferns, cactuses, and other succulent analogues, it is among these we are to look for characters of resemblance; and although many of these remains possess dimensions which baffle our conceptions, when brought in comparison with our pygmy tribes of the present economy, still the latter, insignificant as they appear, may afford us physiological types sufficiently clear to bring us near the truth. A series of observations made on the drying and decomposition of succulent plants has occupied my attention for many years, and I propose to notify some of the changes remarked in the stems and branches of the Sempervivum arboreum L., or tree houseleek, as it is called, in the course of their decay. They will suffice to show the variety of patterns which may be produced, under favourable circumstances, by a single individual of the succulent class. The result of decay in stems of this plant has been the production of exterior markings, presenting a strong analogy to those of fossil plants; and I may venture to say that, if casts had been taken from them, they would have been mistaken for real specimens of these interesting remains. The plants, when chosen for examination, were old, and of extremely luxuriant growth. The stems had acquired the size of a man's arm, and were about five feet in height. They were withdrawn from the hot-house during a sharp frost, by which they were soon killed. The leaves dropped off, leaving the rudiment more prominent and perfect than when their removal happened by natural decay. After the death of the plants, they were kept in the mould they occupied, and placed in a shaded situation, where decomposition was allowed to proceed slowly for the space of two years. During dry seasons they were occasionally watered, and the upper part of the stems was scooped out so as to admit moisture in the interior soft parts when required: thus a partial fermentation was continued until the medullary substance became dissolved. The first change observable in the appearance of the epidermis was a disposition to form regular ridges or risings in the interstices between the marks left by the leaves and subspinous nerve near them, preserving a reticulated pattern. The hardness of the ligneous centre resisted decay for a long time, and allowed the cortical covering to form its pattern with considerable exactness; and at last, by its own shrinking, and the want of farther resistance from the nerve of spine and leaf, the covering contracted in a longitudinal direction, leaving the end of the wood projecting beyond the upper part of the plant. The following drawings of the plants, taken at different stages of decay, will be better understood than a lengthened description. Fig. 7. represents the common appearance of the stem, deprived of its leaves, before decay; the tubercle and rudiment of the leaf, in quincuncial order, scarcely rising above the epidermis. Fig. 8. This figure shows the first angular depression round the tubercle and leaf, caused by the shrinking of the interior parts of the plant: the dried state of the cortex opposing the contraction of its circumference. Fig. 9. is part of the same stem, contracting more equally downwards, it having been more under the influence of moisture than the lower end. It had now commenced a wavy and rhomboidal cancellated figure, the elevated ridges preserving an almost united wavy line from top to bottom. Fig. 10. The same as the last figure, having now attained VOL. VII.- No. 37. D a more decided pattern, by the completing of the above-stated rhomboidal depressions, which had been assisted by a longitudinal cut of a knife, and the careful removal of the woody Fig. 11. Other parts of the plant produced this pattern. It appeared that the spinous processes in the interior had resisted the vertical contraction of the cortex. Here the spaces retained an oblong form, having the elevated ridges prominent, though not united throughout their diagonal direction, but breaking into distinct approximate compartments, much in the same manner as those in the Phytolithus cancellatus. (fig. 21.) Fig. 12. The most general appearance of the plants, where contraction was carried on more equally from both ends of the branch. The elevated ridges surrounding the compartments had now become dry and hard, and might be said to have arrived at the state of hay. Fig. 13. is a vertical section of a branch, to show the ligneous centre, or woody stem, in a fresh state. The Fig. 14. The same after decay had commenced. woody centre, in contracting, had produced lengthened lozengeshaped elevations, retaining the spinous processes rather prominent. Fig. 15. A longitudinal section of a branch, to show the disposition of the medulla, or pith, in the ligneous centre, where it forms transverse cells: the cortical cells or layers, running in a vertical manner, are traversed by the spinous nerve which produced the elevations in the epidermis. Fig. 16. represents the base of the stems of old plants. The epidermis becomes hard and leather-like, and is generally ruptured by the enlargement of the subspinous elevations. In the case under consideration, it formed lozenge-shaped openings round each tubercle, and retained a slight remain of the impression left by the original leaf across the tubercle. Fig. 17. In this last figure decomposition had affected the interior so as to enable the cortical covering to sink between the stiff prominent spines, which resisted decay with greater obstinacy; thus forming rhombic compartments in a reversed order to what had taken place in the younger branches, as seen in fig. 11. A greater variety of figures might have been produced, but as many were the same in character, under a few modifications, and in progress of decay, I have chosen the most decided patterns only. Some interior changes in the epidermis, cutis, and ligneous centre presented very regular markings during the progress of decomposition and desic cation. Fig. 18., taken from the inside surface of the epidermis of the largest stem, will suffice to show the various changes exhibited by a single plant: in this case the cortical parts, under the epidermis, had separated in macerating, and left it at liberty to produce this pattern, although the exterior surface was not much altered from the rest of the plant. In this figure the hollows were numerous and deeply marked. Over each a transverse, sharp, wavy, elevated line passed in regular succession; undoubtedly formed after its separation from the rest of the plant. These lines were not marked on the outside of the epidermis, nor were they at all visible on the remains of the soft part which had been in contact with it. Fig. 19. is a portion of the stem, or lignum, from which, when perfectly dry, its bark has been removed by peeling. It formed a hard and tough cylinder, of moderate thickness, the longitudinal striæ being only interrupted by the slightly elevated spots which held the spinous nerves as they emerged from the wood, and then passed through the bark, where they formed the external ruga. That part of the bark which immediately covered the wood retained the same impressions on its surface, but that in contact with the epidermis had large tubercles corresponding with the outside of the plant; but in no case did this portion of the bark exhibit a pattern dissimilar from it. In fossil plants it has been remarked that the same species may appear under three different states; such as are produced by the epidermal, cortical, and ligneous configurations: the first and the last differing much from each other; and, as Mr. Parkinson states, "it is only close observation that determines that it originates from the same plant." Assuming that fossil succulent plants have undergone changes similar to those here represented, either before they were lodged in their matrix, or when so embedded, it will be easily perceived that one species may appear under several aspects difficult to determine. It is not improbable that, during the progress of maceration in the matrix, a variety of circumstances may have occurred to favour these changes and variations. In contrasting these observations with what is visible in the submarine peat found in these islands, many indications point out a similar decomposition to be going on; and although these trees and vegetables have been buried many centuries, and are of a different description from those under notice, yet they exhibit changes nearly as difficult to recognise. |