fect hexagonal cells are seen, but in another immediately contiguous to it, the cell-walls have been absorbed on two opposite sides, and parallel laminæ result.

In certain plants, especially in the grasses, canes and Equisetacea, the outer framework, which may be truly named the dermal skeleton, is composed of silica, intimately blended with the organic tissue; so that if we subject any of these plants to the action of heat, or boiling nitric acid, the original structure of the tissue remains undestroyed. Silica is most abundant in the canes, but it occurs also, in very large quantities, in the husks of Rice and Wheat, in both of which it not only is met with in the epidermoid cells, but also in the spiral vessels and woody fibres. In the Equisetum (especially Equisetum hyemale), known in commerce as the Dutch Rush, the silica is so abundant, and imparts so great a roughness to its exterior, that it is largely employed by cabinet-makers and sculptors, as a substitute for sand-paper. In the stems of the common grasses of this country the silica is accumulated in the epidermis, and as the stems contain potass and lime, when hay- or corn-ricks are burned, masses of a brittle glassy substance, are often found among the ashes. It is indeed possible, by carefully burning a straw before the blow-pipe, to produce a bead of glass. It will be remembered that the presence of silica in plants was fully considered in the preceding course of Lectures, when it occupied our

attention as a cell-secretion; but I now allude to it again as forming the skeleton of many plants.

Another part of a plant sometimes called the skeleton, is that met with in the leaves, which after maceration and cleansing exhibit a most beautiful lace-like structure, being particularly evident in most of the species of Magnolia and Fig. Many very beautiful specimens of this kind were to be seen in the Great Exhibition, and a large collection of skeleton leaves, prepared in the same way, was purchased many years ago by the Council of this College from Dr. Buchan, the well-known author of "Domestic Medicine." These skeletons, however, are composed of bundles of woody fibres and vessels, and contain no earthy ingredient.

In plants, as I have before stated,* inorganic salts occur in a crystalline form, under the name of raphides; these, however abundant, may be regarded as accidental deposits, since it has been shown that they can

FIG. 2.

be produced by artificial means. For the benefit of those who may not have been present on former occasions, I will give a few examples of the dermal siliceous skeleton of plants.

The first specimen is a por

A portion of the cuticle of tion of the Equisetum hyemale, Equisetum hyemale, after long boiling in nitric acid. Fig. 2, which has been boiled

* Histological Lectures, Vol. I, p. 42.

FIG. 3.

FIG. 4.

b

for a long time in nitric acid, and not only exhibits the cells of the cuticle, with their serrated edges, but also longitudinal rows of oval bodies, which are the stomata. Another good example is a portion of the husk of the Wheat, Fig. 3, in which, in addition to the cells of the cuticle, the spiral vessels, recognized by the coiled-up fibre, also have a skeleton of silica. In the husk of the Rice the peculiar cells of the cuticle are seen, with bundles of woody fibre and vessels below them. The specimen is composed entirely of silica, and there may be noticed in one spot, where the cuticle has been torn, a series of elongated fusiform bodies, with serrated edges, Fig. 4, which are all that remain of the woody fibres, proving that in this plant the silica is not confined to the cuticle. All the fibres, however, are not thus serrated; some, as represented at b, may be seen in bundles, which are both longer and thinner than the first mentioned, with perfectly smooth edges.

Portions of woody fibres from the husk of the Rice.

On the upper surface of the leaf of a plant common in our gardens-the Deutzia scabra-there are numerous stellate hairs, which much resemble Star-fishes in miniature, Fig. 5; these are covered with little tuber

FIG. 5.

Siliceous cuticle

from the under sur

face of the leaf of

Deutzia scabra.

cles, each star being attached to the cuticle by its centre. If the cuticle be removed, and boiled in nitric acid, the stellate hairs may be as plainly seen as in the natural condition of the leaf; the crenated lines found in all parts of the object representing the cell-walls of the cuticle. This specimen will serve to show, which it does in a striking manner, that silica is not confined to the cells of the cuticle, but is equally abundant in the hairs and spines developed upon it.

Such, then, is a slight sketch of the siliceous skeleton of vegetables; there are, however, some few plants in which lime is said to perform the same office as silica. One of these is our common Dianthus, in which the lime occupies the interior of cells. I cannot quit this subject without briefly noticing the Lithophytes, or Stone-plants, with which are included our common Corallines and Nullipores. These plants are remarkable for the large quantity of calcareous material, arranged in a smooth and regular form upon their external surface. The Corallines were originally considered as animals, and polyp cells have been described by Ellis

and Lamarck as existing upon their outer surface; such, however, is not the case, the calcareous material existing in the form of a coating of variable thickness to a mass of cells, evidently of a vegetable nature. If a transverse section of one of these plants be made, the interior will be found composed of a series of cells, some of which contain a green colouring matter, like chlorophylle, analogous in character to starch.

In the Nullipores, which are the densest form of Lithophytes, all the intercellular spaces are filled with lime, so that it would appear that the external surface of the cell-wall possesses the power of separating lime from the sea-water, and arranging it in a certain definite form; in the Corallina officinalis every joint is uniformly coated, and we never meet with anything like an exostosis, or greater abundance of the lime in one part than another, neither are the joints consolidated, but always remain free and flexible. This I consider to be a totally distinct process from that occurring in the common Chara, in which the lime is not unfrequently in a crystalline condition, and also from that termed petrifaction; because, in both these cases, the deposit is continuous, and often so extensive that all trace of vegetable, or filamentous structure is entirely lost, and the mass becomes as solid as stone, whereas, in the Nullipores and Corallines, each filament preserves its original character: the first being a vital, the second an accidental process. As the Corallines and Nullipores have been, and are even now, associated