extremely minute conglomerated crystals, which are rectangular prisms with tetraedral summits, some with a square, others with an oblong base. Crystals of a similar figure have been remarked by the same observer in Rheum palmatum; and their presence, according to him, is sufficient to distinguish samples really from China and Turkey, from those produced in Europe. The former abound in these crystals, the latter have hardly any.

In the above figure, 15 represents the raphides of Aloe verrucosa (from Kieser); 14, those of Cactus peruvianus; 13, those of Rheum palmatum: the two latter from Turpin.

CHAPTER II.

OF THE COMPOUND ORGANS IN FLOWERING PLANTS.

HAVING now explained the more important circumstances connected with modifications in the elementary organs of vegetation, the next subject of enquiry will be the manner in which they are combined into those masses which constitute the external or compound organs, or in other words the parts that present themselves to us under the form of roots, stems, leaves, flowers, and fruit, and that constitute the apparatus through which all the actions of vegetable life are performed. In doing this, I shall limit myself in the first place to Flowering Plants (Introduction to the Natural System, p. 1.); reserving for the subject of a separate chapter the explanation of some of the compound organs of Flowerless plants (Ibid. p. 307.), which differ so much in structure from all others, as to require in most cases a special and distinct notice.

SECT. I. Of the Cuticle and its Appendages.

1. Of the Cuticle.

VEGETABLES, like animals, are covered externally by a thin membrane or cuticle, which usually adheres firmly to the cellular substance beneath it. To the naked eye it appears like a transparent homogeneous pellicle, but under the microscope it is found to be traversed in various directions by lines, which, by constantly anastomosing, give it a reticulated character. In some of the lower tribes of plants, consisting entirely of cellular tissue, it is not distinguishable, but in all others it is to be found upon every part, except the stigma and the spongioles of the roots. Its usual character is that of a delicate membrane, but in some plants it is so hard as almost to resist the blade of a knife, as in the pseudo-bulbs of

certain Orchideous plants. The most usual form of the reticulations is the hexagonal (Plate III. fig. 11.): sometimes they are exceedingly irregular in figure; often prismatical; and not unfrequently bounded by sinuous lines, so irregular in their direction as to give the meshes no determinate figure (fig. 5.).

Botanists have not agreed as to the exact nature of the cuticle; while the greater number incline to the opinion that it is an external layer of cellular tissue in a dry and compressed state; others, among whom are included both Kieser and Amici, consider it a membrane of a peculiar nature, transversed by veins, or vasa lymphatica.

By the latter it is contended, that the sinuous direction of the lines in many cuticles is incompatible with the idea of any thing formed by the adhesion of cellular tissue; that when it is once removed, the subjacent tissue dies, and does not become cuticle in its turn, and that it may often be torn up readily without laceration.

On the other hand, it is contended, that the reticulations of the cuticle are mostly of some figure analogous to that of cellular tissue, and that the sinuous meshes themselves are not so different as to be incompatible with the idea of a membrane formed of adhering cellules. We are accustomed to see so much variety in the mere form of all parts of plants, that an anomalous configuration in cellular tissue should not surprise us. The lines, or supposed vasa lymphatica, are nothing more than the united sides of the cellules, and are altogether the same as are presented to the eye by any section of a mass of cellular substance. It is certain that the cuticle cannot be removed without lacerating the subjacent tissue, with however much facility it may be sometimes separable: on the under surface of the leaf of the Box, for instance, there has plainly been some tearing of the tissue, before the cuticle acquired the loose state in which it is finally found. If the subjacent epidermis never becomes cuticle when the latter is removed, this is no reason why the cuticle itself should not be composed of cellular tissue; for it is an axiom in vegetable physiology, that a part once fully formed is incapable of any subsequent change. Thus, pith never alters its dimensions,

after the medullary sheath that encloses it has been once completed, and a zone of wood never contracts or expands after it has been deposited: new matter may be added to any part, but the arrangement of the tissue, once fixed, remains unchangeable.

The principal argument, however, in favour of cuticle being compressed cellular tissue, is, that in the cuticle of many plants the cellular state is distinctly visible upon a section (Plate I. fig. 2. a); that it even consists occasionally of several layers of cellules, as in many epiphytes of the Orchis tribe; and that, as there is no reason to doubt that Nature is as uniform in the plan upon which cuticle is constructed as in all her other works, in those cases in which the cellular structure is less distinctly visible, we are nevertheless istified by sound philosophy in recognising it; while, on the other hand, it would be highly unphilosophical to suppose that the cuticle is formed in some plants upon one plan, and in others upon a totally different one. It may be farther remarked, that separable cuticle may often be traced into that which, being younger, is both inseparable and undistinguishable from the other cellular substance with which it is in contact, and from which it possesses no organic difference.

There is some reason to suppose that there is occasionally present, on the outside of the cuticle, a transparent, very delicate membrane, having no organic structure, as far as can be discovered with the most powerful microscopes. Something of this kind has been noticed by M. Adolphe Brongniart in the Cabbage leaf, and an analogous structure has been remarked by Professor Henslow in the Digitalis.

2. Of Stomata.

In many plants the cuticle has certain openings of a very peculiar character, which appear connected with respiration, and which are called Stomata. (Plate III. passim.)

STOMATA (Pores of the epidermis; Pores corticaux, allongés, évaporatoires, or grands pores; Glands corticales, miliaires, or epidermoidales; Glandulæ cutanea; Oeffnungen; Stomatia;) are passages through the cuticle, having the appearance of

D

areolæ, in the centre of which is a slit that opens or closes according to circumstances, and lies over a cavity in the subjacent tissue.

There is, perhaps, nothing in the structure of plants upon which it is more difficult to form any satisfactory opinion than these stomata. Malpighi, and Grew, who seems first to have figured them (see his plate xlviii. fig. 4.), call them openings or apertures, but had no exact idea of their structure. Mirbel also, for a long time, considered them pores, and figured them as such; admitting, however, that he suspected the openings to be an optical deception. M. De Candolle entertains no doubt of their being passages through the cuticle. He says their edge has the appearance of a kind of oval sphincter, capable of opening and shutting. The membrane that surrounds this sphincter is always continuous with those which constitute the network of the cuticle: under the latter, and in the interval between the pore and the edge of the sphincter, are often found molecules of adhesive green matter (Organogr. i. 80.); and recently M. Adolphe Brongniart, in his beautiful figures of the anatomy of leaves, would seem to have settled the question beyond all dispute. (Annales des Sciences, vol. xxi.) Nevertheless, there are anatomists of high reputation who entertain a directly opposite opinion; denying the existence of passages, and considering the stomata rather in the light of glands. Nees von Esenbeck and Link deny the existence of any perforation in the stomata, and consider that the supposed opening is a space more pellucid than the surrounding tissue, and that what seems a closed up slit is the thickened border of the space. Link further adds, that the obscuration of the centre of the stomata is caused by a peculiar secretion of matter, as is plainly visible in Baryosma serratum. (Elementa, p. 225.) To the views of these writers is to be added the testimony of Mr. Brown (Suppl. prim. Prodr. p. 1.), who describes the stomata as glands which are really almost always imperforate, with a disk formed by a membrane of greater or less opaqueness, and even occasionally coloured; at the same time he speaks of this disk being, perhaps, sometimes perforated.

In the midst of such conflicting testimony, an observer