3. Latex granules. The interior of the laticiferous vessels is occupied by a fluid rendered turbid by the presence of infinite quantities of extremely minute particles which do not become blue upon the application of iodine, but which have a rapid motion upon their own axis. Of these little more is known than that they exist.

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 which present themselves to us under the form of roots, stems, leaves, flowers, and fruit, and which constitute the apparatus performing all the actions of vegetable life. 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. 395.), 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 Epidermis.

VEGETABLES, like animals, are covered externally by a thin membrane or epidermis, which usually adheres firmly to the cellular substance beneath it. To the naked eye it appears like a transparent homogeneous skin, 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 exposed to the air, except the stigma and the spongelets of the roots. It is, however, as constantly absent from the surface of parts which

live under water. 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 Orchidaceous plants. The most usual form of its reticulations is the hexagonal (Plate III. fig. 11.): sometimes they are exceedingly uncertain 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 were formerly not agreed upon the exact nature of the epidermis; while some inclined to the opinion that it is an external layer of cellular tissue in a compressed state, others, among whom were included both Kieser and Amici, considered it a membrane of a peculiar nature, transversed by veins, or vasa lymphatica.

By the latter it was contended, that the sinuous direction of the lines in many kinds of epidermis 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 epidermis in its turn, and that it may often be torn up readily without laceration.

On the other hand, it was replied, that the reticulations of the epidermis 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 bladders. 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 lymphatic vessels, are nothing more than the united sides of the bladders, 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 epidermis 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 epidermis acquired the loose state in which it is finally found.

There is now no anatomist to be found who doubts the fact of epidermis being cellular tissue. In many plants the cel

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lular state is distinctly visible upon a section (Plate I. fig. 2. a); it even consists occasionally of several layers of vesicles, as in the Oleander and many Orchidacea, and it varies in the density, form, and arrangement of its component cells in different plants, according to the peculiar conditions to which they are exposed.

External to the epidermis is a thin homogeneous membrane, formed of organic mucus (see page 1.) and overlying every part except the stomates and the stigmatic tissue. It was first observed by Adolphe Brongniart in the Cabbage-leaf, afterwards by Henslow in Digitalis, and by myself in Dionæa ; it has subsequently been the subject of more extended observations, and appears to be a universal coating, which is even drawn over the hairs, as if to protect the tender cell forming their interior, and the plexus of capillary Cinenchyma, which is stationed on the outside of the walls of that cell. I have found this cuticular membrane on the delicate petals of Hydrotania Meleagris, from which it may be easily removed after maceration for a few days in spirit of wine; and Ad. Brongniart succeeded in separating it from the leaves of Potamogoton lucens, after very long maceration in water. It is stated to be sometimes covered with a minute granular appearance, the nature of which is unknown, and which is not found at the lines indicating the place where the cuticle was pressed upon the united sides of cells. There are some good observations upon this subject by M. Adolphe Brongniart (Ann. des Sc. 2 ser. 1. 65.), who finds the cuticle by no means uncommon; and imagines that it overlies the stigma in Nymphæa and Mirabilis. It certainly does not cover the stomates, nor the glands found on the surface of the inside of the pitchers of Nepenthes.

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

STOMATES are passages through the cuticle, having the appearance of an oval space, in the centre of which is a slit that opens or closes according to circumstances, and lies above a cavity in the subjacent tissue.

There is, perhaps, nothing in the structure of plants upon which more different opinions have been formed than these stomates. Malpighi and Grew, the latter of whom seems first to have figured them (t. 48., 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. De Candolle entertains no doubt of their being passages through the epidermis. 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 that which constitutes the network of the epidermis: 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 Adolphe Brongniart, in his beautiful figures of the anatomy of leaves, would seem to have settled