gular ones, often having two or three branches, which unite with the limbs of the vesicles next them, and so form a reticulated parenchyma; the spaces between whose vesicles are much larger than the vesicles themselves. It is this reticulated tissue, with large spaces in it (to which the name of cavernous or spongy parenchyma might not improperly be applied), that, in most cases, occupies at least half the thickness of the leaves between the veins. The arrangement of the vesicles is very obvious if the lower cuticle of certain leaves be lifted up with the layer of parenchyma that is applied against it; it may then be seen that these anastomosing vesicles form a net with large meshes,—a sort of grating inside the cuticle. It must not, however, be supposed that this structure, which I have remarked in several ferns, and in a great many dicotyledonous plants, is without exception. In many monocotyledonous and succulent plants we have some remarkable modifications of this structure. Thus, in the Lily, and several plants of the same family, the vesicles of parenchyma that are in contact with the lower cuticle are lengthened out, sinuous, and toothed, as it were, at the sides: these projections join those of the contiguous vesicle; and a number of cavities is the consequence, which render this sort of parenchyma permeable to air. An analogous arrangement exists in the lower parenchyma of Galega. In the Iris, there is scarcely any space between the oblong and polyedral vesicles which form the parenchyma; but it is remarked, that the subjacent parenchyma is wanting at every point where the cuticle is pierced by a stoma. In such succulent plants as I have examined, the spaces between the cellules of parenchyma are very small; but, nevertheless, here and there, there are often larger cavities, which either correspond directly with the stomata, or are in communication with them. The same thing happens in plants with floating leaves, where the stomata placed on the upper surface correspond with the layer of cylindrical and parallel vesicles; in such case there are, here and there, between these vesicles, empty spaces which almost always.correspond to the points where the stomata exist, and which permit the air to penetrate between the vesicles as far as the middle of the parenchyma of the leaf. Thus much M. Brongniart; who adds, that in submersed leaves there is no cuticle, but the whole consists of solid parenchyma alone, in which there are no other cavities than such as are necessary to float the leaves. The veins, being elongations of the medullary sheath, necessarily consist of woody fibre and spiral vessels, to which are sometimes added annular ducts. In submersed leaves spiral vessels are often wanting, the veins consisting of nothing but woody fibre. In these veins M. Schultz finds what he calls vessels of the latex, or of the nutritive fluid; but it is difficult to understand, either from his figures or descriptions, which kind of tissue in particular he means to designate by that name. M. Adolphe Brongniart says, the latex vessels are the vasa propria; but what are the vasa propria of leaves, in which there is nothing but woody fibre, spiral vessels, and ducts? Such are the general anatomical characters of leaves; but it must be borne in mind, that, in different species, they undergo a variety of remarkable modifications. These arise either from the addition of parenchyma when leaves become succulent, or from the non-developement of it when they become membranous, or from the total suppression of it, and even of the veins also in great part, as in those which are called ramentaceous, such as the primordial leaves of the genus Pinus. I have dwelt thus much at length upon the structure of the leaf, because it is by far the most important part of a plant, and that of which the functions are the best ascertained. Let us now turn our attention to the modifications of the leaf. It has already been seen that a leaf may consist of two distinct parts; the petiole, or stalk, and lamina, or leaf itself: both of these demand separate consideration. The lamina, or limbus, as it is called by some, is subject to many diversities of figure and division; most commonly it forms an approach to oval, being longer than broad. When speaking of the leaf, it is usual to take the opportunity of explaining the terms employed by botanists to distinguish varieties of figure; but, as those terms are equally applicable to any other part with a similar dilated surface, it has appeared to me expedient to include them in Glossology, where they will accordingly be found. That extremity of the lamina which is next the stem is called its base; the opposite extremity, its apex; and the line representing its two edges, the margin or circumscription. If the lamina consists of one piece only, the leaf is said to be simple, whatever may be the depth of its divisions: thus, the entire lamina of Box, the serrated lamina of the Apple, the toothed lamina of Coltsfoot, the runcinate lamina of Taraxacum, the pinnatifid lamina of Hawthorn (which is often divided almost to its very midrib), are all considered to belong to the class of simple leaves. But if the petiole branches out, separating the cellular tissue into more than one distinct portion, each forming a perfect lamina by itself, such a leaf is often said to be compound, whether the divisions be two, as in the conjugate leaf of Zygophyllum, or indefinite in number, as in the many varieties of pinnated leaves. Nevertheless, a more accurate notion of a compound leaf is found to consist in its divisions being articulated with the petiole, by which it is much better distinguished from the simple leaf than by the number of its divisions. Thus, the pinnated leaf of a Zamia, and the pedate leaf of an Arum, both in this sense belong to the class of simple leaves; while the solitary lamina of the Orange, the common Berberry, &c. are referable to the class of compound leaves. This distinction is of some importance to the student of natural affinities; for, while division, of whatever degree it may be, may be expected to occur in different species of the same genus or order (provided there is no articulation), it rarely happens that truly compound leaves, that is to say, such as are articulated with their petiole,—are found in the same natural assemblage with those in which no articulation exists. - In speaking of the surface of a leaf it is usual to make use of the word pagina. Thus, the upper surface is called pagina superior; the lower surface, pagina inferior. The upper surface is more shining and compact than the under, and less generally clothed with hairs; its veins are sunken; while those of the lower surface are usually prominent. The cuticle readily separates from the lower surface, but with difficulty from the upper. There are frequently hairs upon the under surface while the upper is perfectly smooth; but there is scarcely any instance of the upper surface being hairy while the lower is smooth. The ramifications of the petiole among the cellular tissue of the leaf are called veins, and the manner of their distribution is termed venation. This influences in a great degree the figure and general appearance of the foliage, and requires a more careful consideration than it generally receives in elementary works. ; The vein which forms a continuation of the petiole and the axis of the leaf is called the midrib or costa: from this all the rest diverge, either from its sides or base. If other veins similar to the midrib pass from the base to the apex of a leaf, such veins have been named nerves and a leaf with such an arrangement of its veins has been called a nerved leaf. If the veins diverge from the midrib towards the margin, ramifying as they proceed, such a leaf has been called a venous or reticulated leaf. This is the sense in which these terms were used by Linnæus; but Link and some others depart from so strict an application of them, calling all the veins of a plant nerves, whatever may be their origin or direction. Till within a few years the distribution of veins in the leaf had not received much attention; the terms just mentioned had been contrived to express certain of the most striking forms of venation; but the application of these was far from being sufficiently precise. Many improvements have been proposed by modern botanists; it however appears to me that the whole nomenclature of venation is essentially defective, and requires complete revision. My ideas upon this subject have been already laid before the public in the Botanical Register for Sept. 1826, page 1004.; and, as I am not aware that any objection to them has yet been taken, I shall repeat them here, in a form better adapted to an elementary work than that under which they first appeared. The objections that I take to the present modes of distinguishing veins are these: 1st, That the veins are very improperly, as I think, called nerves, either in all cases, as by Link, which is bad, or in certain cases only, when they have a particular size or direction, as by Linnæus and his followers, which is worse. Nothing is more destructive of accurate ideas in natural history than giving names well understood in one kingdom of nature to organs in another kingdom of an entirely different kind, unless it is the, perhaps, more reprehensible practice of giving two names conveying totally different ideas to the same organ in the same kingdom of nature. Thus, when the veins of a plant are termed nerves, it is necessarily understood that they exercise functions of a similar nature to those of the nerves of animals: if otherwise, why are they so called? But they exercise no such functions, being, beyond all doubt, mere channels for the transmission of fluid. Again, if one portion of the skeleton of a leaf is called a vein, and another portion a nerve, this apparently precise mode of speaking leads yet more strongly to the belief (especially when such a distinction is seen admitted into works which are said to be of the highest authority in science), that the structure and function of those two parts are as widely different as the structure and function of a vein and a nerve in the animal economy; else why should such nice caution be taken to distinguish them? But it must be confessed that there is no difference whatever, except in size, between the veins and nerves of a leaf. Let us, then, abandon a term which is one of those relics of a barbarous age, which it is the duty of modern science to expel. My second objection is caused by the vague manner in which the veins of leaves are at present described; whence it happens that no precise idea can be attached to the different terms that have been contrived to designate particular forms of venation. A third objection is this, that, while slight modifications in the arrangement of the veins have received distinctive names, others of much greater importance, and of a more decided character, have received no distinctive appellation whatever. For these reasons, the practical weight of which I have long experienced, it has occurred to me that the following changes in the language used in speaking of venation will be found better, at least, than that for which they are substituted, if they are not entirely what could be desired. It has been usual to call that bundle of vessels only which passes directly from the base to the apex of a leaf the costa, or |