108 in one layer in the thallus, which is then said to be heteromerous (the hypha layer being divided into a cortical and deeper part), or they are scattered eaually throughout its Loaves, Fig. 96. FIG. 96. A Lichen (Parme ia), with its cellular expansion (thalius), and its rounded apothecia, or spots of fructification. FIG. 97. Vertical section of an apothecium and thallus of a Lichen. Rounded free cells g, of a green colour, and called conidia, are seen in the centre of the thallus. The apothecium a consists of thecne and paraphyscs. Its upper sur face is often coloured and covered by a peritheciuin. substance and are homceomerous. The gonidia are separ able, and may form independent plants. When they are being detached, the separation begins at the centre of the thallus, so that the middle of the Lichen becomes pulverulent, while its circumference may remain foliaceous or crustaceous. By the continuation of this process it sometimes happens that the whole Lichen becomes a mass of greenish or yellowish powder. The thallus may be crustaceous, in the form of an incrustation upon rocks, trees, &c., with which it becomes closely connected, as in Graphis ; or it may be foliaceous, forming a flattened expansion easily detachable from the substratum to which it is connected by small root- like bodies or rhiziiies, as in Peltigera ; or the lichen may be what is termed fruticose, i.e., composed of a much- branched thallus rising from a single point of attachment, as in Usnea. Kecent observations upon Lichens tend to show that they are in reality composed of two distinct organisms, one, a Fungus, being the hyphal part, and the other, an Alga, the gonidial portion, and that the Fungus is parasitic upon the gonidia. 3. Leaves and their Modifications. In popular language all the green expanded organs borne upon an axis are designated leaves. Investigation, however, has shown that many other parts of a plant which externally appear very different from ordinary leaves are, in their essential particulars, very similar to them, and are in fact their morphological equivalents. Thus the scales on the bulb of the Onion, the various parts of the flower, &c., are all leaves. Assuming, then, that the struc ture ordinarily termed a leaf is the typical form, these latter are designated changed or metamorphosed leaves ; and all structures morphologically equivalent with the leaf are included under the general term phyllome (leaf -structure). Leaves are produced as lateral outgrowths of the stem. This character, common to all leaves, distinguishes them from other organs. In the higher plants we can easily recognize the distinction between stem and leaf. Amongst the lower plants, however, it is found that a demarca tion into stem and leaf is impossible, but that there is a structure which partakes of the characters of both, such is a thallome. The leaves always arise from the outer portion of the primary meris tern of the plant, and the tissues of the leaf are continuous with those of the stem. Every leaf originates as a simple cellular papilla, which consists of a development from the cortical layers covered by epidermis ; and as growth proceeds, the fibre-vascular bundles of the stem art- continued outwards, and finally expand and terminate in the leaf. The increase in length [NUTRITIVE ORGANS. of the leaf by growth at the apex is usually of a limited nature. In some Ferns, however, there seems to be a provision for indefinite terminal growth, while in others this growth is periodically interrupted. It not un frequently happens, especially amongst Monocotyledons, that after growth at the apex has ceased, it is continued at the base of the leaf, and in this way the length may be much increased. Amongst Dicotyledons this is very rare. In all cases the dimensions of the leaf are enlarged by interstitial growth of its parts. The simplest leaf is found in some Mosses, where it Structu: consists of a single layer of cells. Usually it consists of of leave several layers, and amongst vascular plants is distinguish able into an epidermis and ft central parenchyma with fibro-vascular bundles distributed through it, The epidermis (fig. 98, es, ei), composed of cells more or less compressed, has usually a different structure and Section of a Melon-leaf, perpendicular to the surface es, upper epidermis; ti, lower epidermis; p, hairs; $t, stomata ; ps, upper layers of parenchyinatous cells ; pi, lower layers of parcnchymatous cells ; n; mcatus, or canals connected with stomata; they are sometimes called hypostomatic spaces; I, lucunte, or cavities between the loose cells in the cavernous lower parenchyma ; /r, bundles of fibro-vascular tissue, consisting of woody, dotted, spiral, and other vessels. aspect on the two surfaces of the leaf. It is chiefly on the epidermis of the lower surface (fig. 98, ei) that stomata, st, are produced, occupying spaces between the veins, and it is there also that hairs, p, usually occur. The lower epidermis is often of a dull or pale-green colour, soft, and easily detached. The upper epidermis is frequently smooth and shining, and sometimes becomes very hard and dense. Many tropical plants present on the upper surface of their leaves several layers of compressed epidermal cells. These appear to be essential for the preservation of moisture in the leaf. In leaves which float upon the surface of water, as those of the Water-lily, the upper epidermis alone possesses stomata. The parenchyma of the leaf is the cellular tissue surrounding the vessels, and enclosed within the epidermis (fig. 98, 2 )S , 2 } i)- I fc nas sometimes received the names of diachyma, mesophylhim, and diploe. It is formed of two distinct series of cells, each containing chlorophyll or green-coloured granules, but differing in form and arrange ment. Below the epidermis of the upper side of the leaf there are one or two layers of oblong blunt cells, placed perpendicularly to the surface (fig. 98, j)s), and applied so closely to each other as to leave only small intercellular spaces (fig. 98, m], except where stomata happen to be present, On the under side of the leaf the cells are irregular, often branched, and are arranged more or less horizontally (fig. 98, pi), leaving cavities between them, I, which often communicate with stomata. On this account the tissue has received the name of cavernous. In leaves having a very firm texture, as those of Coniferse and Cycadacese, the cells of the parenchyma immediately
beneath the epidermis are very much thickened and