LEAF, the name given in popular language to all the green expanded organs borne upon an axis, and so applied to similar ob jects. Investigation 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. Such are the scales of a bulb, and the various parts of the flower, and assuming that the struc ture ordinarily termed a leaf is the typical form, these other struc tures were designated changed or metamorphosed leaves, a some what misleading interpretation.
Leaves are produced as lateral outgrowths of the stem in defi nite succession below the apex. 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 demarcation into stem and leaf is impossible, but that there is a structure which partakes of the characters of both—i.e., the thallus. The leaves always arise from the outer portion of the primary meri stem of the plant, and the tissues of the leaf are continuous with those of the stem. Every leaf originates as a cellular papilla, which consists of a development from the cortical layers covered by epidermis ; and as growth proceeds, the fibro-vascular bundles of the stem are continued outwards, and finally expand and terminate in the leaf. The increase in length 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 unfrequently 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 dimen sions of the leaf are enlarged by interstitial growth of its parts.
The form and arrangement of the parts of a typical foliage leaf are intimately associated with the part played by the leaf in the life of the plant. The flat surface is spread to allow the maximum amount of sunlight to fall upon it, as it is by the absorption of energy from the sun's rays by means of the chlorophyll contained in the cells of the leaf that the building up of plant food is ren dered possible ; this process is known as photosynthetic assimila tion of carbon dioxide or simply photosynthesis. The first stage is the combination of carbon dioxide, absorbed from the air taken in through the stomata into the living cells of the leaf, with water which is brought into the leaf by the wood-vessels. The wood vessels form part of the fibro-vascular bundles or veins of the leaf and are continuous throughout the leaf-stalk and stem with the root by which water is absorbed from the soil. The palisade layers of the mesophyll contain the larger number of chlorophyll grains (or chloroplasts) while the absorption of carbon dioxide is carried on chiefly through the lower epidermis which is generally much richer in stomata. The water taken up by the root from the soil contains nitrogenous and mineral salts which combine with the first product of photosynthesis—a carbohydrate—to form more complicated nitrogen-containing food substances of a protein na ture; these or their simpler products are then distributed by other elements of the vascular bundles (the phloem) through the leaf to the stem and so throughout the plant to wherever growth or development is going on. A large proportion of the water which ascends to the leaf passes out in the form of water vapour through the stomata—this process is known as transpiration. Hence the extended surface of the leaf exposing a large area to light and air is eminently adapted for the carrying out of the process of photo synthesis and transpiration. The arrangement of the leaves on the stem and branches (see Phyllotaxis, below) is such as to prevent the upper leaves' shading the lower, and the shape of the leaf serves towards the same end—the disposition of leaves on a branch or stem is often seen to form a "mosaic," each leaf fitting into the space between neighbouring leaves and the branch on which they are borne without overlapping.