see Leaf.) The manufacture of food can only take place in cells containing chloroplasts, when these are adequately illuminated. (See Photosynthesis.) The food produced by the nutritive tissue is primarily carbohydrates. This may be used at once for the formation of proteid foods, and since the supply of carbohydrates is most abundant in the leaves, they are also the principal seat of proteid formation. (See Food of Plants.) Foods produced in the leaves may be transported to other parts of the plant and stored for a time. (See Storage.) In those plants which lack foliage leaves the surface of the stem only is occupied by the nutritive tissue, and by its profuse branching it may expose a considerable area of these tissues to light and air. In some cases, however, the necessity for protecting the plant against excessive evaporation is so urgent that the body has no outgrowths, being cylindrical or spherical, as in the cactaceæ; in such a case the nutritive tissue is limited to the surface of the compact stem, and is at a minimum.
(5) The Aërating System. The aërating system consists of irregular passages amongst the cells of the plant body, formed by the separation of the cells as they mature. These passages communicate with the outside air by special opening through the epidermis, called stomata (q.v.). If the epidermis dies and is replaced by cork, as on the surface of twigs, communication of the aërating system with the air is maintained for a few years through lenticels (q.v.). The intercellular spaces are largest in the water plants, where they often form extensive canals easily visible to the naked eye. In most land plants, however, they are narrow, tortuous passages. Through these gases diffuse, but there is little or no flow, i.e., mass movement. Carbon dioxide and oxygen are supplied to the cells in requisite amount by the aërating system, the former for food-making and the latter for respiration. See Aëration.
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Transverse section of the central part of the root of Calamus (Acorus calamus), showing the central cylinder of xylem and phloem bundles and huge air-chambers, separated by plates of cells.
(6) The Secreting System. There is no sharp distinction to be made between secretion and excretion in plants. Many substances, useless to the plant except incidentally, are stored in special receptacles, and are thus removed from the general course of activity. In other cases the material is poured out upon the surface of the plant and is thus gotten rid of. The secreting system consists of (1) single secreting cells or groups of them, called glands (q.v.); (2) receptacles for secretions. Gland-cells differ in appearance from the other cells by the very granular character of their protoplasm. Single glandular cells are common both in the interior of various organs and at the surface. On the surface they are not infrequently raised upon a longer or shorter stalk, in which case they constitute glandular hairs.
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Hairs from leaf of Centaurea, terminating in glands h. At «, a stoma.
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Emergences of the Hemp (Cannabis sativa), capped by glands, showing the cuticle lifted into a vesicle by the secretion.
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A cross section of a resin duct in a young pine leaf, v, receptacle for the resin, formed by the separation of the secreting cells, z. Later the cells surrounding z become thick-walled and prevent collapse of the duct.
Receptacles for secretions are produced either by the unusual enlargement of an intercellular space among the gland-cells, or by the degeneration of the gland itself, leaving the secretion lying amongst the tissues originally enveloping the gland.
(7) The Storage System. The storage system consists of masses of thin-walled cells, which are usually extraordinarily developed in certain organs. Any organ, root, stem, leaf, or even a flower-bud, may thus be specially adapted to storage. The storage-cells retain their vitality, and have the power of organizing the food materials coming to them into permanent storage
