STOMATA (Gr. mouths), are minute openings in the epidermis of leaves and other green' parts of plants exposed to the communicating with intercellular spaces. Their existence was first noticed by Grew, who described trim in his Amami, of Plants in 1082. They ere generally formed by two semilunar cells, which are as lips to the, orifice, and are filled with green matter; but sometimes the cells arranged around them are more numerous. They are generally of an elliptical form, but sometimes circular, and sometimes quadrangular. These differences are very characteristic of particular species, genera. or orders of plants. In a moist state of the atmosphere, they are open; but when it becomes dry they arc closed, or nearly so. It appears that they art organs of transpiration, and that their opening and closing according to the moisture or dryness of the atmosphere regulates it in a manner suitable to the requirements of the plant. They do not appear in any part of the plant covered by the soil, nor in submerged leaves, nor on the lower sides of floating leaves. Succulent plants have very few of them; so that these plants retain for a long time the moisture which they have imbibed, and are thus adapted for living in is dry atmosphere. Stomata are generally more abundant on the
under side of leaves; but in leaves which grow vertically, they are often almost equally numerous on both sides. In general they are irregularly placed; but in grasses and many other endogenous plants with parallel-veined leaves, they are in regular rows; and in some other plants they occur in little groups. The number in a square inch varies from 20 / in the mistletoe, to almost 450,000 in the under side of the leaves of solanunt sane turn.—Stomata arc not found in mosses, lichens, algre, and fungi; but they exist in some of the hepatica, in marchanlia, in which their structure is more complex than in the higher pants: each of them consisting of a kind of shaft, composed of four or five rings placed one upon the other, every ring made up of four or five cells, and the lowest ring apparently regulating the aperture by the contraction or expansion of the cells which form it.