Stoma
(plural, Stomata; from stoma, a mouth). The Stomata are little openings, or mouths, in the outer covering or epidermis of the green parts of plants, through which they may be said to breathe. The air passes into the plant from the outside, bringing with it Carbonic Acid gas. This gas is broken up in the cells containing chlorophyl: all the Carbon, and half of the Oxygen, of the Carbonic Acid are retained by the plant, to be built up into starch, and other foods; and half of the Oxygen escapes from the plant into the outer air, through the Stomata. The air passing out is loaded with vapour of water evaporated from the cells of the leaf; and thus there is a constant escape, through the Stomata, of water from green parts of plants. The form and structure of the Stomata do not vary much in vascular plants; though many groups of these plants exhibit peculiarities in the form and arrangement of the cells of the epidermis that lie round the Stomata, and that are often called the "neighbour-cells"; but it is not necessary here to enter upon a description of the latter. The Stoma, or opening, lies between two sausage-shaped cells, called "guard-cells," which are joined near the ends, but leave a space between them in the middle. This opening leads into an empty space between the cells, below the epidermis, from which space crevices pass in all directions, opening into larger ones among the loosely-arranged cells in the middle of the leaf. There is a constant passage of gases into and out of the cells through the thin cell-walls that border the inter-cellular spaces, with results upon the atmosphere as stated above, if green plants are exposed to daylight. The guard-cells are usually green, owing to the presence in them of chlorophyl. They thus present a contrast to the ordinary cells of the epidermis, which contain few, if any, chlorophyl-bodies in land plants, except in Ferns, and a few others. The guard-cells regulate evaporation from plants, since they swell when full of sap, and become more convex, thus leaving a wider opening between them when a plant contains much sap and would benefit by the evaporation of some of it. In dry weather, when there has been much evaporation, the guard-cells contain less sap, and thus become straighter, and leave a narrower slit; so that evaporation becomes much slower through the Stoma when it requires to be diminished in amount. Stomata seldom occur on parts of plants habitually under ground, or under water, where they would be useless. They are most abundant on leaves, especially on the lower surface, except in some Conifers, and a few other plants, in which they are more numerous on the upper surface. Where the sides of a leaf are equally exposed to light, e.g., in vertical leaves, or phyllodes, the Stomata are disposed in equal numbers on both surfaces. In floating leaves (Nymphaea), they occur only on the upper surface, exposed to the atmosphere. The number varies in different plants from a few hundreds to over 300,000, on a square inch of surface of a leaf, but from 20,000 to 150,000 seem to occur most frequently on that space.
Besides the ordinary Stomata for the passage of air, and of water in the form of vapour, many plants possess others of much larger size, placed singly or in groups along the edges of the leaves, above the ends of the veins. They resemble the other Stomata in form, but their guard-cells cannot move, i.e., cannot alter the width of the slit between them. Their use is to permit the escape of water in drops, instead of in vapour. A familiar example of this action may be observed in Richardia africana, from the tip of whose leaves drops of water often fall. It is also exhibited in the drops of water that are so frequently seen on the tips of the leaves of young Wheat, and of other Cereals, in damp evenings, when evaporation is slow. The water thus exuded often has Carbonate of Lime in solution, and, as the drops evaporate, this is deposited, in such plants, in a white crust around the waterpores, as such Stomata are called.