PLANT 581 the cambium in dicotyledonous stems. Epi- dermal Tissue. The epidermis has been men- tioned as clothing stems of both kinds when young, and indeed it covers all parts of the plant exposed to the air, save the stigma, in which the naked cellular tissue is exposed, but usually covered by a viscid secretion ; the gen- eral structure of the epidermis is illustrated under LEAF, where it is shown to be a layer of flattened empty cells with numerous openings (stomata}, or breathing pores ; in some cases this has a deposit of silex and is very hard ; in others it is covered with a waxy secretion, as in the bloom of the grape and other fruit, and in the leaves of the cabbage. In some cases the exposed cell wall is developed as a cuticle above the proper epidermis. The cells of the epidermis are sometimes in flowers prolonged into papillae, which give a peculiar velvety ap- pearance ; again they are produced as simple hairs, or several cells may unite to form a hair, which may be still more complicated as a sting to exude an irritating fluid ; glands and prickles often belong to the epidermis. Upon the root the epidermis is very thin, and is not upon the tender growing point, but is formed later by the thickening of the exterior cells. Physiology of Plants. In the simple plants mentioned at the beginning, one cell performs all the functions of growth and reproduction ; whatever part of a higher plant be examined, the cell in some form will be found, and growth wherever it takes place in the plant consists in the multiplying of cells. The cell, whether in the soft pith, the hard wood, or the remark- ably strong tissue of the inner bark, without reference to its contents, is chemically the same. The ultimate composition of the cell itself is carbon, hydrogen, and oxygen (CiaHao Oi), and is called cellulose ; it has the same composition in all plants. For the formation of cellulose these elements must be supplied to the plant ; they are not converted directly into cellulose, though this is their ultimate destination, but into dextrine, sugar, gum, and similar principles, either identical in composi- tion with cellulose, or differing from it only in having a few equivalents of oxygen and hydrogen more, or cellulose with the addition of water ; all these principles, including cellu- lose, being essentially carbon and water, are termed carbohydrates. Water is supplied by the soil; carbon is furnished in the form of carbonic acid (00 2 ), which is constantly pres- ent in the atmosphere in the gaseous form, and in the water of the soil in solution. The de- composition of carbonic acid, setting free its oxygen and combining its carbon with the ele- ments of water, is the striking phenomenon attendant upon plant growth. This change is effected only through the agency of chlorophyl (leaf green), and in the leaves or those parts capable of performing the functions of the leaf ; as its name implies, chlorophyl is usually green, but it is sometimes of another color. Giving the green color to leaves, one would suppose it to be present in very large quanti- ties, but an examination of the cells of a leaf by the microscope shows the proportion to be surprisingly small; it is seen in the form of exceedingly minute grains, attached to the walls of the cell or distributed through its con- tents. It has been stated that the foundation of the cell is protoplasm, and that the cell wall is built over that; protoplasm, also called the formative layer, is found in all cells of the growing parts of the plant; and chlorophyl grains are formed in it and belong in it. Pro- toplasm, so essential to the growth of the cell, differs from it in chemical composition, as it contains nitrogen in addition to the elements before mentioned. Other vegetable principles have a similar composition, often with the addition of sulphur and phosphorus, and are called, from their resemblance to albumen, al- buminoids. To form these the plant must be supplied with nitrogen, and although this is present in the atmosphere in such abundance, the most careful experiments fail to show that the plant appropriates it. It enters the plant in combination as nitric acid in the form of ni- trates and as ammonia, these being in solution in the soil. These elements, carbon, oxygen, hydrogen, and nitrogen, when the plant is burned or decays, entering into new combi- nations, disappear in the gaseous form, and hence are termed the volatile elements ; but there is left behind the ash, or non-volatile portion, the source of which must have been the soil. It was formerly supposed that the presence of non- volatile or earthy matters in plants was accidental ; being dissolved in the water of the soil, they were carried into the plant and there deposited. While more of these than is useful may be taken up, and some are of no known use, it is now well estab- lished that others are absolutely necessary ; it is sufficient to say here that potash, lime, iron, phosphoric acid, and some others play an im- portant part in vegetation, promoting the ac- tivity of growth, aiding in the diffusion of al- buminoids, and in other ways; it is also as- certained that chlorophyl, upon which so much of the work of the plant depends, is not form- ed in the protoplasm unless iron be present, though an infinitesimally small quantity is re- quired. These are the essential constituents of plants, furnished in part by the atmosphere, and in part by the soil. The volatile elements of plants may be regarded as coming primarily from the atmosphere, though in good part im- mediately obtained from the decay of former vegetation ; and the vegetable products which cover the earth in the living state, or partially decayed in the soil as humus, or elsewhere as coal, peat, and muck, together represent the amount of materials that plants have taken from the atmosphere. The conversion of the inorganic, lifeless elements into organized mat- ter and structure endowed with life (assimila- tion) is done in the leaf, and a reference to its structure (see LEAF) shows how well it is
