`COPE. As a science, plant physiology con cerns it-elf with every inquiry relating to the functions and behavior of living plants. Certain aspects of physiology, namely the relation of liv ing plants to the environment. physical (includ ing soil. water supply. temperature. etc.) and biological (i.e. other plants and animals), have recently been separated from physiology proper and named ecology (q.v.). Physiology proper restricts itself to the activities of the individual plant. Its study demands a previous knowledge of the anatomy (q.v.) and histology (q.v.) of plants, since familiarity with plant mechanism must precede the knowledge of plant function.
The functions of plants may be divided for convenience into certain groups. First, processes connected with the absorption of materials by the plant from the surrounding medium. Every thing which enters the plant body must do so in solution. The plant depends, therefore, funda mentally upon water, which, with an infinite variety of solutes• both solid and gaseous, it absorbs. (See ABSORPTION.) Necond. processes by which materials escape from the plant body. Gases, especially carbon dioxide and oxygen. are evolved (see .AERATIoN; OSMOSIS) ; a great variety of materials are eliminated from the plant body by various processes of secretion (q.v.) ; and water is evaporated in great quan tity by the surfaces of land plants. ( See TRAN SPIRATION.) Third, processes by which water and foods are moved about from the points of tion or manufacture to the points of use, storage, or loss. But in spite of the fact that the ment of water was almost the first subject that attracted the attention of observers, no satisfac tory explanation has yet been devised. (See CONDUCTION.) Fourth, processes of nutrition. Carbohydrate foods are made only by green plants under the influence of light, a power which is of the utmost importance. since the commercial supply of food and energy depends at present almost wholly upon green plants. The making
of carbohydrates is fully described in the article PHOTOSYNTHESIS. (See CHLOROPHYLL; CHLOROPLASTS.) All plants. however, make pro teid foods, when the necessary supply of car bohydrates, nitrates, and salts is furnished. Pro teid synthesis is discussed in the article FOOD or PLANTS. When proteid foods are at hand, either through manufacture or absorption. they undergo various changes before they become a part of the protoplasm. (For what little is known of these processes, see ASSIMILATION.) Fifth. processes concerned with the release of energy in the plant body or adjacent to it. in such a way that the kinetic energy can be applied to such work as the plant must do. For most plants, these processes constitute respiration. (See TION IN PLANTS.) For a few plants fermentation (q.v.) seems to replace. in part at least, the normal respiration. Sixth. processes of repair and growth. One of the important of the plant is the formation of new material or the rearrangement of old into new parts. The verse operations involved in this constitute growth (q.v.). Scrcath. processes of perception and response. Every function of the plant is more or less influenced by the changes occurring in its vicinity, whether these be entirely outside of the body or only in adjacent parts. In order to adjust itself to these changes the plant must receive impressions of them. and these impres sions must result in appropriate adjustment. The changes in the protoplasm corresponding to changes in the surrounding medium and the chain of effect: thus initiated, aro said to manifest ritability (q.v.). The visible alterations of the rate of growth (q.v.) or in movement (q.v.) are most striking evidences of irritability. See