A'ERA'TION (Lat. ai'r, air). In botany, the exchange of gases between living plant tissue and the surrounding medium. This exchange is manifested by two processes. In one of these, viz., the manufacture of foods (see PHOTOSYNTHESIS ) , carbon dioxide is required by the plant and oxygen must be eliminated. On the contrary, in the other prod.ess, viz.. respira tion (q.v.), oxygen is necessary and carbon diox ide must be eliminated. The former process is confined to green plants.; the latter is essential to all except a few of the lowest and simplest type (anaerobic bacteria). A Inoue: the smaller plants, and those whose bodies are made up of interwoven filaments (Fungi), the go souls ex changes can take place directly, since almost every part of the body is in contact with the air or with water. In the former ease, the outside gases diksolve in the constituent water of the eell-wall and are then free to enter; or, arising sisting of irregular passages, i, between the inte rior cells, which communicate with the outer air through microscopic openings, s, between the sur cells (sec STOMATA) ,or through larger breaks in the corky layers of tissue on the surface of the stems. (See LENTICELS. ) The intercellular pas sages and stomata are formed by the partial separation of the cells as they mature. In land
plants they are most abundant, and largest in the green parts, because the gas exchanges in food-making, photosynthesis, exceed those in respiration. In water plants, however, whose opportunity for securing gases from the air is more limited. the aeratin." system reaches its highest development. (See HyonommEs.) The tissues may present to the eye a spongy appear ance, and in some cases the canals in stems and leafstalks may even be large enough to be easily seen with the naked eye (fig. 2). The internal atmosphere pervading these canals is voluminous enough to permit both considerable exchanges between it and the adjacent tissues and the freer diffusion of gases entering from the restricted area of organs exposed to the air. The compo sition of the internal atmosphere is always dif ferent from that of the outer and varies from time to time. During the day the internal at mosphere contains legs carbon dioxide and more oxygen than the external atmosphere, at night the reverse being the case. At all times the in ternal atmosphere contains a larger amount of water vapor. because the wetness of the cell-wall, which is necessary for gaseous exchanges. per mits evaporation. See TRANSPIRATION.