FACTORS ON WHICH GROWTH IS DEPENDENT.
The conditions of growth arc four: First, an adequate supply of constructive materials, which are necessary for the formation of a new organic substance, protoplasm, cell-wall, etc.; second, an adequate supply of water, which is necessary for the enlargement of the cells; third, access of free oxygen, which is necessary for respira tion, by which the requisite energy is set free; fourth, a temperature within definite limits, which, however, vary with different plants, the minimum being approximately 0° C. (32° F.), and the maximum 40° C. (104° F.). The best aver age temperature for the growth of most of the higher plants lies between 25° and 32° C. (77° and 89.6° F.).
The effects of external agents upon growth are numerous and profound. Scarcely any change in external conditions can occur without modify ing the growth of the plant. These effects may he unequal in different tissues, or in different parts of an organ, or they may be uniform, so that the total growth of the part is retarded or accelerated. The most noticeable effects, natural ly, are those which produce enrvature and conse quent displacement of certain parts. These are described under CI I EMOTROPI S M ; E LECTROTRO P ; GEOTROPISM; HELIOTROPISM ; HYDROTRO.
PISM ; RHEOTROPISM ; THERMOTROPISM ; and TIII0 MOTROPISM. The most important effects upon the general rate of growth are those produced by temperature and light. In general, growth is ac celerated by increased temperature up to the op timum point, but further increase beyond the optimum retards it. On the other hand, growth seems to be definitely retarded by light. Since under ordinary conditions plants are subject to daily variations in temperature and light, there arises a corresponding daily variation or 'daily period' in the rate of growth (Fig. 6). Since the temperature rises during the day and diminishes during the night, the general effect will be to accelerate growth during the day and retard it during the night. As light, however, retards growth during the day, the final effect will be a resultant between these two opposing factors, the effect of light as a rule predominating. The min imum usually occurs in the afternoon and the maximum after midnight, and this variation per sists for many days, if plants be kept in contin uous darkness.