82 SYMPOSIUM II FACTORS AFFECTING SOIL MOISTURE-

The gross effects of deficient and of excessive soil moisture on plant growth are well known, but controversy has existed for many years around the question whether the so-called “'available moisture ” is equally available for plant growth or available only with such increasing difficulty that plant growth functions are retarded before the wilting point is reached. Existing viewpoints are illustrated by schematic diagrams and the probable effects of plant, soil, weather, and some other miscellaneous factors on. soil rnoisture-plant growth relations, discussed. Considered are the theory that plants can obtain water with equal facility between field capacity and permanent wilting percentage; the notion that growth diminishes progressively as soil moisture falls below field capacity; and the theory that plant functioning is directly related to the tenacity with which water is held by soil. Also considered are views that physiological activity may be related to time integrated soil moisture stress or to maximum soil moisture stress." Various measurable aspects of plant 'growth ' (elongation, increase in fresh or dry weight and vege-tative versus reproductive development) do not respond in the same manner to increasing moisture stress. Appreciable shifts may occur m the relative abundance of certain chemical constituents within plants. Indicated are possibilities of so controlling soil moisture stress over the growing season as t