SALIENT FEATURES OF THE ROOT SYSTEM RELATIVE TO THE PROBLEM OF SALT ABSORPTION

Investigations on salt accumulation which utilize tissue derived from root systems have a double interest. Not only may they contribute to the elucidation of the mechanism of accumulation regarded as a problem of cell physiology, but also to the further question of the absorption of salts envisaged, not merely in terms of individual cells, but of the root as the most effective of salt absorbing organs. The former aspect has been discussed by HOAGLAND and BROYER (10), and the factors which determine the accumulation of inorganic salts by the cells of barley roots in particular have been outlined. This first discussion concerned only a composite mass of root material which, though accurately reproducible, still comprised cells at all stages of growth and development and of necessity no attempt was made to evaluate their relative efficiency in absorption or to localize along the axis of an individual root those regions most responsible for accumulation. It is the latter aspect of the general problem which is now to be discussed. The extensive interest (32) in the gross morphology and growth of root systems under field conditions is clearly inspired mainly by their more conspicuous role of water absorption. In this connection it is generally recognized that the severe delimitation of the absorbing region for water-due to the presence of fat-impregnated tissues (endodermis, exodermis, or even cork) in the older portions of the root system-involves a clear connection between the ability of a root for growth and its efficiency as an absorbing organ. The maintenance of a root surface free from these restrictions is required. A similar general relation must obtain with respect to salts. With particular reference to water, the extent of the absorbing zone has been discussed notably by POPEscO1 (16), and, with emphasis upon anatomical and developmental features, by SCOTT and PRIESTIEY (25). We are not here concerned with the extent of the absorbing zone for water apart from the expectation that, at least in so far as mere penetration of the superficial tissues is concerned, the absorbing zone for salts would be as restricted as that for water. Statements to the contrary (7), based solely upon the disappearance of solutes from external solutions without examination of the tissues of the root, probably fail to discriminate between the 1 This paper may be consulted for citations to earlier work.