ALTHOUGH THERE HAS BEEN a considerable amount of research on the problem of precisely where minerals and water enter roots, plant physiologists have not yet satisfactorily answered the vexing question. Many physiologists will agree that most attempts to obtain information pertaining to the place of entrance of minerals have, in reality, resulted in data relating to the place of their accumulation. In recent years some progress has been made in designing experimental methods that have yielded information, some of which appears to be contradictory, concerning the actual place of entrance of minerals and water (Hayward et al., 1942; Hayward and Spurr, 1943; Overstreet and Jacobson, 1946; Jacobson and Overstreet, 1947; Kramer and Wiebe, 1952). In connection with this problem, question of the role played by various root tissues in the movement of minerals and water centripetally through the root has been raised. Before the relative importance of each root tissue can be assessed, it is necessary to know precisely the spatial relationships of these tissues in roots growing in various environments. If it were found that the spatial relations differed in roots growing in different environments, a new experimental tool would be at hand. It could be used in designing experiments to give us more information regarding the relative influence of various root tissues on the entrance or rate of movement of minerals and water into and through roots. Heimsch (1951) called attention to the need for such data in his statement if the range of differentiation and maturation of vascular tissues could be modified experimentally, it might be possible to identify the substances or conditions which control this aspect of growth and determine their specific influences. A great deal of speculation appears in both anatomical and physiological literature concerning the relationships of (1) the level of differentiation of one tissue to that of others and (2) the level at which a tissue differentiates (or matures) to the level at which certain physiological processes occur. Since we have relatively few data concerning the levels of tissue differentiation and maturation in roots, it seemed desirable to discover as many of these relationships as feasible for the roots of a particular plant. MATERIAL AND METHODS.-The common garden and commercial canning pea, Pisuim sativum, was