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The primary path of salt movement from root to shoot of vascular plants is through the xylem, which constitutes a group of conduits contained within the central cylinder, the stele. In roots the stele is normally delineated by a specialized layer of cells, the endodermis, and surrounded by a multilayered sheath of cortex parenchyma which lies beneath the epidermis." 2 In passing radially from the root surface to the xylem, salts may be absorbed by cells at the root surface and pass centripetally through a cytoplasmic continuum, the symplasm,3' 4 or may diffuse into the cortex through the water-free space5 of the cell walls, there to be absorbed into the symplasm of the cortical cells. Radial movement by free diffusion through the free space is for practical purposes terminated at the boundary of the stele by the nonporous Casparian strips2 which impregnate the radial walls of the endodermis. Thus ion entry into the stele must be through the symplasm. Plasmodesmata-cytoplasmic connections between contiguous cells-traverse the encompassing cellulosic walls, including the tangential walls of the endodermis, and provide an internal path of ion movement from epidermal cells and cortex to within the central cylinder.' With respect to the deliverance of ions to the xylem, the large vacuoles of the cortical and stelar parenchyma represent a diversionary sumpa salt repository which is in parallel with the xylem.6 7 The concentration of salt, hence of cations and anions alike, in the xylary fluid is frequently well in excess of that in the surrounding medium.'1 89 The foregoing phenomenon is the basis of root pressure. Since mature xylem is nonliving, and in effect represents a standpipe, it is evident that osmotic work must be done between the root surface and the xylem when the total concentration of salt in the xylem exceeds that of the external solution. The choice regarding the site of osmotic work has involved primarily two alternatives. On the one hand, it has been suggested that the endodermis or neighboring stelar parenchyma represents secretory tissue which pumps ions into the xylem.7' 10 11 On the other hand, it has been proposed that ions are accumulated in the thermodynamic sense in, the cytoplasm of the epidermal and cortical cells, following which, movement into the stele takes place passively through the symplasm in response to a gradient of chemical potential."' 12 In the latter view ions must ultimately leak from stelar parenchyma into the vessels of the xylem. In both cases loss of salt by diffusion from the vascular conducting elements to the environment is deemed to be prevented by the Casparian strips of the endodermis. When Crafts and Broyerl first examined the question of salt movement into the xylem of roots, they proposed that ions leak into the xylem from contiguous living cells within the stele, and that the leaky condition of stelar cells is the consequence of diminished respiratory metabolism brought about by an unduly low 02 and high CO2 concentration thought to obtain in the center of the root. The latter inter-

THE DEVELOPMENT OF DIFFERENTIAL PERMEABILITY IN ISOLATED STELES OF CORN ROOTS