The paper provides a mathematical discussion, illustrated with graphs, of the relationship be-tween pressure gradients required for the flow of water through soil and through roots at different soil depths, radius of root and of xylem vessels of grasses (Lolium perenne, Phleum pratense and Dactylis glomerata), and the upward flow of soil water to replenish that lost by transpiration. The discussion has a bearing on the point at which water enters the grass roots from the soil and explains why, in the grassland studied, little water was found in roots below a depth of 10 cm.-D.B. (Abstract retrieved from CAB Abstracts by CABI’s permission)