Root Pressure Theory

Root pressure is a real, measurable positive pressure that builds up inside the tracheary elements of the xylem, generated purely by the root's own metabolic activity rather than by any pull exerted from above. It was first observed by Stephen Hales, often described as the father of plant physiology, who noticed that a well-watered plant, if cut a few inches above ground level, exudes sap with some visible force - a phenomenon he called sap exudation or bleeding; Stoking later, in 1956, gave the effect its formal definition. Despite being real, root pressure theory has serious limitations as a general explanation for the ascent of sap: it is entirely absent in gymnosperms, a group that includes some of the very tallest trees on Earth; there is no consistent link between its size and the actual speed of sap ascent, since ascent is fastest in summer even though root pressure is then at its lowest, and slowest in winter even though root pressure then peaks; sap continues rising even in cuttings and grafts that have no roots at all; and, most tellingly, it …