<P> Xylem vessels sometimes empty over winter . Root pressure may be important in refilling the xylem vessels . However, in some species vessels refill without root pressure . </P> <P> Root pressure is often high in some deciduous trees before they leaf out . Transpiration is minimal without leaves, and organic solutes are being mobilized so decrease the xylem water potential . Sugar maple accumulates high concentrations of sugars in its xylem early in the spring, which is the source of maple sugar . Some trees "bleed" xylem sap profusely when their stems are pruned in late winter or early spring, e.g. maple and elm . Such bleeding is similar to root pressure only sugars, rather than ions, may lower the xylem water potential . In the unique case of maple trees, sap bleeding is caused by changes in stem pressure and not root pressure (1). </P> <P> It is very likely that all grasses produce root pressure . In bamboos, root pressure is correlated with maximum height of a clone. <4> </P>

Relationship between casparian strip and the development of root pressure