|Xylem in Plants|
Xylem sap always moves from the roots to the leaves. It travels by bulk flow, like water in a series of pipes, rather than by diffusion through cells. Three things cause xylem sap to flow:
• The soil solution is more dilute than the cytosol of the root cells. Thus, water moves osmotically into the cells, creating root pressure. Even under optimal conditions, root pressure can only lift water a couple of feet.
• Capillary action helps sap to flow up the narrow tracheids.
• Another cause of xylem sap flow is transpirational pull. This is the reverse of root pressure, caused by the transpiration of water from leaves.
In perennial plants, xylem is laid down in multiple phases. Primary xylem is one of the tissues left behind by the apical meristem. Secondary xylem is laid down by vascular cambium on the outside of the xylem column.