Plants amange to raise large amounts of water to considerable heights. The precise mechanisms by which they do this have remained controversial over the centuries, which capillary forces, osmotic ...tensions, transpiration, and cohesion being alternatively invoked as the prime agents. We argue that root pressure is essential for most, if not all, plants, and that it cannot be explained by any combination of these four forces: mechanical pumps are often required to achieve the reverse osmosis occuring in young root tips. The pumps are singlecell, osmosis- plus contraction-pressurized, plasmodesma-valved chambers, located in the endodermis (and sometimes exodermis). They act as heat pumps, cooling the sap by less then 0,2K. Their cell walls are strengthened by undulating Casparian strips; their valves are pit fields, each traversed by a large number of plasmodesmata in the outer perclinal cell walls of the endodermis, whose sub-micron foldings allow them to serve as flexible pistons. In this way, the root pressure of a large plant is established by numerous cell-sized water pumps working at frequencies of about 1 Hz - like the human heart.
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