To appreciate the combined effects of root pressure and transpirational pull, imagine a vertical tube in which water is both pumped under pressure at its base and pulled by suction from above. The water would move at a considerable speed; but for the flow to continue, water must also be removed from the top of the system. In most plant species, about 98 percent of the water entering the roots is lost in the form of transpired water vapor from the leaves. One can readily perceive the outpouring of transpiration when, on a hot, dry day, the air beneath a shady tree feels cooler and is more
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