Water Within the Plant
We have seen that the plumbing system of the plant consists essentially of a more or less vertical column of parallel tubes dispersing into discrete branches at both ends (the xylem) and ending in close association with thin-walled cells (of the roots, and of the leaf mesophyll). The whole system is full of water. Commonly in trees the vertical distance between the extremeties of the tubes is many metres, equivalent to hydrostatic water pressure of the same height. To raise water to the top of an ordinary tree would require a pressure of 10 to 20 bars (1–2 MN m-2), far more than recorded in the most successful of root pressure demonstrations. The rate of exudation from a detopped plant is usually only of the order of 5 per cent of the previous transpiration rate of the intact plant. Thus osmotic uptake of water into the roots cannot explain the flow of water to the tops of trees.
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