The Physical Nature of Transpirational Pull

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Abstract
Physiology texts state that "transpirational pull" initiates sap ascent by diffusion of liquid water from the top of vessels in the leaf to the evaporating surface of mesophyll cells. This concept is shown to be impossible because under commonly found conditions, diffusion rate of water vapor may be 600,000 times that of liquid water. The objection of Curtis and Clark that it would require a pressure of 100,000 atms to produce a flow through microcapillaries of the cell wall is also shown to be in error. Actually, less than 0.5 x 10-8 atms would be needed. A consistent, physically sound theory of sap ascent is possible only if it is assumed that surface tension forces initiate the rise[long dash]i.e., that adhesive (imbibitional) forces are increased at the surface due to evaporation and this causes the rise of the whole column due to cohesion between water molecules.

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