Ever since botanists knew that sap was carried from the roots of a tall tree to the leaves at its top they have wondered what force was responsible for this transportation. One theory held was that pressure in the roots acted as a pump from below. The trouble with this was that no pressures could be measured higher than 1.4 atmospheres, which would not do for trees taller than 46 feet. Lately accepted as the most satisfactory explanation is the cohesion theory, in which it is supposed that suction created at the top by...
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