High Pressure Sap

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 evaporation is transmitted through a cohering column of water under tension all the way down to the roots.

Last week Dr. Philip Rodney White of the Rockefeller Institute's Princeton laboratories took a long stride toward solving the problem, and incidentally spilled much wind from the sails of the cohesion theorists, by announcing that he had found enormous pressures in the roots of tomato plants--pressures high enough to serve tomato plants hundreds of feet tall. The trouble with previous pressure experiments, it appeared, was that they were made on dead or dying roots. At Princeton, Dr. White has an apparatus which keeps detached roots alive indefinitely by supplying them with nutrient fluid. When he attached glass tubes carrying columns of mercury to his tomato roots, the mercury went up until it indicated a pressure of more than eight atmospheres (125 Ib. per sq. in.), at which point the powerful roots broke his apparatus.

The cohesion theory cannot be entirely shelved, because water will rise in stalks from which the roots have been cut away, indicating suction from above. But Dr. White, speaking at the American Association for Advancement of Science convention in Indianapolis, showed clearly enough that root pressure could serve as an adequate explanation for sap transportation in the tallest known trees, and to that extent botanical theory must be revised. His colleagues considered his announcement so important that they awarded him the annual $1,000 prize conferred for the best paper among hundreds at the A. A. A. S. midwinter meeting.

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