THE TRANSPIRATION STREAM. By Henry H. Dixon. (University of London
Press. 2s. 6d. net.) Professor Dixon has been arguing for years in favour of a mechanical theory of the ascent of sap in trees and plants, and he now reviews the evidence' for and.against the theory and deals with the criticisms that have been ad,vinced, not always very suavely, in dispute of his findings. ft is quite pleasant to And that all acrimony has not departed from academic discussions ; we remember from our schooldays how delighted we were to find in the footnotes " Putidissime bic Scaliger and other such outbursts of abuse. But. Professor Dixon is moderation-and suavity itself : his riposte to Herr Ursprung must be much more effective than Herr Ursprung's own bludgeon-work. Professor Dixon gently remarks : " His theory is remarkable ; and in view of experimentally ascer- tained facts is untenable." The " Cohesion Theory of the Ascent of Sap," to which Professor Dixon subscribes, is roughly this : the tiny columns of water which reach from root to leaf are normally unbroken ; they have a compara- tively large cohesion or elasticity ; when the water at the top of the column evaporates, the Column is in a state of tension and the water is pulled upwards, hanging from the upper termination. It is interesting to learn that " even in leaves which have completely wilted from loss of water intact water-columns may be microscopically observed." The problem is of interest, too, in determining how food materials are transported inside the plant. The opposing theory is that the sap ascends, in some way, through the organic action of the plant cells.
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