i66 THE LOSS OF WATER FROM PLANTS 



of necessity approximations, although, not of course, without their utility 

 in the consideration of certain types of problems. 



The results of such calculations indicate that sufficient water may transpire 

 from maize plants during the course of a season to cover the field in which 

 they are growing to a depth of 15 inches (Transeau, 1926). Similarly 

 Minckler (1936) has calculated that in a growing season water equivalent to 

 4.8 inches of rainfall may transpire from an acre of American elm trees and 

 that an acre of red maples, growing in a very moist habitat, may lose water 

 equivalent to 28.3 inches per acre. In evaluating all such data it must be 

 remembered, however, that the magnitude of transpiration varies tremendously 

 with the available soil water supply. 



Significance of Transpiration. — Opinions regarding the significance of 

 transpiration have ranged all the way from those which would put it prac- 

 tically on a par with such processes as photosynthesis and respiration, to those 

 which would relegate it to the category of a "necessary evil" (Curtis, 1926). 

 The principal roles which have been ascribed to the transpiration of plants 

 can be summarized under the following three headings: 



1. Supposed Role in the Movement of IFater. — It is often claimed or 

 assumed that the movement of water through the plant requires the occurrence 

 of transpiration from the leaves. That this concept is entirely erroneous 

 will be clear from the discussion of the mechanism of the conduction of water 

 through plants in Chap. XV. Under conditions of high transpiration the 

 movement of water through plants is, in general, more rapid than under con- 

 ditions of low transpiration. The mechanism causing ascent of water through 

 a plant operates in such a way that any decrease in the turgidity of the meso- 

 phyll cells favors a more rapid movement of water towards those cells. Hence 

 a rapid transpiration rate, which invariably results in a considerable loss of 

 turgidity by the mesophyll cells, usually speeds up the rate at which water 

 ascends through the plant. However translocation of water to the extent 

 that it is used in photosynthesis, growth or other metabolic processes con- 

 tinues even if the transpiration rate is negligible. 



2. Supposed Role in the Absorption and Translocation of Mineral Salts. — 

 It has often been assumed that the more rapid the rate of transpiration, the 

 greater the rate of absorption of mineral salts. This view implies that the 

 dissolved mineral salts are swept into the plant along with the absorbed 

 water, a postulation which ignores much evidence that the mechanisms operat- 

 ing in the absorption of water and the absorption of mineral salts are very 

 different (Chap. XVII ; Chap. XXIV). 



The results of certain experiments do indicate that a somewhat larger 

 quantity of mineral salts accumulates in plants under conditions favoring high 



