REVIEWS 115 



tion in the wilting coefficient, but some of his conclusions are open 

 to question. Following Hannig*' he had taken the average osmotic 

 pressure of the root cell sap to be equivalent to from 7 to 8 atmos- 

 pheres. Since the water-holding power of the soil at the wilting coeffi- 

 cient is only 3 to 4 atmospheres, and furthermore since seeds in soil 

 at the wilting coefficient take up within a few per cent of as much 

 moisture as they do when placed in water, he concludes that wilting 

 at the wilting coefficient is not due to lack of moisture. The cause 

 of wilting, he says, is that the rate of movement of water in the soil 

 becomes too slow to meet the needs of the plant. This, he continues, 

 explains why Briggs and Shantz, working under uniform conditions 

 of evaporation, found the same wilting coefficient for all kinds of plants 

 in a given soil, regardless of root sap concentration and other variable 

 factors. It also explains, he adds, why with high evaporation wilting 

 occurs before the water has been reduced to the wilting coefficient as 

 in the work of Caldwell^ and of Livingston and Shive** ; it is because 

 the whole matter is a "question of rates." 



Here Shull has been influenced by a misconception which Cald- 

 well and Livingston have introduced into the application of the wilting 

 coefficient. Caldwell found that when plants which have been grown 

 under conditions of moderate, or even fairly high, evaporation are 

 placed under conditions of intense evaporation they wilt before reduc- 

 ing the water to the wilting coefficient.^ His results gave rise to the 

 idea that the wilting coefficient is dependent on the rate of evapora- 

 tion ; and Livingston and Shive went so far as to plot a curve of the 

 relation between evaporation and the wilting coefficient, showing that 

 the water left in the soil at permanent wilting increases with the rate 

 of evaporation. 



Briggs and Shantz show, on page 62 of Bulletin 230, that field 



s Hannio;, E. "Untersuchun8;en uber die Verteilung des osmotischen Drucks 

 in der Pflanze in Hinsicht auf die Wasserleitung." Ber. Deutsche. Bot. Gesells, 

 30: 1Q4-204, 1912. 



^ Caldwell, T. S. "The Relation of Environmental Conditions to the Phenomenon 

 of Permanent Wilting in Plants." Physiological Researches, Vol. I, No. 1, 1-56, 

 Baltimore, Md., 1916. 



^ Shive, J. W., and Livingston, B. E. "The Relation of Atmospheric Evaoorat- 

 ing Power to Soil Moisture Content at Permanent Wilting in Plants." Plant World, 

 17:81-121,1914. 



5 Caldwell does not say much about the conditions under which his plants 

 were grown before making the wilting tests, but it appears that some were grown 

 in the open. If this open had the same evaporation and insolation as that of the 

 wilting tests, the explanation which the reviewer gives below does not hold for all 

 of Caldwell's work. It seems probable, however, that in most cases the wilting 

 tests in the open were made under conditions more severe than those under which 

 the plants had been grown. 



