PERMANENT WILTING IN PLANTS 83 



Caldwell,^ working at the Desert Laboratory of the Carnegie 

 Institution of Washington, carried out numerous experiments 

 bearing on the question whether the residual water content of a 

 given soil remains constant when the wilting of the plants rooted 

 therein occurs under widely varying atmospheric conditions, 

 and whether, therefore, the amount of this moisture can be sat- 

 isfactorily calculated from the water holding power of the soil, 

 according to the formula apparently established by the experi- 

 mentation of Briggs and Shantz. The results of Caldwell's work 

 led him to the following conclusions, among others not bearing 

 closely upon the present discussion. (1) The amount of water 

 left in any given soil at the time of permanent wilting of plants 

 rooted therein is not generally to be regarded as independent of 

 the atmospheric conditions under which wilting has occurred. 

 (2) The magnitude of this water residue for any given soil is 

 determined largely by the evaporating power of the air during the 

 period of wilting. (3) If the period of wilting be one of low evap- 

 oration rates, so that the rate of water loss by transpiration from 

 the plant never greatly exceeds that of its gain by root absorp- 

 tion, wilting occurs only when the water supply fails at the sur- 

 face of the absorbing organs. (4) If the period of wilting is 

 characterized by high rate of evaporation, the plant loses water 

 by transpiration at a rate which greatly exceeds that of absorp- 

 tion by the roots, and under these conditions wilting should 

 occur long before the water ceases to enter the latter organs at 

 a considerable rate. In such a case the failure of the water supply 

 which brings about wilting occurs somewhere between the absorb- 

 ing and transpiring surfaces, and soil moisture residues observed 

 under these conditions are higher than those observed with wilting 

 under conditions of low evaporation intensities. (5) In order that 

 the soil moisture residue may be determined solely by soil con- 

 ditions, it is only necessary that the evaporating power of the 

 air do not exceed a certain limit. 



Caldwell clearly points out that the limit just .mentioned is 

 deserving of attention and that when this limit becomes known 



^ Caldwell, J. S., The relation of environmental conditions to the phenomenon 

 of permanent wilting in plants. Physiol. Res. 1 : 1-56. 1913. 



