230 BRIGGS AND SHANT2 : WILTING COEFFICIENT OF PLANTS 



not usually exceed one part in a hundred, which is comparable 

 with the accuracy with which the moisture retentiveness of the 

 soil used can be determined by purely physical methods. 



The wax seal method is also particularly adapted to the study 

 of transpiration, since all loss of water is avoided except that 

 taking place through the plant. « 



Wilting coefficient determinations have been made in a series 

 of twenty soils, ranging from sands to clays. In this work, in- 

 volving about 1300 determinations, a large number of varieties of 

 the different crop plants have been tested, as well as many native 

 plants from the Great Plains. 



The results obtained show that species differ only slightly as 

 regards the soil moisture content at which permanent wilting 

 first takes place. Taking 100 to represent the average wilting 

 coefficient, the different species tested give an extreme range 

 (excepting Colocasia and Isoetes) from 92 for Japan rice to 106 

 for a variety of corn. Most of the species and varieties tested 

 differ much less than this. On the same scale, the great crop 

 plants gave the following values, obtained by combining the dif- 

 ferent varieties: corn, 103; wheat, 99; oats, 99; sorgo, 98; millet, 

 97; barley, 97; rye (one variety only) 94; rice, 94; grasses, 97; 

 legumes, 101. 



We are, then, led to conclude that the differences exhibited by 

 crop plants, in their ability to reduce the moisture content of 

 the soil before wilting occurs, are so slight as to be without prac- 

 tical significance in the selection of crops for semi-arid regions. 

 Furthermore, we believe that the slight differences which we have 

 observed are largely due, not to the ability of one variety to exert 

 a greater tractive force upon the soil moisture than another, but 

 to the more perfect root distribution of one variety as compared 

 with another. Drought resistance in certain plants cannot, 

 then, be attributed to their ability to exert a greater force upon 

 the soil moisture, and so gain an additional water supply. 



A series of comparisons have also been made of the relative 

 time of wilting of different plants grown together in the same pot. 

 In practically every case, wilting occurred simultaneously, which 

 is in harmony with the above conclusions. 



