132 WILLIAM H. BROWN 



wide range of intensities of the evaporating power of the air, 

 we may expect a given evaporation rate to produce wilting in dif- 

 ferent plants growing in the same soil, with about the same soil 

 moisture content. This is not in agreement with what is usually 

 stated, but has been convincingly substantiated by the work of 

 Briggs and Shantz, 7 who have arrived at the same conclusion 

 as that just given from an extensive series of experiments with 

 many plant forms, well distributed in the natural system. 



That high soil temperature may affect the percentage of 

 residual soil water at the time of wilting, is shown by the follow- 

 ing observations. When plants grown in zinc cans were allowed 

 to wilt in the open, where they were exposed to the direct rays 

 of the sun, the temperature of the soil was always higher than 

 in flower pots similarly placed. Under these conditions the per- 

 centage of residual water was greater for the plants in cans than 

 for those in pots. When, however, the plants wilted in the room 

 or in the moist chamber, both the temperature of the soil and 

 the percentage of residual moisture were practically the same in 

 the two cases. From this it appears that excessive soil temper- 

 atures, like high evaporation rates, increase the amount of soil 

 moisture which is present at the time of wilting. 



If the soil in which the plants were growing was very moist 

 when they were transferred from the open to the room or to the 

 moist chamber, it took several days for the soil to become dry 

 enough to produce wilting. In such cases the plants always 

 wilted with a higher percentage of water in the soil than in the 

 cases where the soil was sufficiently dry at the time of transfer 

 so that wilting occurred within twenty four hours. This shows 

 how capable these plants are of acclimatization, and emphasizes 

 the importance of the internal conditions of the plant in deter- 

 mining the soil moisture content at the time of wilting. 



Briggs and Shantz (loc. cit.), in their thorough-going treat- 

 ment of certain phases of wilting in relation to soil moisture, 

 found that rather marked differences in humidity and sunlight 



' Briggs, L. J., and Shantz, H. L., The wilting coefficient for different plants 

 and its indirect determination. Bulletin 230, Bureau of Plant Industry, U. S. 

 Dept. of Agric. 1912. 



