BRIGGS AND SHANTZ: WILTING COEFFICIENT OF PLANTS 231 



A balancing method has been developed for determining the 

 wilting coefficient for plants in which wilting [ s difficull to observe, 

 owing to structural peculiarities. The plant is potted in a glazed 

 pot and the soil surface sealed with wax as before. The potted 

 plant is then mounted horizontally in a frame balanced on knife 

 edges. The water transpired by the plant is at first replaced 

 from the supply in the soil. During this time the soil end of t lie 

 system tends to move upward, owing to the loss of water, and 

 is kept balanced by adjusting a counterpoise. A short period 

 then occurs during which the water lost thru transpiration is 

 supplied both from the soil and from the storage tissues of the 

 plant. During this time the system appears sluggish. Finally 

 the amount supplied from the soil is insufficient to offset the in- 

 creasing amount lost from the storage tissues of the plant, and 

 the direction of motion of the system is reversed, the plant mov- 

 ing upward. The soil moisture content at this time corresponds 

 to the wilting coefficient of that soil for the particular plant 

 used, since the loss of water from the plant tissues would be accom- 

 panied by wilting, except for structural peculiarities. 



We have shown by this method that the olive and Opuntia 

 undergo a reduction in the moisture content of their aerial parts 

 at the time other plants wilt, altho no outward evidence of such 

 loss is apparent. 



That part of the soil moisture content that is available for 

 plant growth is represented by the difference between the actual 

 water content and the wilting coefficient. The latter determi- 

 nation is consequently essential in any critical study of the rela- 

 tion of plant growth to soil moisture. The desirabilhVy of a rapid 

 determination of the wilting coefficient in connection with field 

 work led to an investigation to determine whether it could be 

 computed from physical measurements of the moisture retentivity 

 of the soil. A comparison of the wilting coefficient has been made 

 with the moisture equivalent, hygroscopic coefficient, moisture 

 holding capacity, and the mechanical analysis (all expressed in 

 percent), using a series of soils ranging from sand to clay. From 

 this comparison a series of linear relationships have been estab- 

 lished as expressed in the following equations, which thus provide 



