METHOD FOR PLANTS HAVING NO DEFINITE WILTING POINT. 47 



rooted and fine-rooted plants in the preceding determinations. In 

 other words, the wheat plants in the pots occupied also by coarse- 

 rooted plants would tend to give a higher wilting coefficient than 

 those in the pots occupied wholly by fine-rooted plants. This is in 

 accordance with the results obtained, the corn and some of the 

 legumes giving a mean wilting coefficient a little above the average. 

 The results obtained by observing the relative time of wilting of 

 two different plants growing in the same soil mass are then in accord 

 with the conclusions drawn from a comparison of the wilting coeffi- 

 cients obtained with different plants as indicators, namely, that the 

 differences exhibited by crop plants as regards their ability to reduce 

 the moisture content of the soil before wilting occurs are so small 

 as to be of little practical significance in the selection of crops for 

 semiarid regions. There is no evidence that drought resistance in 

 a plant is due to an additional water supply made available for 

 growth by virtue of a greater ability on the part of that plant to 

 remove moisture from the soil. 



METHOD FOR DETERMINING THE WILTING COEFFICIENT FOR 

 PLANTS HAVING NO DEFINITE WILTING POINT. 



The wilting coefficient of a given soil can be found for most plants 

 by the method already described. In the case of plants having 

 aerial water-storage tissues or thick, heavy leaves this procedure can 

 not be followed, since they have no well-defined wilting point. The 

 method herein described makes it possible to determine when a 

 plant of this kind is no longer able to obtain water from the soil as 

 rapidly as it is given off to the air by the plant. This condition is 

 analogous to wilting in the case of plants without water-storage 

 tissues. 



Let us consider a fleshy plant such as a cactus, the roots of which 

 are well established in a mass of soil contained in an impervious 

 pot, with the surface of the soil sealed to prevent loss of moisture 

 except through the transpiration of the plant. Consider this system 

 supported on knife-edges located on opposite sides of the pot, and 

 suppose the system balanced with suitable counterweights so that 

 it is in stable equilibrium when the plant is in a horizontal position, 

 but with the center of gravity high enough to form a system of some 

 sensibility. This system if set in motion will oscillate about the 

 point of equilibrium. As the plant begins to lose water through 

 transpiration, water will move from the soil to replace the water 

 lost from the plant and the soil end of the system will become lighter 

 and rise in consequence. This process will be repeated as often as 

 the balance of the system is readjusted to the original zero point, 

 until finally the soil is no longer able to supply water to the plant at 



230 



