116 JOURNAL OF FORESTRY 



crops growing under the high rates of evaporation prevalent at the 

 western edge of the Great Plains reduce the soil water to the wilting 

 coefficient before they wilt. Furthermore, Burr^° has shown, in charts 

 VII and X-XV, that under natural conditions in western Nebraska, 

 where the evaporation rate is high, field crops reduce the soil water 

 to the wilting coef^cient during their period of growth before perma- 

 nent wilting takes place. 



The explanation of the apparent discrepancy between the point 

 of view of Briggs and Shantz and of Caldwell is probably to be found 

 in Caldwell's failure to distinguish between the effects of high evapora- 

 tion and the effects of sudden changes in evaporation. When a plant 

 is moved from moderately sheltered surroundings into the full sun- 

 light, it wilts, regardless of the moisture in the soil. Wilting in this 

 case is due to the change of environment rather than to the severity of 

 the last environment. 



The wilting of plants which are suddenly moved into an environ- 

 ment characterized by high transpiration is not entirely due to slow- 

 ness of water movement through the soil, as Shull implies, but largely 

 to the inability of the root system to supply the sudden increase de- 

 manded by the increased transpiration. This is shown by the fact that 

 a plant can be made to wilt in a water culture by a sudden change in 

 the conditions governing transpiration. In this case wilting is obviously 

 not a question of the rate of water movement outside of the plant. 



This does not, however, explain why plants wilt when their roots 

 can still exert a pull 3 or 4 atmospheres greater than the "back-pull" 

 of the soil. The reason for this, Shantz is inclined to believe," is 

 that the wilting coefficient determination is more a matter of soil 

 physics than of a balance between the "back-pull" of the soil and 

 the pull of the plant. He is "still not entirely sure that the wilting 

 coefficient determination represents anything more (nor less, we might 

 add) than a point in the water-content of the soil at which the water 

 practically ceases to move along the film (on the soil particle), no 

 matter how sharp the gradient at the edge of this film." An indication 

 that this may be the case is found in the fact that seeds placed in 

 soil which has been reduced to the wilting coefficient cannot absorb 

 any great amount of water; but when the soil is tumbled over them 

 (as in Shull's experiments) the seeds absorb as much as in a very 



1" Burr, W. W. "The Storage and Use of Soil Moisture." Research Bui. No. 5, 

 Agr. Expt. Sta. of Nebraska, 1914. 

 " Shantz, in a letter to the reviewer. 



