206 



THE AMERICAN NATURALIST [Vol. XLYI 



of supplying moisture to the absorbing regions of the 

 plant. 



It would seem, a priori, that a flooded soil should offer 

 the least possible resistance to water movement, but such 

 a soil appears indirectly to reduce water entrance in 

 many forms by influencing (probably directly or indi- 

 rectly in a chemical way) the internal conditions of the 

 plant, and it is only with a soil in a considerably drier 

 condition than the flooded one, that we find the optimum 

 subterranean environment for ordinary plant processes. 

 As the soil becomes drier, its direct resistance to water 

 intake by the roots increases, slowly at first, then rapidly, 

 and at a certain stage (for any given complex of aerial 

 conditions, and hence for any given transpiration rate) 

 the combined resultant of the movement of soil moisture 

 to the root surfaces and that of these surfaces through 

 the soil (by growth) falls to a magnitude so low that the 

 processes of transpiration and of growth, etc., remove 

 water from the tissues more rapidly than it enters below. 

 This condition of the substratum is approximately what 

 is usually termed the wilting point, and the remaining 

 water in the soil is said to be unavailable for plants. 



In researches which have yet to be published, my asso- 

 ciates and I have shown that this wilting point is not the 

 constant which it has been supposed to be, for either soil 

 or plant. It is possible to cause the lower limit of "avail- 

 able" water in the soil to assume almost any magnitude, 

 within a broad range, for any given plant, merely by 

 altering the rate of transpiration, — through proper 

 changes in the evaporating power of the air and the 

 intensity of the impinging solar radiation. The wilting 

 point thus ceases to have any meaning at all, unless the 

 corresponding rate of transpiration is known, or unless, 

 indeed, the aerial environment is known to be the same 

 throughout any series of cultures the data from which 

 are to be compared. 



The primary problem, then, which must be quantita- 

 tively solved if we are to place the soil water relation in 



