26 BOTANICAL GAZETTE [;uly 



The most careful, extensive, and valuable study of the osmotic 

 pressure of the sap of root cells under ordinary conditions of soil 

 moisture which has been made up to the present is that by Hannig 

 (15). He finds that the average root cell sap pressure for 64 species 

 of plants is o. 21M. KN0 3 , or equivalent to 7 or 8 atmospheres. It 

 is seen, therefore, that the water-holding power of the soil at the 

 wilting coefficient is only about half that of the average osmotic 

 pressure of the sap of the root cells. Certainly wilting at the wilt- 

 ing coefficient cannot be due to lack of water, for seeds come within 

 a few per cent of taking up as much moisture at the wilting co- 

 efficient as when placed in water itself. Nor can it be due to 

 equalization of forces between root hair and soil water, for there 

 is still a gradient of 4 atmospheres of force in favor of the plant. 

 Moisture and gradient for movement of water toward the plant 

 are both present, and yet the plant wilts! 



Even in cases where the soils are drier than the wilting coefficient, 

 the accommodation of the root hairs mentioned above would prob- 

 ably maintain this gradient of a few atmospheres in favor of mois- 

 ture intake. This idea is strongly supported by unpublished work 

 of Miss Edith A. Roberts, who, working in this laboratory, has 

 shown that seedlings of mustard and radish grown in sugar solu- 

 tions develop root hairs with osmotic pressures usually about 4 

 atmospheres in excess of the medium in which they grow, the same 

 amount of excess as this gradient at the wilting coefficient. This 

 relationship of internal to external forces was maintained, in her 

 work, up to volume molecular solutions of cane sugar. It is exceed- 

 ingly probable, therefore, that as soils dry out beyond the wilting 

 coefficient the root hairs maintain an osmotic pressure a few 

 atmospheres in excess of the soil forces until those forces become 

 relatively very high. Nevertheless, permanent wilting occurs 

 within a narrow range of soil moisture under moderate conditions 

 of evaporation. 



There seems to be but one reasonable explanation for this situ- 

 ation. The wilting of plants at the wilting coefficient of the soil must 

 be due to the failure of water movement from soil particle to soil 

 particle, and from these to the root hairs, rather than from lack 

 of moisture or gradient. This does not mean complete cessation 



