26 BOTANICAL GAZETTE [JULY 
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. KNO,, 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 This idea is strongly supported by unpublished work 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 Epita 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 
