28 BOTANICAL GAZETTE [july 



solutions, and (b) vapor pressure equilibrium. The osmotic method 

 is at present the more reliable. 



2. The air-dry seeds of Xanthium show an initial attraction for 

 water of nearly iooo atmospheres. 



3. The attraction which exists at any moisture content of the 

 seed between air-dry and saturation can be approximated. Table 

 III gives the data. 



4. The seeds have in turn been used to measure the complex 

 moisture-holding forces of soils, with the following results: 



a) The air-dry subsoil of the Oswego silt loam holds its hygro- 

 scopic moisture with about the same force as an air-dry seed, that 

 is, about 1000 atmospheres. 



b) As the moisture content of the soil increases, the surface 

 force decreases rapidly. When about 3 . 5 per cent of water has 

 been added to the air-dry soil, the force remaining is about 375 

 atmospheres. When the soil moisture reaches 6 per cent above 

 air-dry in this soil, the moisture is held with a force of 130 or more 

 atmospheres. At n per cent above air-dry the holding power has 

 fallen to 22.4 atmospheres. 



c) At the wilting coefficient of the soil (13.3 per cent above 

 air-dry in the Oswego silt loam subsoil) the "back pull" of the soil 

 particles amounts to not more than that of ao.iM. NaCl solution, 

 that is, not more than about 4 atmospheres. This is shown to hold 

 true for a number of types of soil with widely varying wilting 

 coefficients. 



5. This water-holding power of soils at the wilting coefficient 

 is less than the osmotic pressure of the root hairs of many kinds of 

 plants, as shown by Hannig and others. 



6. The wilting of plants at the wilting coefficient of the soil 

 cannot be due to lack of moisture in the soil, nor to lack of a gradient 

 of forces tending to move water toward the plant. 



7. The view is held, therefore, that the wilting at this critical 

 soil moisture content must be due to the increasing slowness of 

 water movement from soil particle to soil particle, and from these 

 to the root hairs, the rate of movement falling below that necessary 

 to maintain turgidity of the cells of the aerial parts, even under 

 conditions of low transpiration. 



