28 BOTANICAL GAZETTE [JULY 
solutions, and (0) vapor pressure equilibrium. The osmotic method 
is at present the more reliable. 
2. The air-dry seeds of Xanthium show an nigel attraction for 
water of nearly 1000 atmospheres. 
3. The attraction which exists at any moisture content of the 
seed between air-dry and saturation can be approximated. Table 
IIT 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’ suctae 
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 11 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 a o.1M. 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 ee 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 Hannic 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. 
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