1916] SHULL—SOILS 21 
approximately 49 per cent, and this average agrees rather closely 
with the intake in the Oswego silt loam subsoil and in the no. 2/o 
sand at their wilting coefficients. This probably means that the 
wilting coefficient represents a fairly definite water-holding power 
for-the soil particle, regardless of its size. By comparison of the 
results given in table VIII with those given in tables III and VI, 
it is seen that the ‘‘back pull” of the soil particles, the force with 
which they withhold water from seeds and plants at this critical 
moisture content, is not more than the equivalent of o.1M. NaCl 
solution, or 3-4 atmospheres. This value is surprisingly low. 
V. Discussion 
In this paper the term internal forces of seeds is used to designate 
the resultant of all forces within the seed tending to cause intake 
of moisture; and by surface forces of soils is meant the resultant of 
all the forces of adhesion, cohesion, surface force, etc., as determined 
by size, chemical composition, and character of the surface of the 
soil particles, which cause the soil to retain moisture. 
In order to make clear the nature of the problems involved in 
this work we shall first consider rather fully the moisture relations 
between seeds and their environment, and then the moisture rela- 
tions of soils to seeds and to the root hairs of living plants. A 
careful, study of the moisture relations existing between organic 
bodies, like seeds, and the atmosphere, the soil, and osmotic solu- 
tions, if the seeds have semipermeable coats, convinces one that 
the entrance or exit of water from the seed is due to the interplay 
of such internal forces as capillarity, imbibition force, colloid 
hydration forces, etc., with external forces such as atmospheric 
evaporation, the surface forces active on soil particles, osmotic 
pressure, etc., according to the environment of the seed. Moisture 
flows into or out of the seed as one or the other of these sets of 
forces is the greater. Movement of water continues only until 
the two forces, unequal at the start, become equal. This establishes 
moisture equilibrium, and further movement of water must be 
consequent to some disturbance, external or internal, of the bal- 
anced condition of forces. Moisture equilibrium may obtain at 
any moisture content of the seed, if only the two forces are equal. 
