iqi6] 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. 2/0 

 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. iM. 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. 



