ioi6] 



SHULL— SOILS 



25 



to particles become very great. This force would reach its maxi- 

 mum presumably when the particles were surrounded by a film of 

 water just one molecule thick. The curve for the sand is very simi- 

 lar, except that the period of slow increase of the soil forces is very 

 short, and that the whole curve lies much nearer 

 to the absolutely dry condition. These rela- 

 tions, of course, are conditioned mainly by the 

 size of the particles. 



Let us see now how the soil forces at the 

 wilting coefficient compare in value with the 

 average osmotic pressure of the root hairs. A 

 few years ago Hill (18) showed that the root 

 hairs of Salicornia can in a measure accommo- 

 date themselves to changes in the osmotic con- 

 centration of the surroundings, through increase 

 or decrease of the cell sap concentration parallel 

 to the changes in the environment. That the 

 cell sap of leaves and other exposed parts in- 

 creases in concentration with xerophytic habitat 

 has been shown by Drabble and Lake (13), 

 Fitting (14), and others. The general conclu- 

 sion reached by Drabble and Drabble (12), 

 that the osmotic strength of cell sap varies 

 with the physiological scarcity of water in any 

 area, seems most reasonable, and it doubtless 



807. 



60 



40 



20 



/Urns 

 1000 



400 



•S00 



700 



600 



500 



■ H00 



■ 300 



,200 



100 







Fig. 5. — Curves showing increase in the surface forces of soils as drying proceeds; 

 to the left, for subsoil of the Oswego silt loam; to the right, for no. 2/0 sand. 



holds true of root cells under xerophytic soil conditions. As 

 the soil becomes deficient in water supply, and the surface forces 

 are rapidly increasing in magnitude, we might then expect this 

 increased force of the soil to be paralleled by increase in the osmotic 

 forces of the root hairs, owing to progressive concentration of the 

 cell sap. Such increase might amount to many atmospheres. 



