WATER OF SOIL IN ITS RELATION TO PLANTS 257 



the viscosity increases very rapidly and cuts down the 

 capillarity to such a point that the needs of the plant 

 are unsatisfied. Wilting therefore occurs simply be- 

 cause the soil is unable to move the water rapidly 

 enough for crop needs. As the viscosity of the water 

 increases very rapidly after the point of lento-capillarity 

 is reached, the wilting coefficient is a figure somewhat 

 less than the percentage representing the lento-capillarity ; 

 also, it is greater than the hygroscopic coefficient, since 

 wilting due to viscosity occurs before it is possible 

 for the film to become thinned to the zone of hygro- 

 scopicity. Not only all the hygroscopic water is unavail- 

 able, then, but also a certain small quantity of the 

 capillary water lying between the point of wilting and 

 the hygroscopic film. This relationship is shown by 

 data from the work of Heinrich and of Briggs and Shantz ; 

 men working at widely different times and under entirely 

 different conditions. 



Relation of the Wilting Point to the Hygroscopic Co- 

 efficient. Heinrich x 



1 Heinrich, R. Ueber das Vermogen der Pflanzen den Boden 

 an Wasser zu erschopfen. Jahresbericht der Agri.-chem., 

 Band 18, Seite 368-372. 1875. 



