122 ENVIRONMENTAL CONDITIONS. 



As the soil adjoining the absorbing membranes becomes drier, the 

 surface tension of the capillary films about its particles increases until 

 it finally equals or surpasses the imbibition attraction for moisture 

 exerted by the exposed walls of the absorbing-cells. These capillary 

 phenomena are the main factor in the attraction of the soil for water, 

 and it is this capillary force against which the forces that produce 

 water-entrance into plant roots must operate. It therefore appears 

 that, at the time when absorption ceases, we may expect to find the 

 vapor-tension of the exposed root-membranes just balanced by that 

 of the soil solution. 



The amount of water remaining in different soils, with different 

 plants, has been determined by various workers, and it has been 

 taken to vary with the nature of the plant and with that of the soil. 

 It is lower in sandy soils than in heavier ones, depending thus upon 

 the specific attraction of the soil for water, a variable which depends 

 largely upon the size of the soil particles. Non-available soil-moisture 

 has often been treated briefly and sunmiarily in texts and monographs 

 as a soil constant.^ This it assuredly is not,^ for with a given plant 

 and a given soil, this factor may be made to vary within wide limits, 

 according to the status of the other conditions. It depends, indeed, 

 for any soil and any plant, upon the transpiration-rate for the period 

 during which wilting occurs. The higher the rate of water-loss from 

 a plant the more water will there be in the soil about its roots when 

 permanent wilting occurs. Plants in a moist room remove more 

 water from the soil in which they are potted before permanent wilting 

 occurs than do other similar ones in a dry room or in the open. This 

 state of affairs might have been inferred from the principles already 

 brought out, that the drier the soil becomes the less rapidly will it 

 conduct moisture to the roots of a plant, and that when the transpira- 

 tion-rate surpasses that of intake, there must be a tendency toward 

 wilting. The residual moisture content of a soil, with reference to a 

 certain plant when wilting occurs, is simply the amount of water which 

 that soil contains when the rate of water absorption and conduction 

 to the foliage have been, for an adequate period, less than the rate 

 of loss from the leaves. The length of the period of lag which elapses 

 between the time when the rate of foliar water-supply first falls below 

 that of transpiration and the time when permanent wilting ensues 



1 The best general treatise on soils and their relation to plants is, so far as we are aware, 

 Mitscherlich, E. A., Bodenkunde fur Land und Fortswirte, Berlin, 1913. The work can not be 

 too highly commended as, in general, a logically and physically sound treatise on this, one of the 

 most difficult of biological subjects. On the general phenomena of capillarity and the complex 

 principles upon which these depend the reader may be referred to Freundlich, H. Kapillarchemie, 

 Leipzig, 1909. 



2 Livingston, B. E., Present problems of soil physics as related to plant activities, Amer. 

 Nat., 46: 294-301, 1912. — Briggs and Shantz 1912, Brown 1912, Caldwell 1913.— Shive and 

 Livingston 1914. 



