THE PHYSIOLOGICAL ROLE OF WATER IN PLANTS. 7 



where the soil particles are largest. The following- table' shows the 

 coeiticieiits of absorption, liygroseopicity, and evaporation for various 

 types of soil : 



Cocfficietitfi of absorption, hiigroscopivitji, and eraporation in xoils. 



Absorptive capacity : 



Sand 25. 



Clay 10.0 



Lime 70. 



Garden soil 89. 



Hiimns 190. 



Coefficients of liygroscopicity : 



Calcareous sand 1. .^ 



Garden soil 2G. 



Clay 17.5 



Humus 60. 



Coefficients of evaporation : 



Sand 90. 



Fine garden soil 80. 



Lime " 65. 



Clay 35.0 



Hnmus 20.0 



To sum up, water is a physical agent which modities tlie texture of 

 the soil and influences its aeration, density, cohesion, etc. Water also 

 acts chemically upon the constituents of the soil. In aerating the soil 

 it at the same time introduces ammonia and carbon dioxid, two essen- 

 tials to fertility, which facilitate the solution of the organic and mineral 

 materials necessary to the jdant. The organic materials are rapidly 

 destroyed by the oxygen of the air and the nitrogenous nnitter is trans- 

 formed into nitrates, which are partly absorbed by the plant. The 

 mineral constituents undergo modifications no less important. Phos- 

 phate of lime is dissolved and the silicates are decomposed by the water 

 charged with carbon dioxid. 



It is possible to conduct interesting experiments on this action of 

 water by analyzing at different times the solutions which exist in 

 irrigated soil. By systematically repeating these analyses through a 

 series of years it will without doubt be found that irrigation water 

 exerts a steadily diminishing power. 



Water is not only a solvent which sets free certain oxids, alkalies, 

 phosphoric acid, and silica, but is a vehicle for the fertilizing elements 

 intended for the roots of plants. Water, therefore, is essential to the 

 utilization of fertilizers. A soil responds very differently to chemical 

 fertilizers under different conditions. The fertility will be considerably 

 increased if there be enough water i)resent to act as a vehicle for the 

 fertilizing substances, while there will be little improvement if the soil 

 is subjected to an extreme drought. Moreover, for certain fertilizers 

 too abundant irrigation is injurious in that the fertilizers are washed 



lEdmoud Gain, Rev. g^n. Bot., 7 (1895), p. 123, 



