292 THE PRINCIPLES OF SOIL MANAGEMENT 



The difference in absorbing power may be due to 

 either one or both of two causes: (1) A larger absorbing 

 system. (2) A more active absorbing system. The 

 former is determined by the extent of the root-hair 

 surfaces; the latter by the intensity of the osmotic action. 



150. Extent of absorbing system. Plants with 

 large root systems may, therefore, be expected to absorb 

 the larger amounts of nutrients from the soil, and such 

 is usually the case, although the extent of the root-system 

 is not necessarily proportional to the total area of the 

 absorbing surfaces of the root-hairs. 



151. Osmotic activity. The osmotic activity of a 

 plant under any given condition of soil and climate 

 depends upon: (1) The rapidity and completeness with 

 which the plant elaborates the substances taken from 

 the soil into plant substance or otherwise removes them 

 from solution. (2) The extent to which the exudations 

 from the root-hairs act upon the soil particles to in- 

 crease the density of the solution between the root-hair 

 and the soil-particle. 



The first of these is a function of the vital energy 

 of the plant and its ability to utilize sunshine and carbon 

 dioxide to produce organic matter. It may be com- 

 pared to the property which enables one animal to do 

 more work than another animal of the same weight on 

 a similar ration. 



The removal from the ascending water current in 

 the plant of substances derived from the soil is accom- 

 plished in the leaves. By the dissociation of these, ions 

 are constantly furnished for metabolism into materials 

 that may be built into the tissues of the plant. The 



