FUNCTION OP HOOTS. 9 



capable of attacking some of the solid ingredients of the 

 soil, which they render soluble and then appropriate. 

 This important action of roots exists in different degrees 

 with different plants. The action takes place only at the 

 points of contact between the root-hairs and the particles 

 of the soil, and is brought about by the acid sap which 

 the roots contain. This action of the roots plays an 

 important part in the supply of phosphoric acid and 

 potash to the plant, as these substances, especially the 

 former of them, exist in the soil in difficultly soluble 

 forms, and are rarely found in solution in the water pre- 

 sent in soils. 



An apparently selective power is exerted by plants, 

 some soluble ash constituents being taken up in much 

 larger quantity than others, which may actually be more 

 abundant in the soil. A striking example of this is the 

 assimilation of potassium in preference to sodium salts. 

 This selective action of the roots is quite explainable by 

 the known laws of diffusion. When ingredients of the 

 sap are removed out of solution by becoming part of the 

 tissues of the plant, the diffusion of such substances from 

 the soil will continue : while salts not appropriated by the 

 tissues can continue to enter by diffusion only so long as 

 the solution in the soil is stronger than that in the plant. 



2. Assimilation of Nitrogen. Besides furnishing the 

 plant with its ash constituents, the root has the impor- 

 tant function of supplying nitrogen; this is nearly 

 always taken up in the form of nitrates. A plant 

 is capable of making use of nitrogen in the form of 

 nitric acid or ammonia ; it is also, according to 

 several experimenters, able to assimilate nitrogen when 

 in the form of urea, uric and hippuric acids, and 



