8 THE CHEMISTRY OF THE FARM. 



The roots take up apparently all the diffusible sub- 

 stances (those capable of passing through a membrane) 

 which are present in the water which they draw from the 

 soil. The plant may thus receive a number of substances 

 not actually required for its nutrition. 



The feeding power of roots is not, however, confined to 

 the taking up of ready-formed solutions, they are also 

 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 rootlets and the particles of the 

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

 contain. This action of roots probably 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 present in soils. 



Besides furnishing the plant with its ash constituents, 

 the root has the important 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 also, according to several ex- 

 perimenters, is able to assimilate nitrogen when in the 

 form of urea, uric and hippuric acids, and several other 

 amide bodies. The facility, however, with which ammonia, 

 and other nitrogenous substances, are converted into nitric 

 acid in the soil is so great that nitrates become by far the 

 most important source of nitrogen at a plant's disposal. 

 Most plants are unable to assimilate the nitrogenous 

 humus contained in soil. 



The very w^eak solutions taken up by the roots are 



