12 THE CHEMISTRY OF THE FARM 



where they are absent ; indeed, in the former case, the 

 quantity of nitrogen in the crop and sand at harvest 

 much exceeds that originally present in the sand, seed, 

 and added soil. This gain of nitrogen has been derived 

 from the free nitrogen of the atmosphere. These facts 

 supply a much-needed explanation of. the remarkable 

 power of assimilating nitrogen possessed by leguminous 

 plants. 



When two organisms grow together for their mutual 

 advantage, the case is said to be one of symhiosisj or 

 joint life. 



Destination of the Ash Constituents.— The very weak 

 solutions taken up by the roots are concentrated iii 

 the upper parts of the plant, the water being rapidly 

 evaporated by the leaves, as already mentioned. The 

 essential ash constituents are employed in the forma- 

 tion of new tissues, and are stored up in the seed. 

 The non-essential ash constituents which have been 

 taken up by the roots are partly disposed of in a solid 

 form as a permanent incrustation of the older tissues. 

 The soluble salts which are not otherwise disposed of 

 at first accumulate in the sap : they are finally more 

 or less removed from straw, and probably from other 

 old tissues, by the washing effect of rain. 



The deposition of sihca upon the external tissues of 

 wheat, barley, and other graminaceous plants is a 

 familiar example of the excretion of a non-essential 

 ash constituent. SiHca is also abundant in the old 

 leaves, and in the outer bark of many trees, and is 

 commonly found as an incrusting constituent of 

 old tissues. Insoluble calcium salts, frequently the 

 oxalate, are also deposited as incrusting matters in 



