DESTINATION OF THE ASH CONSTITUENTS. 11 



been derived from the free nitrogen of the atmosphere. 

 These very remarkable results have been obtained by 

 several independent investigators. They supply a much- 

 needed explanation of the remarkable power of assimil- 

 ating nitrogen possessed by leguminous plants. 



Destination of the Ash Constituents. The very weak 

 solutions taken up by the roots are concentrated in 

 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. The non-essential ash constituents 

 which have been taken up by the roots are partly 

 disposed of in a solid form, as a permanent incrus- 

 tation of the older tissues. The soluble salts which are 

 not thus disposed of at first accumulate in the sap. They 

 are finally more or less removed from straw, and prob- 

 ably from other old tissues, by the washing effect of rain. 



The deposition of silica upon the external tissues of 

 wheat, barley, and other graminaceous plants is a 

 familiar example of the excretion of a non-essential ash 

 constituent. Silica 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 old tissues. These incrustations 

 are indirectly of service to the plant, as they tend to harden 

 the tissues, and thus protect them from injury. 



Soluble non-essential ash constituents, as chloride of 

 sodium, are found abundantly in the succulent parts of 

 plants when such ash constituents have been present in 

 the soil. They generally diminish in quantity as the 



