DESTINATION OF THE ASH CONSTITUENTS 13 



old tissues. These incrustations are indirectly of 

 service to the plant, as they tend to harden the 

 tissues, and thus protect them from injury. 



It is in succulent crops, as meadow grass, clover, 

 and mangels, that we find the greatest variation in 

 the amount and composition of the ash, depending on 

 variations in the character of the soil, the manure, and 

 season. Similar variations will be observed in the ash 

 constituents of the straw of grain crops. In such 

 plants, or parts of plants, we find very variable 

 quantities of soluble sodium and potassium salts, 

 depending on the abundance of these in the soil. 

 The amount of lime present is also largely dependent 

 on the composition of the soil. In clover hay, and bean 

 straw, lime or potash will preponderate in the ash 

 according to the character of the soil on which the crop 

 grows. 



Of the particular action of the ash constituents 

 within the plant little is known. Phosphoric acid and 

 potash are undoubtedly the most important of the 

 ash constituents ; they are always found concentrated 

 in those parts of the plant where cell growth is most 

 active — as, for instance, in a growing bud, or in the 

 growing layer (cambium) between the wood and bark 

 of a tree ; they are also abundantly stored up in the 

 seed, as a provision for a new generation. 



Silica being the most abundant ash constituent of 

 wheat, barley, oats, and other graminaceous plants, 

 was long supposed to be essential for their growth, and 

 to be the ingredient on which the stiffness of their 

 straw chiefly depended. It has been shown, however, 

 that maize and oats may be successfully grown without 

 any supply of silica, and with no perceptible di^erence 



