12 THE CHEMISTRY OF THE FARM. 



plant matures, and are never stored up in the seed 

 Both the amount and composition of the ash of suc- 

 culent plants, as meadow grass, clover, and mangel, are 

 greatly influenced by the character of the soil, and the 

 manure applied. The ash of a seed, on the other hand, 

 is very constant in composition, the result of the 

 selective power of the plant. 



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 constitu- 

 ents ; 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 layer (cambium) 

 between the wood and bark of a tree ; they are also 

 abundantly stored up in the seed. 



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 difference as to 

 the stiffness of the stem. The grass growing on peat 

 bogs also contains scarcely any silica, though silica is 

 abundant in ordinary hay. Silica may, however, dis- 

 charge useful functions. In Wolff's experiments, 

 although the presence of silica made little difference in 

 the weight of the oat plant, it considerably increased the 

 proportion of corn. 



Germination. A seed is constructed with the purpose 

 of developing a young plant. It contains the " embryo," 



