198 HOW CROPS GROW. 



In the ash of the wood of trees, it usually ranges from 

 1-3%, but is often found to the extent of 10-20%, or 

 even 30%, especially in the pine. In leaves, it is usually 

 more abundant than in stems. The ash of turnip leaves 

 contains 3-10% ; of tobacco leaves, 5-18% ; of the oat, 

 11-58%. (Arendt, Norton.) In ash of lettuce, 20% ; of 

 beech leaves, 26% ; in those of oak, 31% have been 

 observed. (Wicke, Henneberg's Jour., 1862, p. 156.) 



The bark or cuticle of many plants contains an extra- 

 ordinary amount of silica. The cauto tree of South 

 America (Hirtella silicza) is most remarkable in this 

 respect. Its bark is very firm and harsh, and is difficult 

 to cut, having the texture of soft sandstone. It yields 

 34% of ash, and of this 96% is silica. (Wicke, loc. cit., 

 p. 143.) 



Another plant, remarkable for its content of silica, is 

 the bamboo. The ash of the rind contains 70%, and in 

 the joints of the stem are often found concretions of 

 hydrated silica, the so-called Tabashir. 



The ash of the common scouring rush (Equisetum hye~ 

 male) has been found to contain 97.5% of silica. The 

 straw of the cereal grains, and the stems and leaves of 

 grasses, both belonging to the botanical family Grami- 

 nacce, are specially characterized by a large content of 

 silica, ranging from 40-70% of the ash. The sedge and 

 rush families likewise contain much of this substance. 



The position of silica in the plant would thus appear 

 to be, in general, at the surface. Although it is present 

 in other parts of the plant, yet the cuticle is usually rich- 

 est, especially where the content of silica is large. Davy, 

 in 1799, drew attention to the deposition of silica in the 

 cuticle of the grasses and cereals, and advanced the idea 

 that it serves these plants an office of support similar to 

 that enacted in animals by the bones. 



In case of the pine (Pinus sylvestris), Wittstein has 

 obtained results which indicate that the age of wood or 



