186 HOW CROPS GROW. 
Chem., xlviii, 474-7.) Such instances might be greatly 
multiplied. 
The idea that a part of the silica is accidental is further 
sustained by the fact observed by Saussure, the earliest in- 
vestigator of the composition of the ash of plants, (Re- 
cherches sur la Vegetation, p. 282,) that crops raised on a 
silicious soil are in general richer in silica than those grown 
on a calcareous soil. Norton found in the ash of the chaff 
of the Hopeton oat from a light loam 56.7 per cent, from 
a poor peat soil 50.0 of silica, while the chaff of the potato- 
oat from a sandy soil gave 70.9 per cent. 
Salm-Horstmar obtained some remarkable results in the 
course of his synthetical experiments on the mineral food 
of plants, which fully confirmed him in the opinion that 
silica is indispensable to vegetation. He found that an 
oat plant, having for its soil pure quartz, (insoluble silica,) 
with addition of the elements of growth, soluble silica ex- 
cepted, not only grew well, but contained in its ash 23°], 
of silica, or as great a proportion as exists in the plant 
raised under normal conditions. This silica may, however, 
have been mostly derived from the husk of the seed, for 
the plant was a very small one. 
Sachs, in 1862, was the first to publish evidence indi- 
cating strongly that silica is not a necessary ingredient of 
maize. He obtained in his early essays in water-culture a 
maize plant of considerable development, whose ashes con- 
tained but 0.7°|, of silica. Shortly afterwards, Knop pro- - 
duced a maize plant with 140 ripe seeds, and a dry-weight 
of 50 grammes, (nearly 2 oz. av.,) in a medium so free from 
silica that a mere trace of this substance could be found in 
the root, but half a milligramme in the stem, and 22 milli- 
grammes in the 15 leaves and sheaths. It was altogether 
absent from the seeds. The ash of the leaves of this plant 
thus contained but 0.54 per cent of silica, and the stem 
but 0.07 per cent. Way & Ogston found in the ash of 
maize, leaf and stem together, 27.98 per cent of silica, 
