ON THE ROLE OF SILICA IN THE WHEAT-PLANT. 345 



Tliis explanation amounting to the cstablislnnent of an a priori 

 probability as to the siliceous wheats being comparatively rust- 

 proof is corroborative of the experience that they possess this 

 character, and we can only point then to the desirability of 

 growing wheats of this class. It is not merely necessary for us to 

 sow hard wheats, for the character of the succeeding generations 

 derived from this seed will possess this character, or not, according 

 to the nature of the soil on which they are raised. That wheats 

 may be made to, and do, differ in this respect might be regarded 

 as certain, even if it had not been demonstrated to be the case by 

 actual experiment, when it is considered that all the varieties of 

 wheat have arisen from a few — at most — different kinds, and 

 chiefly under the influence of change of soil. We are, however, 

 enabled by way of making this appear more evident, to state, on 

 the authority of Lyon Play fair, F.R.S., and by reference to a 

 "Table showing the Composition of the Ashes of Wheat," which 

 he quotes as follows: — The grain of a wheat — " Hopetoun 

 Wheat" — on analysis by Messrs. Way and Ogden was found to 

 contain 5.91 per cent, of silica, and on being used as seed gave, 

 when grown on one soil, 1.42 per cent, of silica, and 2.84 percent, 

 (or just double the amount) on another ; finally the f-ame 

 Hopetoun Wheat when grown on a sandy soil became so siliceous 

 that the silica in its grain amounted then to 5.63 per cent. 

 Analyses of the grain of another wheat, viz. : " Red straw W^hite 

 Wheat," showed that the silica in it ranged from 2.05 to 9.71 per 

 cent., according as it was grown on " loamy soil in the Greensand" 

 or "calcaseous soil in the Magnesian Limestone formation.* Fi'om 

 this it may be seen that wheat originally rich in silica, on cultiva- 

 tion, may become poor in that mineral, and that the propoi'tion 

 in whicli silica is present depends on the nature of the soil — a 

 nature which can be modified by ordinary agricultural processes. 



The manner, however, in which a soil poor in assimilable silica, 

 may be rendei-ed well endowed with respect to this mineral will 

 be best indicated on consideration of the process by which the 

 plant obtains its silica component from the ground. The direct 

 assimilation of soluble silicates being as a theory dismissed tor 

 reasons which I shall immediately adduce, we may state that it 

 is now usually held that silicon is taken up by plants in the form 

 of a very dilute solution of silicic acid, or as gelatinous or colloidal 

 silica, and not directly in that of the soluble silicates of soda or 

 potash. These salts, even if they could exist in a soil poor in 

 uncombined silicic acid, being decomposed at the points of contact 

 of the root hairs with the soil and by the carljon dioxide which 

 they inhale. This is the view taken by the Justly celebrated 

 plant physiologist, Julius Sachs, Professor of Botany in the 

 University of Wiirzburg. He adds, that by far the larger part of 



*"A Cyclopedia of Agriculture," edited by John C. Morton, 8. v. "Ashes," VoL I., p. 143. 



