PROPORTION OF SILICA IN THE ASH CF PLANTS. 83 



eompanying their increase in size — or from a portion of the saline and 



earthy matters they contained heing again rejected by the roots. But 



if the former be the true explanation, the relative proportions of the 



several substances of which the ash itself consisted, in the several cases, 



should have been the same at the several periods w^hen the experiments 



were made. But this was by no means the case. Thus, to refer only 



to the quantity of silica contained in the ash left by each of the above 



plants at the several stages of their growth, the ashes of the 



Before flowering. In flower. Seeds ripe. 



per cent. per cent. per cent. 



Vetch contained ... 1*5 1-5 1'75 



Golden rod 1-5 1-5 3-5 



Turnsol 1-5 1-5 3-75 



Wheat 12-5 26-0 51-0 



If, then, the proportion of silica in the ash increased in some cases 

 four-fold, while the whole quantity of ash left by the plant decreased, it 

 appears evident that some part of that which existed in the plant during 

 the earlier periods of its growth must have bee.^ excreted or rejected by 

 the roots, as it advanced towards maturity. 



2°. Tills conclilsion is confirmed and carried farther by another con- 

 sideration. The quantity of ash left by the ripe wheat plant, in the 

 above experiments of De Saussure, amounted to 3-3 per cent. ; — of 

 which ash, 51 per cent., or rather more than one-half, was silica. This 

 silica, it is believed, could only have entered into the circulation of the 

 plant in a state of solution in water, and could only be dissolved by the 

 agency of potash or soda. But, according to Sprengel, the potash, soda, 

 and silica, are to each other in the grain and straw of wheat, in the pro- 

 portions of — 



Potash. Soda. Silica. 



Grain .... 0-225 0-24 0-4 



Straw .... 0-20 0-29 2-87 



Or, supposing the grain to equal one-half the weight of the straw— 

 their relative proportions in the whole plant will be nearly as 21 potash, 

 27 soda, 205 sihca, or the weight of the silica is upwards of four times 

 the weights of the potash and soda taken together. 



Now silica requires nearly half its weight of potash to render it solu- 

 ble in water,* or 'three- fifths of its weight of a mixture of nearly equal 

 parts of potash and soda. The quantity of these alkaline substances 

 found in the plant, therefore, is by no means sufficient to have dissolved 

 and brought into its circulation the whole of the silica it contains. One 

 of two things, therefore, must have taken j)lace. Either a portion of 

 the potash and soda present in the plant in the earlier stages of its 

 growth must have escaped from its roots at a later stage, f leaving the 

 silica behind it — or the same quantity of alkali must have circulated 

 through the plant several times — bringing in its burden of silica, deposit- 



• A soluble glass may be made by melting together in a crucible for six hours 10 parts of 

 carbonate of potash, 15 of silica, and 1 of charcoal powder. 



t De Saussure does not state the exact relative quantities of potash and soda at the several 

 periods of the growth of wheat, though they appear to have gradially diminished. It 

 seems, indeed, to be true of many plants, that the potash and soda thej^ contain diminishes 

 in quantity as their age increases. Thus the weight of potash in the juice of the ripe or 

 Bweet grape, is said to be less than in the unripe or sour grape— and the leaves of the potato 

 have been found more rich in potash before than after blossoming (Liebig). 



