78 *OOD OF THE VEGETATING PLANT. CHAP. II. 



The water expended was 2493 grains, and the in- 

 crease as 1 : 95 + At the commencement of the 

 experiment the third plant weighed Q2 grains, at 

 the end 376 grains, being augmented by 284 grains. 

 The water expended was 14Q50 grains, and the 

 increase as ] : 52 + .* 



From the greater proportional augmentation of 

 the plant to which the mould had access, we may 

 infer the beneficial effect of the earths as applied to 

 the root, and perhaps the absorption of a part ; par- 

 ticularly as it is known that the proportion of earths 

 contained in the ashes of vegetables depends upon 

 the nature of the soil in which they grow. The 

 ashes of leaves of the Rhododendron ferrugineum, 

 growing on Mount Jura, a calcareous mountain, 

 yielded 43*25 parts of earthy carbonate, and only 

 0'75 of silica. But the ashes of leaves of the same 

 plant, growing on Mount Breven, a granitic moun- 

 tain, yielded two parts of silica, and only 16*75 of 

 earthy carbonate. 



It is probable, however, that plants are not in- 

 debted merely to the soil for the earthy particles 

 which they may contain. They may acquire them 

 partly from the atmosphere. Margray has shewn 

 that rain water contains silica in the proportion of a 

 grain to a pound; which, if it should not reach the 

 root, may possibly be absorbed along with the water 

 that adheres to the leaves. 



But although the earths are thus to be regarded 

 * Phil. Trans, vol. xxi. p. 200. Saus. sur la Vcg. chap. ix. sect. iii. 



