IN WHAT MANNEK PLANTS ABSOEB FOOD. Ill 



If the examination of the water in the soil shows it 

 to contain half the quantity of potash required by a 

 turnip crop, there is no need to explain how the dis- 

 solved half of the potash has passed into the turnip- 

 plant, but in what form and manner the plant has ab- 

 sorbed the other half deficient in the -water. 



If, again, by the examination of the water in other 

 fields, we find 'that it contains only J ; nay, only -J-, ^ 

 or -^ of the quantity of potash found in a turnip crop 

 grown upon it ; and if we further ascertain that in a 

 soil, favourable for the growth of turnips, the plant 

 always takes up the same quantity of potash from the 

 ground, no matter how much or how little of that sub- 

 stance the water circulating in the soil dissolves from 

 the earth ; it follows, that as the water, the soil, and the 

 plant, can alone come into consideration here, the direct 

 power of the water to dissolve potash is of no impor- 

 tance to the plant ; and that the plant itself, by the 

 help of water, must have rendered the needful potash 

 soluble. 



What is here asserted of one constituent, holds good 

 for all. If, therefore, we find, that by treating a soil 

 with rain-water we can dissolve from it potash, phos- 

 phoric acid, and ammonia or nitric acid, in sufficient 

 quantity to account for the presence of these substances 

 in the cereal plants grown on such a soil ; while, on the 

 other hand, we find that the plant contains a. hundred 

 times more silicic acid than the water could possibly 

 have supplied ; the cause of the absorption of silicic 

 acid, which clearly is not in the water, must again here 

 be sought for in the plant itself. Again, if other cases 

 show that an equally abundant crop of corn is obtained 

 on fields, from which water fails to extract phosphoric 

 acid or ammonia, here, too, we are led to the conclusion 

 that the nutritive substances dissolved in the water are 

 of no special importance to the plants in question ; but 

 that, as an indispensable requisite, these elements must 

 possess the form most suitable for the action of the root, 

 be this what it may. 



The beautiful experiments on vegetation made con- 



