12 OF THE ASSIMILATION OF CARBON. 



If, now, we suppose that the whole quantity of 

 this rain is taken up by the roots of a summer plant 

 which ripens four months after it is planted, so that 

 not a pound of this water evaporates except from 

 the leaves of the plant ; and if we further assume 

 that the water thus absorbed is saturated with hu- 

 mate of lime (the most soluble of the humates, and 

 that which contains the largest proportion of humic 

 acid); then the plants thus nourished would not 

 receive more than 300 Ibs. Hessian of humic acid, 

 since one part of humate of lime requires 2500 

 parts of water for solution. 



But the extent of land which we have mentioned 

 produces 2580 Ibs. Hessian of corn (in grain and 

 straw, the roots not included), or 20,000 Ibs. Hes- 

 sian of beet-root (without the leaves and small 

 radicle fibres). It is quite evident that the 300 

 Ibs. of humic acid, supposed to be absorbed, cannot 

 account for the quantity of carbon contained in 

 the roots and leaves alone, even if the supposition 

 were correct, that the whole of the rain-water was 

 absorbed by the plants. But since it is known 

 that only a small portion of the rain-water which 

 falls upon the surface of the earth evaporates 

 through plants, the quantity of carbon which can 

 be conveyed into them in any conceivable manner 

 by means of humic acid must be extremely trifling 

 in comparison with that actually produced in vege- 

 tation. 



Other considerations, of a higher nature, confute 



