THEORIES OF ITS USE. 165 



cannot be considered as a confirmation of the theory 

 of Decandolle, for they leave it quite undecided 

 whether the substances were extracted from the soil, 

 or formed by the plant itself from food received from 

 another source. It is certain, that the gummy and 

 resinous excrements observed by Macaire-Princep 

 could not have been contained in the soil, and as we 

 know that the carbon of a soil is not diminished by 

 culture, but, on the contrary, increased, we must 

 conclude that all excrements which contain carbon 

 must be formed from the food obtained by plants 

 from the atmosphere. Now, these excrements are 

 compounds, produced in consequence of the trans- 

 formations of the food, and of the new forms w^hich 

 it assumes by entering into the composition of the 

 various organs. 



M. Decandolle's theory is properly a modification 

 of an earlier hypothesis, which supposed that the 

 roots of different plants extracted different nutritive 

 substances from the soil, each plant selecting that 

 which was exactly suited for its assimilation. Ac- 

 cording to this hypothesis, the matters incapable of 

 assimilation are not extracted from the soil, whilst 

 M. Decandolle considers that they are returned to it 

 in the form of excrements. Both views explain how 

 it happens that after corn, corn cannot be raised 

 with advantage, nor after peas, peas ; but they do 

 not explain how a field is improved by lying fallow, 

 and this in proportion to the care with which it is 

 tilled and kept free from weeds ; nor do they show 

 how a soil gains carbonaceous matter by the cultiva- 

 tion of certain plants such as lucern and sainfoin. 



Theoretical considerations on the process of nutri- 

 tion, as well as the experience of all agriculturists, 

 so beautifully illustrated by the experiments of Ma- 

 caire-Princep, leave no doubt that substances are 

 excreted from the roots of plants, and that these 

 matters form the means by which the carbon received 

 from humus in the early period of their growth is 

 restored to the soil. But we may now inquire wheth- 



