164 THE INTERCHANGE OF CROPS. 



The experiments ofMacaire-Princep are positive 

 proof that the roots, probably of all plants, expel 

 matters, which cannot be converted in their 

 organism either into woody fibre, starch, vegetable 

 albumen, or gluten, since their expulsion indicates 

 that they are quite unfitted for this purpose. But 

 they cannot be considered as a confirmation of the 

 theory of Decandolle, for they leave it quite unde- 

 cided 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 ; arid 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 transformations of the food, 

 and of the new forms which it assumes by entering 

 into the composition of the various organs. 



M. Decandolle' s theory is properly a modifica- 

 tion 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. According to this hypothesis, the 

 matters incapable of assimilation are not extracted 

 from the soil, whilst M. Decandolle considers that 



