162 THE INTERCHANGE OF CROPS. 



alone deserves to be mentioned as resting on a firm 

 basis. 



Decandolle supposes that the roots of plants 

 imbibe soluble matter of every kind from the soil, 

 and thus necessarily absorb a number of substances 

 which are not adapted to the purposes of nutrition, 

 and must subsequently be expelled by the roots, 

 and returned to the soil as excrements. Now as 

 excrements cannot be assimilated by the plant 

 which ejected them, the more of these matters 

 which the soil contains, the more unfertile must it 

 be for plants of the same species. These excre- 

 mentitious matters may, however, still be capable of 

 assimilation by another kind of plants, which would 

 thus remove them from the soil, and render it again 

 fertile for the first. And if the plants last grown 

 also expel substances from their roots, which can 

 be appropriated as food by the former, they will 

 improve the soil in two ways. 



Now a great number of facts appear at first sight 

 to give a high degree of probability to this view. 

 Every gardener knows that a fruit-tree cannot be 

 made to grow on the same spot where another of 

 the same species has stood ; at least not until after 

 a lapse of several years. Before new vine-stocks are 

 planted in a vineyard from which the old have been 

 rooted out, other plants are cultivated on the soil for 

 several years. In connexion with this it has been 

 observed, that several plants thrive best when 

 growing beside one another ; and on the contrary, 



