THEORIES OF ITS USE. 163 



that others mutually prevent each other's develop- 

 ment. Whence it was concluded, that the bene- 

 ficial influence in the former case depended on a 

 mutual interchange of nutriment between the 

 plants, and the injurious one in the latter on a 

 poisonous action of the excrements of each on the 

 other respectively. 



A series of experiments by Macaire-Princep 

 gave great weight to this theory. He proved 

 beyond all doubt that many plants are capable of 

 emitting extractive matter from their roots. He 

 found that the excretions were greater during the 

 night than by day (?), and that the water in which 

 plants of the family of the Leguminosce grew, acquired 

 a brown colour. Plants of the same species, placed 

 in water impregnated with these excrements, were 

 impeded in their growth, and faded prematurely, 

 whilst, on the contrary, corn-plants grew vigor- 

 ously in it, and the colour of the water diminished 

 sensibly ; so that it appeared, as if a certain quan- 

 tity of the excrements of the Leguminosce had 

 really been absorbed by the corn-plants. These 

 experiments afforded as their main result, that the 

 characters and properties of the excrements of dif- 

 ferent species of plants are different from one 

 another, and that some plants expel excrementitious 

 matter of an acrid and resinous character ; others 

 mild (douce) substances resembling gum. The 

 former of these, according to Macaire- Princep, may 

 be regarded as poisonous, the latter as nutritious. 



M 2 



