THEORIES CONCERNING SOIL FERTILITY 311 



"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 

 Leguminosa grew, acquired a brown color. 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 vigorously in 

 it, and the color of the water diminished sensibly ; so that it appeared, as if a 

 certain quantity of the excrements of the Leguminosa had really been absorbed 

 by the corn plants. These experiments afforded as their main result, that the 

 characters and properties of the excrements of different 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. 



"The experiments of Macaire-Princep are positive proof that the roots, prob- 

 ably of all plants, expel matters, which cannot be converted in their organism 

 either into woody fiber, starch, vegetable albumen, or gluten, since their ex- 

 pulsion 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 undecided whether the substances were extracted from the soil, or formed 

 by the plant itself from food 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 ob- 

 tained 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 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. According to this hypothesis, the matters -incapable of as- 

 similation 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 cultivation of certain 

 plants, such as lucern and esparsette. 



"Theoretical considerations on the process of nutrition, as well as the ex- 

 perience of all agriculturists, so beautifully illustrated by the experiments of 

 Macaire-Princep, leave no doubt that substances are excreted from the roots of 

 plants. . . ." 



