164 THE ALTERNATION OF CROPS. 



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 ex- 

 tractive 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 

 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 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 different species of plants are different 

 from one another, and that some plants expel excre- 

 mentitious matter of an acid and resinous character ; 

 others mild substances resembling gum. The former 

 of these, according to Macaire-Princep, may be re- 

 garded as poisonous, the latter as nutritious. 



The experiments of Macaire-Princep afford posi- 

 tive proof that the roots, probably of all plants, ex- 

 pel matters, which cannot be converted in their or- 

 ganism either into woody fibre, starch, vegetable al- 

 bumen, or gluten, since their expulsion indicates that 

 they are quite unfitted for this purpose. But they 



ous to plants of similar species, has been inferred from the fact, that a 

 soil, in which peach or apple trees have grown, is unfit for young shoots 

 of the same description, so as to render it a necessary rule in practice, 

 that a piece of ground should be occupied by forest and by fruit trees 

 alternately. 



Reference has also been made to a circumstance, which most travel- 

 lers in the United States have remarked, and which I myself, during 

 my tour in that country, had frequent opportunities of substantiating, 

 namely, that where a forest of oak or of maple has been destroyed, the 

 trees, that are apt to shoot up spontaneously in their place, are of the 

 fir-tribe ; whereas, if a pine forest be cut down, young oaks and other 

 allied species will make their appearance afterwards. — Daubeny's 

 Lectures on Agriculture, 



For an account of experiments on this subject now in progress at Ox- 

 ford, see Appendix. 



