56 



AGRICULTURAL CHEMISTRY. 



rrements 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 

 substances resembling gum. The former of 

 these, according to Macaire- Princep, may 

 be regarded as poisonous, the latter as nu- 

 tritious. 



The experiments of Macaire-Princep af- 

 ford 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 confir- 

 mation of the theory of Decandolle, for they 

 leave it quite undecided whether the sub- 

 stances 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, 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 

 obtained by plants from the atmosphere. 

 Now, these excrements are compounds, 

 produced in consequence of the transforma- 

 tions of the food, and of th,: new forms 

 which it assumes by entering into the com- 

 position of tiie various organs. 



M. Decandolle's theory is properly a 

 modification of an earlier hypothesis, which 

 suppo-ed that the roots of different plants 

 extracted different nutritive substances from 

 the soil, each plant selecting that which 

 was exactly suited for its assimilation. Ac- 

 cording to this hypothesis, the matters in- 

 capablt of assimilation are not extracted 

 from the soil, whilst M. Decandolle consi- 

 ders 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 im- 

 proved by lying fallow, and this in propor- 

 tion 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 lucerne 

 and sainfoin. 



Theoretical considerations on the process 

 of nutrition, as well as the experience of all 

 agriculturists, so beautifully illustrated by 

 the experiments of Macaire-Princep, leave 

 no doubt that substances are excreted from 

 the roots of plants, and that these matters 

 form the means by which the carbon re- 

 ceived from humus in the early period of 

 their growth is restored to the soil. But 

 we may now inquire whether these excre- 

 ments, in the state in which they are ex- 

 pelled, are capable of being employed as 

 food by other plants. 



The excrements of a carnivorous animal 

 contain no constituents fitted lor the nou- 



rishment of another of the same species j 

 but it is possible that an herbivorous animal, 

 a fish, or a fowl, might find in them undi- 

 gested matters capable of being digested in 

 their organism, from the very circumstance 

 of their organs of digestion having a different 

 structure. This is the only sense in which 

 we can conceive that the excrements of one 

 animal could yield matter adapted for the 

 nutrition of another. 



A number of substances contained in the 

 food of animals pass through their alimentary 

 organs without change, and are expelled 

 from the system ; these are excrements but 

 not excretions. Now a part of such excre- 

 mentitious matter might be assimilated in 

 passing through the digestive apparatus of 

 another animal. The organs of secretion 

 form combinations of which only the ele- 

 ments were contained in the food. The 

 production of these new compounds is a 

 consequence of the changes which the food 

 undergoes in becoming chyle and chyme, 

 and of the further transformations to which 

 these are subjected by entering into the 

 composition of the organism. These mat- 

 ters, likewise, are eliminated in the excre- 

 ments, which must therefore consist of two 

 different kinds of substances, namely, of the 

 indigestible constituents of the food, and of 

 the new compounds formed by the vital pro- 

 cess. The latter substances have been pro- 

 duced in consequence of the formation of 

 fat, muscular fibre, cerebral and nervous 

 substance, and are quite incapable of being 

 converted into the same substances in any 

 other animal organism. 



Exactly similar conditions must subsist in 

 the vital processes of plants. When sub- 

 stances which are incapable of being em- 

 ployed in the nutrition of a plant exist in 

 the matter absorbed by its roots, they must 

 be again returned to the soil. Such excre- 

 ments might be serviceable and even indis- 

 pensable to the existence of several other 

 plants. But substances that are formed in 

 a vegetable organism during the process of 

 nutrition, which are produced, therefore, in 

 consequence of the formation of woody fibre, 

 starch, albumen, gum, acids, &c., cannot 

 again serve in any other plants to form the 

 same constituents of vegetables. 



The consideration of these facts enables 

 us to distinguish the difference between the 

 views of Decandolle and those of Macaire- 

 Princep. The substances which the former 

 physiologist viewed as excrements, belonged 

 to the soil; they were undigested matters, 

 which although not adapted for the nutrition 

 of one plant might yet be indispensable to 

 another. Those matters, on the contrary, 

 designated as excrements by Macaire-Prin- 

 cep, could only in one form serve for the 

 nutrition of vegetables. It is scarcely ne- 

 cessary to remark that this excrementitious 

 matter must undergo a change before another 

 season. During autumn and winter it be- 

 gins to suffer a change from the influence 

 of air and water j its putrefaction, and a* 



