ROTATION. 345 



generally. In effect, plants the most opposite in botanical character 

 and properties, alimentary as well as poisonous, will live and flourish 

 on the same mound of earth, and with the same manure. Moreovei 

 these plants reciprocally withdraw nourishment from one another, 

 which could not occur did each species need different elements of 

 nutrition.* 



When it was taken for granted that the organs of plants elaborate 

 a common nourishment derived from the manure, then vegetables of 

 diverse organizations were supposed endued with the faculty of 

 searching at different depths for the nutritive matter contained in the 

 soil, by reason of a more or less considerable extension and develop- 

 ment of their roots. This served to explain how a plant with long 

 and perpendicular roots could, as a sequel to corn, derive benefit 

 from manure situate in the undermost layers of ploughed land. It 

 is possible that an action of this kind may take place under certain 

 circumstances, but the explanation can never be generally re- 

 ceived. 



Another explanation of the necessity for alternate crops is based 

 upon properties assigned to the excretions of the roots, as compared 

 to animal excrements. 



The excretion of roots, first observed by Brugman in the Viola 

 arvensis,\ has been confirmed by the recent observations of M. Ma- 

 caire. This physiologist obtained the matter exuded from certain 

 plants by keeping their roots in water ; but, strange to say, could not 

 discover it in silicious sand in which certain vegetables had been 

 grown. J I myself likewise failed in detecting sensible traces of 

 organic matter in sand which had served as soil during several 

 months to wheat and clover ; a result which renders the fact of 

 radicular excretion doubtful. The excretion consequent upon im- 

 mersion in water is perhaps the effect of disease. 



Be that as it may, upon the assumption of the excretion from 

 roots, Messrs. A"on Humboldt and Plenck have exj)lained the cause 

 of the attractions and repulsions of certain plants. § More recently 

 M. de Candolle has reproduced this idea as the basis of a theory of 

 rotation of crops. If it be supposed, in flict, that the excretion from 

 the roots represents vegetable excrements, it may be easily imagin- 

 ed that these excretions once deposited in the soil may be as pre- 

 judicial to the plant which produced them as would be the excrement 

 of an animal presented to it as food. On the other hand, by change 

 of species, the plant newly implanted may profit by the excretions- of 

 the preceding crop, absorbing them as nourishment. This ingenious 

 hypothesis is deficient in the groundwork, inasmuch as the fact of 

 radicular excretion is not suflSciently established. Again, admitting 

 the excretion, several facts concur to demonstrate that plants may 

 thrive in soil charged wiffi their own excrements. 



The culture of corn, for example, may proceed uninterruptedly, 

 as we find in the triennial rotation. I have seen in the table-landa 



* De Candolle, t. i. p. 248. t Ibid. t. ii. p. 1497. 



X Ibid. t. iii. p. 1474. $ De Candolle, l, iii. p. J474 



