16 Investigation of the Theory 



" The bean grows pretty well in pure water. It was found, on 

 trial, that the water continued clear, but assumed a yellow tint. 

 Chemical tests and evaporation seemed to detect a matter similar 

 to gum, and a little chalk. Another bean was placed in this 

 liquor, and would not thrive : and then, in order to determine 

 whether this was occasioned by the want of carbonic acid, or by 

 the presence of some exuded matter, plants of wheat were placed 

 in the water. They lived well, the yellow colour of the fluid 

 became less intense, the residuum less considerable, and it was 

 evident that the new plants absorbed a portion of the matter 

 discharged by the first. Hence, the practice of cropping wheat 

 after beans is justified by this experiment." 



Tlie potato scarcely coloured water wherein it was placed, 

 left little residuum, and gave but little taste. " This experiment," 

 M. Macaire observes, " was made upon a plant at an early stage 

 of developement. The experiment would lead to the inference, 

 that the potato is not a very good preparation for corn crops, 

 which is known to be the case in practice, unless it is assisted by 

 an extraordinary quantity of manure. All these facts tend to 

 prove the theory of the rotation suggested by M. de Candolle." 



From the foregoing passages, the reader may draw some infer- 

 ence concerning the theory, and the nature of the experiments 

 recorded. I am by no means disposed to retract what I had 

 written upon the philosophy of the rotation of crops ; on the 

 contrary, I rejoice to feel myself supported by physiologists of 

 so much eminence. To know, beyond a doubt, that a Lindley 

 and a De Candolle have adduced a theory exactly in accordance 

 with that which some years past impressed my own mind, is at 

 least highly gratifying : to ascertain that the direct experiments 

 of another man of science have tended, as far as they have been 

 carried, to confirm it, is still more so. But I must not neglect 

 to say, that the practice of the rotation of crops may be, and is, 

 carried too far. There can, I think, be no doubt that, whenever 

 a crop fails upon repetition, that failure ought to be attributed to 

 an unhealthy (specifical) saturation of the soil : but rotation, as a 

 sine qua non, an indispensable and never to be omitted practice, 

 ought not to be insisted upon. They who have boldly ventured 

 to persist for a time in recroppings have not found a certain 

 deterioration. As to the potato, it is no uncommon thing to 

 hear of the same ground being planted and replanted, year after 

 year, for a great period of time. M. Macaire's experiment with 

 the potato bears upon the assertion, for it tends to show that it 

 does not produce much radical matter. That plants, in many 

 instances, give forth a considerable portion, may be inferred from 

 the peculiar odour which they impart to the soil ; and also from 

 the colour, the change of tint, which the ground acquires from 

 croppings. Let new-turned maiden earth be put into a garden 



