ON ROTATION OF CROPS. 173 



seek its nourishment near the surface, while the other 

 penetrates deeper into the soil; thus they would both be 

 fed without interfering with each other. 



Emily. But you do not consider, Caroline, that the 

 plough overturns your theory, in overturning the soil ; for 

 it brings the lower part to the surface, and mixes the 

 whole so well together that the nutritive particles must 

 be pretty equally diffused throughout. 



Mrs. B. Your observation is very just ; and we find 

 that clover, which has very superficial roots, will not 

 thrive after lucerri, whose roots are very long. This 

 theory, however, is applicable to simultaneous crops 

 whose roots are of different lengths. Another theory has 

 been suggested, which is, that the fall of the leaf of the 

 first crop fertilises the earth for a second : this is un- 

 doubtedly true to a certain extent ; but the foliage can 

 fertilise the earth only by being converted into manure, 

 which would equally afford nourishment for a second 

 crop of the same nature. 



Caroline. Nay ; one would even suppose that a green 

 crop, ploughed into the soil, would afford more appro- 

 priate food for a second crop of the same description 

 than for one of a different family ; that the leaves of straw 

 would yield the best nutriment for a future crop of corn, 

 and of grass for that of grasses ; whilst the fact, you say, 

 is exactly the reverse. 



Mrs. B. The theory which M. De Candolle is most 

 inclined to favor, if indeed he is not its author, is the fol- 

 lowing. A plant, being under the necessity of absorbing 

 whatever presents itself to its roots, necessarily sucks up 

 some particles which are not adapted to its nourishment, 

 and in consequence, after having elaborated the sap in 

 its leaves, and re-conducted it downwards through all its 

 organs, each of which takes in the nourishment it re- 

 quires ; after having extracted from it the various pecu- 

 liar juices, and in a word, turned it in every possible way 

 to account, finds itself encumbered with a certain resi- 

 due, consisting of the particles it had unavoidably ab- 



950. How does she think the different lengths of the roots may oper- 

 ate in doing it 1 ? 951. How does Emily reply to Caroline'? 952. 

 What does Mrs. B. say of clover and lucern in illustration of the subject'? 

 953. What other theory has been suggested of fertilization from the 

 leaves'? 954. What does Caroline say of this'? 955. What is the 

 theory of Candolle'? 



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