174 ON ROTATION OF CROPS. 



sorbed, and which were not adapted to its nourishment, 

 these particles, having passed through the system with- 

 out alteration, are exuded by the roots which had absorb- 

 ed them, and thus return into the soil, which they dete- 

 riorate for a following crop of the same species of plant, 

 but improve and fructify for one of another family ; thus 

 affording an admirable proof of the wise economy of Na- 

 ture, in multiplying her vegetable produce by feeding 

 different plants with different substances, and enabling 

 beings, incapable of distinguishing or selecting their food, 

 to obtain that which is appropriate to them. 



Emily. It is, indeed, admirable ! Then, though the 

 roots of plants can make no choice, their organs are in 

 some measure capable of selecting, since they reject, and 

 will not elaborate, substances which are not adapted to 

 the nourishment of the plant, 



Mrs. B. If we cannot exactly allow them the nice 

 discrimination of the chemist, we must at least suppose 

 their laboratory to be so arranged as to act only on bodies 

 congenial to the plant. 



Caroline. And the rejected substances, which would 

 be poison to one family of plants, when transfused into 

 the soil, is greedily devoured by a succeeding crop of a 

 different family. 



Emily. Yet, Mrs. B., there is land in the Vale of 

 Glastonbury, in Somersetshire, which is celebrated for 

 growing wheat for many years together without any ma- 

 nure ; and I have heard that in the neighborhood of the 

 Carron iron-works, in Scotland wheat has been raised 

 above thirty years, without injury either to the crops or 

 the soil. 



Mrs. B. Those soils must not only abound with veg- 

 etable nourishment, but the land be particularly well 

 adapted to growing wheat ; consequently the roots would 

 have little or nothing to exude, and successive crops of 

 wheat might be raised so long as the land was not ex- 

 hausted. This explanation would reconcile your difficul- 

 ty to the theory of exudations ; but, interesting and 

 plausible as this theory is, it requires the confirmation of 

 facts to rest on a solid foundation : few experiments have 



956. What in this theory affords admirable proof of the wise econo- 

 my of nature 1 ! 957, What cases are mentioned by Emily of the 

 growing of wheat 1 958. How does Mrs, B, account for them! 



