REPORT ON THE PHILOSOPHY OF BOTANY. 57 



poisonous matter by their roots. For instance, a plant of Mer- 

 curialis had its roots divided into two parcels, of which one was 

 immersed in the neck of a bottle filled with a weak solution of 

 acetate of lead, and the other parcel was plunged into the neck 

 of a corresponding bottle filled with pure water. In a few days 

 the pure water had become sensibly impregnated with acetate 

 of lead. This, coupled with the well known fact that plants, 

 although they generate poisonous secretions, yet cannot absorb 

 them by their roots without death, as, for instance, is the case 

 with Atropa Belladonna, seems to prove that the necessity of 

 the rotation of crops is more dependent upon the soil being 

 poisoned than upon its being exhausted. 



This is a part of vegetable physiology of vast importance to 

 an agricultural country like England, and may possibly cause 

 a total revolution in our system of husbandry. 



All that M. Macaire can be said as yet to have done, is to 

 have discovered the fact and to have pointed out certain strong 

 examples of it ; but if the discovery is to be converted to a 

 practically useful purpose, we require positive information upon 

 the following points : — 



1. The nature of the faecal excretions of every plant culti- 

 vated by the farmer. 



2. The nature of the same excretions of the common weeds 

 of agriculture. 



3. The degree in which such excretions are poisonous to the 

 plants that yield them, or to others. 



4. The most ready means of decomposing those excretions 

 by manures or other means. 



It would be superfluous to point out what the application 

 would be of such information as thi;?, ; but I cannot forbear ex- 

 pressing a hope that a question upon which so many deep inter- 

 ests are involved may be among the first to occupy the atten- 

 tion of the chemists of the British Association. 



