PhilosojJitcfd Characfer of DecandoUe. 241 



tion ascribed to the roots, fi'om which our author has deduced 

 an explanation of the deterioration which a plant is subject to, 

 when sown year after year upon the same ground, as well as 

 of the consequent advantages of a due rotation of crops in 

 agriculture. 



All, I apprehend, will be disposed to allow his theory on 

 this subject to be ingenious and plausible, and to admit that 

 he has evinced considerable dexterity and skill in removing 

 the objections that lie upon the face of such an hypothesis, 

 and in particular in reconciling the permanent existence of 

 forest trees on a particular locality, with the assumed neces- 

 sity for a frequent change in the soil from which they draw 

 their nourishment. 



I conceive also, that subsequent observations have tended to 

 confirm, rather than to invalidate, his fundamental position, 

 that the roots are exci'etory, as well as secretory organs ;* and 

 the experiments of Macaire and others seem to place beyond 

 doubt the fact, that in plants which contain poisonous juices, 

 the excretions are also poisonous. 



It is likewise very probable, that the juices emitted by the 

 roots of a plant, may be better adapted for the nourishment 

 of certain species than of others ; and that hence each culti- 

 vated crop, and perhaps each kind of forest tree, may foster 

 its own class of Aveeds, as well as its own particular parasites. 



But it yet remains to be proved, whether the advantage of 

 a frequent interchange of crops be connected in any degree 

 with the emission of juices, which are more injurious to the 

 plants of the same kind, or of similar conformation, than they 

 are to others ; or whether, on the contrary, a different set of 

 causes may not serve to account for the result. 



The doubts, which I myself entertained at the time the 

 theory was first propounded, have led me to originate a series of 

 experiments, which may, I hope, tend eventually to throw some 

 light upon the question ; and I must confess that these doubts 

 have since been much increased by the results to which the 



• Thus, for instance, in Mr Hyctt's experiments on tlio absorption of liquid 

 solutions by growing timber, which are detailed in vol. xiv. p. 535 of the 

 Transactions of the Highland Society, the neighbouring trees were found 

 by liim, as I understand, to have been affected by the poisonous matters 

 imbibed, under circumstances, wliich seem to imply, that they had been ex- 

 creti'd by the roots of the plants to the trunks of whicli they were applied. 



