Dr. Meyen's Researches in Physiological Botany, 139 



species, are injurious ; but if of a different genus, are, if not ex- 

 actly favourable to their growth, still certainly not hurtful, as 

 in the former case." This theory is to be found, it is true, in 

 the most celebrated botanical works, but in the newer phy- 

 siological ones it is circumstantially enough proved, that this 

 theory is nothing better than an hypothesis, for the known 

 experiments on which it has been founded have been shown 

 to be incorrect ; and therefore I cannot agree with those views 

 according to which the advantageous influence of the chan- 

 ging plants is explained by M. Nietner. The several instances 

 which are adduced as proving the correctness of the above 

 theory, can be explained in a different manner ; particularly 

 the luxuriant growth of rye after three years' cultivation of 

 sorrel, in which case the soil requires no manure. I do not 

 suppose it is necessary to assume here an excretion from the 

 sorrel roots which is beneficial to the rye, which moreover has 

 by no means been proved ; but one must look for this excel- 

 lent manure in the roots and stubble of the sorrel plants. 



Moreover, M. Nietner remarks, that carrots, parsnips? 

 (weisse Ruben), and other bulbous plants acquire a bitter un- 

 pleasant taste, and become scarcely edible when cultivated on 

 a soil which in the previous year has borne tobacco. This 

 may however be explained by the great mass of the tobacco 

 plants which always remains on such a field ; these masses, 

 abounding in alkaloids and still imperfectly decomposed ex- 

 tractive matters, pass over more or less into those plants which 

 follow next. 



It has at length been acknowledged in France that the 

 results of the experiments of Macaire on the excretions of 

 the apices of the roots of plants, on which so important 

 theories have been founded, cannot be correct. M. H. Bra- 

 connot of Nancy has opposed the conclusion drawn by Ma- 

 caire from his experiments. M. Braconnot* planted a large 

 specimen of Nerium grandiflorum in a pot which had no open- 

 ing at the bottom, and let it grow therein for three years, and 

 when the earth was examined at the expiration of that time, 

 it was found that there was nothing therein beyond the usual 

 salts, and none of that peculiar poisonous sharp principle pe- 

 culiar to Nerium. In the same manner the root-excretions of 

 Inula Helenium, Scabiosa arvensis, Carduus arvensis, and of se- 

 veral Euphorbiacece and Cichoriacece were examined, but with- 

 out satisfactory results. Hereupon some of Macaire's own 

 experiments were repeated ; but instead of Chondrilla muralis 



* " Recherches sur l'lnfluence des Plantes sur le Sol." — Annales de 

 Chemie et de Physique, Septembre, 1839, pp.27 — 40. 



