Ri OG Seay 
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 Riiben), 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 Neriwm 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 Neriwm. In the same manner the root-excretions of 
Inula Helenium, Scabiosa arvensis, Carduus arvensis, and of se- 
veral Kuphorbiacee and Cichoriacee were examined, but with- 
out satisfactory results. Hereupon some of Macaire’s own 
experiments were repeated ; but instead of Chondrilla muralis 
* “ Recherches sur l’Influence des Plantes sur le Sol.”—Annales de 
Chemie et de Physique, Septembre, 1839, pp. 27—40. 
