Vegetable Physiology. 139 
But the roots of all plants, at the period of germination, must be in 
the same stratum, and of course be equally dependent upon it; and. 
besides, the culture of the farmer turns up and mixes the various. 
layers of the soil together, so as to render them, in all probability, ho- 
mogeneous. It is known also that plants of the same family, such as 
clover (tréfle) and lucerne do‘not prosper in succession, although their 
roots are of very different lengths. 
The true explanation of the necessity of rotation, appears to lia 
founded on the fact stated by Brugmans, and more fully exposed by 
De Candolle, that a certain portion of the juices which are absorbed 
by the roots of plants, are, after the salutiferous portions have been 
extracted by the vessels of the plant, again thrown out, by exudation, 
from the roots and deposited in the soil. It is probably the exist- 
~ ence of this exuded matter, which may be regarded, in some meas- 
ure, as the excrement of the preceding crop of vegetables, - that 
proves injurious to a succeeding vegetation. It has been compared to 
an attempt to feed animals upon their own excrements. The parti- 
cles which have been deleterious to one tribe of plants cannot but 
prove injurious to plants of the same kind, and probably to those of 
some other kinds, while they may furnish nutriment to miei 
of vegetables.* 
‘The author endeavored to subject these theoretic views to the teif 
of expetiment. After various attempts to raise plants in pure sili- 
ceous sand, pounded glass, washed sponge, white linen, &c. he de- 
cided upon pure rain water. After cleansing and washing the roots 
thoroughly, he placed them in vials with a certain quantity of pure 
water. After they had put forth leaves, expanded their flowers and 
flourished for some time, he ascertained, by the evaporation of the 
water, and the use of chemical reagents, that the water contained 
- which had exuded from the roots. He satisfied himself that 
this is the fact with respect to nearly all the p! 
Several plants of Chondrilla muralis, perfectly clean, were speed 
with their roots in pure water. At the end of a week, the water was 
yellowish and emitted an odor like opium, and had a bitter taste. 
Sub-acetate and acetate of lead produced a brownish flocculent pre- 
Cipitate, and a solution of gelatine disturbed its transparency. As a 
* [have been assured, by farmers, of a fact somewhat analogous in relation to an- 
Hay, which has been left in the manger of a horse, or which has otherwise 
ss the impregnations of his breath, will not be touched by ee — but 
be freely eaten by cows or sheep. dead 
