FOOD OF PLANTS. 
589 
contrary; and pure vegetable substances are doubted to con¬ 
tain any azote. Such is the uncertainty that attends the 
most plausible theory on this subject. 
It has been supposed by several eminent physiologists, 
that plants eject faeces like animals, and that the excrements 
are converted into humus by exposure and cultivation. It 
has also been supposed that the fecal exudations of one plant 
supply food to another that is of a wholly different kind, and 
that the benefits of alternate cropping are derived from that 
source. But no instance is known, in the animal world, of 
any one individual deriving its support directly from the ex¬ 
crements of another; it seems to be repugnant to the laws of 
nature, until the substance has undergone many modifications, 
and has been converted by the process of assimilation into 
animal and vegetable food. The theory is only a modification 
of the old hypothesis, that plants have a power of choosing 
and rejecting, and that different plants require different food. 
The excrements are supposed to restore to the soil the carbon 
they derived from the humus during the early period of 
growth, and alkalies and ashes may hasten the decomposition ; 
but it is still uncertain if the exudations be derived from the 
soil, or from some other source, or formed by the plant itself; 
and until that be ascertained no certainty can exist of the 
justness of the theory. 
Experiment has not yet decided if soluble animal and 
vegetable matter passes unaltered into plants, and becomes a 
part of their organization. If it does pass, it must be by 
many combined and unseen agencies ; and it is very probable 
that water, air, and earth, although they do not singly afford 
the food of plants, all of them operate in the process of 
vegetation. It is probable that we are yet unacquainted 
with any of the true elements of matter; for general sub¬ 
stances, that were formerly thought simple, have been decom¬ 
pounded, and the most recent discoveries are by no means 
sufficient to penetrate into the deep mysteries of organized 
life, and it is doubtful if the propagation of vegetables has 
been in any degree rendered more intelligible or easier of com¬ 
prehension. The original vegetables—lichens and mosses— 
that grew on the naked decomposing rock, could have little 
other food than water and atmospheric air ; and though 
chemists yet regard carbon as a simple and uncompounded 
substance, and have found it in water and in air only in a 
very minute degree, but by some supposed to be decomposable, 
we may very reasonably think that water and air are the chief 
ingredients of the food of plants ; and, if the conclusion be 
true, it is more curious in speculation than in practice, for it 
