53G 
On the. Food of Plants. 
ledge of the methods by which these necessary supplies of food 
may be most economically and effectually rendered to the plants 
we wish to cultivate, and of the external circumstances which 
assist or interfere with the absorption and assimilation of such 
principles. 
Two things in connexion with this suljject have been known 
from the remotest ages : namely, that the same plants grown over 
and over again in the same soil rai)idly degenerate, and at length 
fail entirely ; and in the second place, that this evil can be, to a 
very considerable extent, diminished — this exhaustion of the soil, 
as it has been called, be in part repaired by spreading over the 
land the excrements of animals. It has also long been known 
that exhaustion of the soil, to the extent described, may be avoided, 
and the impoverishing effects of c:ultivation diminished, by occa- 
sionally leaving the land uncropped for a season, exposed to the 
action of the rain and air, after which it is found to have regained 
a portion at least of its former fertility. 
More recently it has been discovered that this unproductive 
" fallowing ' may be entirely avoided, except so far as it is re- 
quired to cleanse the land from weeds, hy cultivating a particular 
succession of plants, so that two crops of the same kind may be 
always separated by a certain interval of time. This is what is 
known by the term, " system of rotation of crops," and is in 
general use throughout Europe, although the particular order of 
this rotation, as well as the period of time involved by it, are 
scarcely in any two places the same. 
This fact, so familiar to all engaged in practical agriculture, 
received some years ago, at the hands of the celebrated De 
Candolle, an extremely beautiful explanation, which obtained 
what was long considered a kind of proof from a set of experi- 
ments made by M. Macaire.* The theory in question supposes 
that all plants throw out from their roots certain peculiar matters 
analogous to the excrements of animals, which their system cannot 
assimilate, and which, if retained in the plant, would occasion its 
destruction by a kind of poisoning as surely as the suppression of 
the urinary secretion causes the death of an animal. These vege- 
table excrements, thus accumulated in the soil during the life of 
one particular set of plants, cannot of course supply nourishment 
to those individuals, but at the same time they do not produce 
upon them any very notable amount of injury, because the roots 
of these plants are constantly lengthening, and thus removing the 
spongioles, by whose aid absorption is supposed to occur, Irom 
the spots occupied by the deposited poison ; but let another race 
of the same plants be cultivated on the ground immediately after 
* Ann. Chim. et Phys., 52, 225. 
