On Ihc Food of Plants. 
537 
the removal of tlie first, and these poisonous principles are brought 
into active operation to tlu; detriment of the new-comers. 
The theory further supposes, that althoufrh tlio excrements of 
one plant are incapable of affording nourishment to another of 
the same kind, and would even inflict upon it positive mischief, 
yet to a jdant of a totally different family these very substances 
may serve as nutritious food ; that the fecal matters accumulated 
in the soil during the growth of a crop of wheat, although injuri- 
ous to another crop of grain, may afford food to a fresh set ot 
plants, such as clover, which in their turn excrete substances 
hurtful to themselves, but exceedingly nutritious to corn. 
Macaire, in his investigation before referred to, observed that, 
when plants were made to grow in pure water for several days, 
soluble matter was certainly emitted from the roots, which could 
be detected both by its colour, taste, and smell, and also by che- 
mical I'e-agents, and that this soluble matter differed very much 
with the kind of plant. He further observed, that water charged 
with the excrements of a leguminous plant, although decidedly 
injurious to another of the same kind, suffered a plant of wheat, 
whose roots were immersed in it, to live perfectly well, while at 
the same time the yellow colour of the water diminished in in- 
tensity. 
It is a very great objection to such experiments that they are 
made under conditions never fulfilled in nature. It is impossible 
that a plant thus situated can maintain a hoalthy state even for a 
few days. From what we now know of the nature of vegetable 
nutrition, plants so treated must be living upon their own resources 
to a great extent, like hyacinth bulbs grown in glasses of water. 
Moreover, attempts to obtain these specific excretions, by causing 
the same plants to grow in moist sand, completely failed. Some 
important observations, since made by Braconnot,* throw great 
doubt (ju the fact of specific substances being ejected from the 
roots of plants, of such a nature as to act in the manner de- 
scribed. 
There are two especial reasons for the rejection of this theory 
at the present moment. The small chance such substances would 
have of escaping destruction in the soil, by the oxygen of the air, 
even for a very brief period ; and the fact, now rendered pretty 
certain, that bodies of this kind are quite incapable of nourishing 
plants by direct absorption : they can only furnish food by their 
decomposition into carbonic acid, and water, and ammonia. 
A better explanation is to be found in the ancient idea of ex- 
haustion of soil, aided by what little knowledge we already pos- 
sess of the kind of nourishment given by the ground to each class 
* Ann. Chim. et Phys., 72, 27. 
