f branches, in its first two hundred years, and live for five
or six hundred years longer in a state of comparative rest. It seems to
grow no more, simply because it has exhausted too much of the material
for its nourishment from the ground around its roots. At least, we know,
that, when we have cut it down, not oaks, but pines, will germinate
in the same soil,--pines, which, having other necessities and taking
somewhat different food, find a supply in the ground, untouched by
their predecessor. Hence the rotation of crops, so much talked of by
agriculturists. Before the subject was so well understood, the ground
was allowed to lie fallow for a year or two, when the crops began to
grow small, that it might recover from the air the elements it had lost.
We now adopt the principle of rotation, and plant beans this year where
last year we put corn.

It is not merely that plants deprive themselves of their future support
by exhausting the neighboring earth of the elements they require. Some
of them put into the ground substances which are poisonous to themselves
or other plants. Thus, beans and peas pour out from their roots a very
notable amount of a certain gum which is not at all suited to their
own nourishment,--so that, if we plant beans in the same spot several
successive seasons, they thrive very poorly. But this gum appears to be
exactly the food for corn; if, therefore, we raise crops of beans and
corn alternately, they assist each other. Liebig gives the results of a
series of experiments illustrating the reciprocal actions of different
species of plants. Various seeds were sprouted in water, in order to
observe the nature of the excretions from their roots. It was found
"that the water in which plants of the family of the _Leguminosae_
(beans and peas) grew acquired a brown color, from the substance which
exuded from their roots. Plants of the same species, placed in water
impregnated with these excrements, were impeded in their growth, and
faded prematurely; whilst, on the contrary, corn-plants grew vigorously
in it, and the color of the water diminished sensibly, so that it
appeared as if a certain quantity of the excrements of the _Leguminosae_
had really been absorbed by the corn-plants." The oak, which is the
great laboratory of tannin, not only lays up stores of it in its bark
and leaves, but its roots discharge into the ground enough of it to tan
the rootlets of all plants that venture to put down their suc