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 suction-hose into the same region, and their spongioles are so effectually closed by this process, that they can no longer perform their office, and the plant that bears them dies. Plants whose roots ramify among the roots of poppies become unwilling opium-eaters, from the exudation of this narcotic principle into the ground, and are stunted, like the children of Gin Lane.