The air which we expel from the lungs at every breath has a large proportion of carbonic acid. Let a man be shut up in an air-tight room for a day, and he will have changed nearly all the oxygen in it into this carbonic acid, and rendered it unfit for animal life. Dogs, cats, and birds would die in it. But, poisonous as it is to man and other animals, it is a feast to plants. They want it all day and every day; not in the night,--at that time they have a taste for oxygen. This effete air, which men and animals exhale, so charged with carbonic acid, the plants drink in through every pore. They take it from the mouth of man, appropriate it to their daily uses, and in time render it back to him mingled with other ingredients in wholesome fruit. Carbonic acid is death when it combines with the blood,--as it does when we inhale it; but not so when it enters the stomach in small quantities. One inspiration of it is enough to make us dizzy,--as when we enter an old well or stoop over a charcoal fire; but a draught of water fully charged with it is exhilarating and refreshing, as we know by repeated experiences at marble fountains that meet us on so many city-corners.
If plants had souls, they would be pure ones, since they can bear such contamination and not be harmed,--nay, since even from such foul food as we give them they can evolve results so beautiful. We give them our cast-off and worn-out materials, and they return us the most beautiful flowers and the most luscious fruits.
Beside carbonic acid, there are two other principal materials, which are every day passing off in an effete state, though capable of being transferred to the uses of plants. But when an animal dies, the whole substance is then at Nature's disposal. We must set aside a great deal of it for the ants and flies, who will help themselves in spite of us. If any one has never seen a carcass rapidly disappearing under the steady operations of the larvae of the flesh-fly, he has yet to learn why some flies were made. The ants, too, carry it off in loads larger, if not heavier, than themselves. But carcasses of animals may go to decay, undisturbed by the ravages of these useful insects. That is, the limited partnership of Oxygen, Hydrogen, & Co., under which they agreed to carry on the operations of sheep, fox, or fish, having terminated by the death of the animal, the partners make immediate use of their liberty and go off in inorganic form in search of new engagements, leaving sulphur, phosphorus, and the other subordinate elements of the animal, to shift for themselves. They were in the employ of a sheep; they will now carry on a man or an oak-tree, a colony of insects, or something else. Under the form of carbonate of ammonia, the four elements diffuse themselves through the air, or are absorbed by the earth, and offer themselves at once to the roots and leaves of the trees, as ready to go on with their vivifying operations as they were in behalf of the animals. There are some plants which seem not to be left to the chances of securing their nourishment from the carbonate of ammonia that the air and the soil contain, but are contrived so as to entrap living animals and hold them fast while they undergo decomposition, so that all their gases may be absorbed by them alone. Thus, "the little Sundew exudes a gluey secretion from the surface of its leaves, which serves to attract and retain insects, the decay of whose bodies seems to contribute to its existence." And the Dionaea, or Venus's Fly-trap of the Southern States, has some leaves which fold together upon any insect that alights upon their upper surface; and by means of a row of long spines that fringes the leaves, they prevent his escape. The more active the struggles of the captive, the closer grows the hold of the leaf, and speedily destroys him. The plant appears to derive nutriment from the decomposition of its victims. "Plants of this kind, which have been kept in hot-houses in England, from which insects were carefully excluded, have been observed to languish, but were restored by placing little bits of meat upon their traps,--the decay of these seeming to answer the same purpose."