The green plant in other words, raises non-living into living matter, while the animal can only transform living matters into its like.  This is why the plant is called a constructive organism, while the animal is, contrariwise, named a destructive one.  The result of the plant’s existence is to build up, that of the animal’s life is to break down its substance, as the result of its work, into non-living matter.  The animal’s body is, in fact, breaking down into the very things on which the green plant feeds.  We ourselves are perpetually dissipating our substance in our acts of life and work into the carbonic acid, water, ammonia, and minerals on which plants feed.  We “die daily” in as true a sense as that in which the apostle used the term.  And out of the debris of the animal frame the green plant builds up leaf and flower, stein and branch, and all the other tokens of its beauty and its life.

If, then, an animal can only live upon living matter—­that is to say on the bodies of other animals or of plants—­with water, minerals and oxygen gas from the air thrown in to boot, we might be tempted to hold that in such distinctive ways and works we had at last found a means of separating animals from plants.  Unfortunately, this view may be legitimately disputed and rendered null and void, on two grounds.  First of all, the mushrooms and their friends and neighbors, all true plants, do not feed as do the green tribes.  And secondly, many of the green plants themselves can be shown to have taken very kindly to an animal mode of diet.

A mushroom, thus, because it has no green color, lives upon water, oxygen, minerals, and organic matter.  You can only grow mushrooms where there is plenty of animal matter in a state of decay, and as for the oxygen, they habitually inhale that gas as if they were animals.  Non-green plants thus want a most characteristic action of their green neighbors.  For the latter in daylight take in the carbonic acid gas, which is composed of carbon and oxygen.  Under the combined influence of the green color and the light, they split up the gas into its two elements, retaining the carbon for food and allowing the oxygen to escape to the atmosphere.  Alas! however, in the dark our green plant becomes essentially like an animal as regards its gas food, for then it is an absorber of oxygen, while it gives off carbonic acid.  If to take in carbonic acid and to give out oxygen be held to be a feature characteristic of a plant, it is one, as has been well said, which disappears with the daylight in green plants, and which is not witnessed at all in plants that have no green color.