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Life as a whole depends entirely upon this property of chlorophyll; for every atom of organic matter in your body or mine was originally so manufactured by sunlight in the leaves of some plant from which, directly or indirectly, we derive it.

To be sure, in order to make up the various substances which compose their tissues–to build up their wood, their leaves, their fruits, their blossoms–plants require hydrogen, nitrogen, and even small quantities of oxygen as well; but these various materials are sufficiently supplied in the water which is taken up by the roots, and they really contribute very little indeed to the bulk of the tree, which consists for the most part of almost pure carbon. If you were to take a thoroughly dry piece of wood, and then drive off from it by heat these extraneous matters, you would find that the remainder, the pure charcoal, formed the bulk of the weight, the rest being for the most part very light and gaseous. Briefly put, plants are mostly carbon and water, and the carbon which forms their solid part is extracted direct from the air around them.

How does it come about then that a careless world in general, and more especially the happy-go-lucky race of gardeners and farmers in particular, who have to deal so much with plants in their practical aspect, always attach so great importance to root, soil, manure, minerals, and so little to the real gaseous food stuff of which their crops are, in fact, composed? Why does Hodge, who is so strong on grain and guano, know absolutely nothing about carbonic acid? That seems at first sight a difficult question to meet. But I think we can meet it with a simple analogy.

Oxygen is an absolute necessary of human life. Even food itself is hardly so important an element in our daily existence; for Succi, Dr. Tanner, the prophet Elijah, and other adventurous souls too numerous to mention, have abundantly shown us that a man can do without food altogether for forty days at a stretch, while he can’t do without oxygen for a single minute. Cut off his supply of that life-supporting gas, choke him, or suffocate him, or place him in an atmosphere of pure carbonic acid, or hold his head in a bucket of water, and he dies at once. Yet, except in mines or submarine tunnels, nobody ever takes into account practically this most important factor in human and animal life. We toil for bread, but we ignore the supply of oxygen. And why? Simply because oxygen is universally diffused everywhere. It costs nothing. Only in the Black Hole of Calcutta or in a broken tunnel shaft do men ever begin to find themselves practically short of that life-sustaining gas, and then they know the want of it far sooner and far more sharply than they know the want of food on a shipwreck raft, or the want of water in the thirsty desert. Yet antiquity never even heard of oxygen. A prime necessary of life passed unnoticed for ages in human history, only because there was abundance of it to be had everywhere.

Now it isn’t quite the same, I admit, with the carbonaceous food of plants. Carbonic acid isn’t quite so universally distributed as oxygen, nor can every plant always get as much as it wants of it. I shall show by-and-by that a real struggle for food takes place between plants, exactly as it takes place between animals; and that certain plants, like Oliver Twist in the workhouse, never practically get enough to eat. Still, carbonic acid is present in very large quantities in the air in most situations, and is freely brought by the wind to all the open spaces which alone man uses for his crops and his gardening. The most important element in the food of plants is thus in effect almost everywhere available, especially from the point of view of the mere practical everyday human agriculturist. The wind that bloweth where it listeth brings fresh supplies of carbon on its wings with every breeze to the mouths and throats of the greedy and eager plants that long to absorb it.