WE have talked about the necessity of power development in all living things, and have seen that power development depends on the oxidation of fuel. Of course, our fuel is the food that we eat. No substance is suitable for fuel unless it contains power which can be gotten out by oxidation, and unless, in addition, it is suited to the particular kind of oxidation that goes on in the body, and can be handled by the body. Wood is excellent fuel for some purposes, but as a food for man it has no value, even when ground fine and mixed with flour as was done in some European countries during the Great War, because wood cannot be handled by the body in the way in which a usable fuel must be. Although wood is not good food, closely related materials are, and in fact make up the bulk of it. All fuel food is either vegetable or animal. All animal food traces back finally to the vegetable world, and it is an interesting fact that we do not usually care to eat flesh that is more than one remove from the vegetable kingdom. Animals that are flesh eaters are not considered fit for food, except in the case of fish and birds, and the flesh that these eat is not commonly thought of as being such, since it consists mostly of the flesh of insects, frogs, and fish themselves.

The real sources of food, then, are in the vegetable world. Of the countless thousands of kinds of plants that exist a few dozen have proven to be of enough use for human food, or for food for the animals on which human beings feed, to justify us in taking the trouble to raise them on our farms and in our gardens. There must be something about these particular plants to make us prefer them. If we look into the reason for the preference we shall understand something of the qualities which make plants good for food in the first place. At the beginning of the chapter were set down the things which make a substance fit for food. These are: the ability to yield power by oxidation, and a composition suitable to be used by the body. The ability to yield power involves the possession of a store of it. Power, or energy, which means the same thing in our present use of the words, is never present anywhere except as the result of an earlier exhibition of power. It is not made out of nothing. The sun is a reservoir of energy on which the earth draws, and we do not know with any certainty from whence the sun got its power. The heated center of the earth itself is a reservoir of power on which we may draw at some time in the future, when cheaper sources are used up. Except for energy from these sources and for trifling amounts that may be brought in by meteorites, there is none on the earth’s surface that has not always been here. On the other hand, the earth is constantly losing energy into space. The amount that reaches us from the sun balances our losses into space, so that the total energy present holds fairly steady. The energy that comes to us from the sun is chiefly in the two forms of heat and light. In actual horsepower the heat far outweighs the light, but in importance to mankind one stands about on a par with the other, for while without the sun’s heat the earth would become so cold that we would all die, without its light there would be no food and we would all starve. This is another way of saying that the energy that plants store up, and that we get when we eat them, comes originally from the light of the sun. Plants, like animals, are made up of cells. Those with which we are familiar consist of a great many cells, of a good many different kinds. Some are in the roots, others in the stems, still others in the leaves; the blossoms, fruits, and seeds are made up, likewise, of cells. The cells near the surface of the leaves, and, in many kinds of plants, near the surface of the stems as well, contain a green substance known as chlorophyll. This substance enables the cells in which it is present, although we do not know just how, to manufacture sugar, utilizing the energy of the sunlight for the purpose.

Sugar is composed of three very common chemical elements, carbon, hydrogen, and oxygen. As we all know, hydrogen and oxygen in combination of two atoms of hydrogen to one of oxygen form water; the most familiar of chemical symbols is that expressing this combination, namely H{2}O. Carbon, which we know in an almost pure state in anthracite coal, and in even purer form in diamonds, forms a combination with oxygen known as carbon dioxide. This is a gas; it makes up a small fraction of the air. The amount in the air is increased whenever coal or any other carbon-containing material is burned, since carbon dioxide is the product of the oxidation of carbon. Except in the arid regions of the earth there is always some water in the soil a greater or less distance below the surface of the ground. Water and carbon dioxide between them contain all the elements of which sugar is composed. The chlorophyll-containing cells are the factory; the sunlight is the power; and the carbon dioxide and water are the raw materials. Sugar is the finished product, and wherever sunlight is falling on green plants, whether directly or through a layer of cloud, its manufacture is going on. Sugar will oxidize readily, and in so doing will yield abundant power. The energy which it contains was derived by transformation from the energy of the sunlight. With the exception of a few kinds of bacteria every living thing on the earth depends for its food, and so for its energy, either directly or indirectly on the sugar which green plants manufacture. Since sugar dissolves in water it cannot easily be held in storage, so by a simple chemical process the plant changes it to starch, and it is in this latter form that we get it, except in the case of a few plants, like sugar cane.

The green parts of plants are the only places where sugar is made. We eat a certain amount of green food in lettuce and asparagus and similar vegetables, but for the most part the sugar or starch we eat comes from parts of plants that are not green. There is evidently a transportation from the point of manufacture to points of storage. The means of transport is in the sap; since starch is not soluble in water, it must be changed back into sugar. This is done, and then, by the movement of sap the dissolved sugar is carried to the points of storage, roots in such vegetables as beets, underground stems in potatoes, above ground stems in sugar cane, fruits or seeds in orchard and grain crops. In such of these as are sweet, the sugar itself is held in storage; in most kinds it is changed back into starch. Where the storage is in the form of starch the vegetable ordinarily keeps better than when sugar is the substance on deposit.

A few kinds of plants—olives, peanuts, and cocoanuts, for example—convert the sugar into oil and store their surplus material in that form. The chemical elements in oils and fats are the same as in starch and sugar, although the proportions are not the same. Weight for weight oil has more than twice the energy value of sugar; in making a given amount of peanut oil the peanut vine used up more than twice the amount of starch or sugar; but since energy value is what counts rather than bulk the plant is just as well off, and perhaps better on account of the smaller bulk occupied by the stored material. One of the very interesting examples of oil storage is found in the very tiny plants, called diatoms, which abound in the water of the ocean. Each tiny diatom stores within itself an even more tiny drop of oil. Although by themselves single oil drops would make no impression, if enough of them could be brought into one place a respectable accumulation of oil would result. This is precisely what the geologists tell us has happened in past ages; the bodies of diatoms have accumulated through thousands of years, and finally the oil accumulations have been covered over with sediment of one kind or another. When we tap through the sediment we strike into the “oil sand,” which contains this residue of the diatoms, and an oil well results.

Since we depend for our food, and so for our life, on the sugar-making activities of green plants, it will be worth our while to think for a moment of the slowness with which the process goes on. The slice of bread which we may eat in a dozen bites represents the result of a season’s growth of several wheat plants, every one of which was absorbing the sun’s energy and laying up starch grains during every daylight minute throughout the growing season. From the standpoint of the plant which does the storing the material which serves us as food is the excess over the plant’s own daily needs. In most cases it would be utilized at the beginning of the next season’s growth before the plant had put out a leaf system, if the course of events were not disturbed to satisfy the needs of man.

In addition to starch, sugar, and fat there is another kind of food material manufactured by plants, known as protein. This substance is much more complex chemically than any of the others; it contains, in addition to the three chemical elements—carbon, oxygen, and hydrogen—that are found in them, the element nitrogen, and usually some phosphorus and sulphur. These materials are dissolved in the soil water in the form of simple chemical substances, and are taken up by the plant along with the water which enters the roots and flows as sap up to the leaves. The same cells of the plant that make sugar have the power to make protein, using as raw materials some of the sugar along with the substances brought in with the soil water. The energy for the manufacture of protein comes from the oxidation of some of the sugar or starch in the leaf. The finished protein has about the same energy value, weight for weight, as has the starch from which it was mainly derived.

When an animal eats a plant or part of one, he is eating for the sake of the sugar or sugar products which the plant has made. There is one sugar product that is useful as food for many animals, but not for man, except possibly to a very slight extent. That is the woody substance, cellulose, which is formed in plants mainly as a support to the delicate living protoplasm. Cotton fiber is nearly pure cellulose. Cellulose is very similar to starch chemically, and is an excellent fuel wherever it can be burned. The human digestive tract is unable to handle it in a manner to make it usable, although grazing animals do so quite efficiently. A good many plants make products that are either disagreeable in flavor or actually poisonous. Of course, in such cases the plants become useless as food unless a treatment can be devised that will remove the objectionable material or convert it into something harmless. The few dozen kinds of plants that we raise for food are those that are free from harmful substances and that yield large quantities of stored food materials, or in some cases that taste especially good, even though they may not have much food value. Tomatoes, lettuce, and the like, come in this latter class. The world has been pretty well ransacked for food grains, fruits, and herbs, but probably there are others yet to be found besides those we now have.