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.