=Source of Plant Energy.=—We notice that green plants thrive only in the light, while animal growth is largely independent of light. In fact, in the salt mines of Poland there are churches and villages below the ground, and children are born, become adults, and live all their lives below ground, without seeing the sun. (That these people are not very strong is doubtless due more to want of fresh air and other causes than to want of sunlight.) [Illustration: FIG. 1.—SURFACES OF A LEAF, magnified. ] [Illustration: FIG. 2.—A LEAF STORING ENERGY IN SUNLIGHT. ] _The need of plants for sunlight shows that they must obtain something from the sun._ This has been found to be _energy_. This enables them to _lift_ their _stems_ in growth, and _form_ the various structures called _tissues_ which make up their stems and leaves. It is noticed that they take in food and water from the soil through their roots. Experiments also show that green plants take in through pores (Fig. 1), on the surface of their leaves, a gas composed of carbon and oxygen, and called _carbon dioxide_. The _energy in the sunlight_ enables the plant _to separate out the carbon, of the carbon dioxide_ and to build mineral and water and carbon into organic substances. The oxygen of the carbon dioxide is set free and returns to the air (Fig. 2). Starch, sugar, oil, and woody fibre are examples of substances thus formed. Can you think of any fuel not due to plants? =How Animals obtain Energy.=—You have noticed that starch, oil, etc., will _burn_, or _oxidize_, that is, _unite with the oxygen of the air_; thus the sun’s energy, stored in these substances, is changed back to heat and motion. The oxidation of oil or sugar may occur in a furnace; it may also occur in the living substance of the active animal. [Illustration: FIG. 3.—COLOURLESS plants, as A GREEN LEAF, even after it is cut, MUSHROOMS, give off no oxygen. gives off oxygen (O) if kept in the sun. ] Fortunately for the animals, the plants oxidize very little of the substances built up by them, since they do not move about nor need to keep themselves warm. We notice that animals are constantly using plant substances for food, and constantly drawing the air into their bodies. If the sunlight had not enabled the green plant to store up these substances and to set free the oxygen (Fig. 3), animals would have no food to eat nor air to breathe; hence we may say that the sunlight is indirectly the source of the life and energy of animals. Mushrooms and other plants without green matter cannot set oxygen free (Fig. 3).