Living things are distinguished from the not living by a series of processes, or changes (feeding, growth, development, multiplication, etc.), which together constitute what is called life. These processes are called _functions_. Both plants and animals have certain parts called _organs_ which have each a definite work, or function; hence animals and plants are said to be organized. For example, men and most animals have a certain organ (the mouth) for taking in nourishment; another (the food tube), for its digestion. Because of its _organization_, each animal or plant is said to be an organism. Living things constitute the _organic kingdom_. Things without life and not formed by life constitute the _inorganic_, or _mineral, kingdom_. Mark I for inorganic and O for organic after the proper words in this list: granite, sugar, lumber, gold, shellac, sand, coal, paper, glass, starch, copper, gelatine, cloth, air, potatoes, alcohol, oil, clay. Which of these things are used for food by animals? Conclusion? =Energy in the Organic World.=--We see animals exerting energy; that is, we see them moving about and doing work. Plants are never seen acting that way; yet they need energy in order to form their tissues, grow, and raise themselves in the air. =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 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. (See Part I, Chap. XIII.) 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 under side of their leaves, a gas composed of carbon and oxygen, and called _carbon dioxid_. The _energy in the sunlight_ enables the plant _to separate out the carbon of the carbon dioxid_ and build mineral and water and carbon into organic substances. The oxygen of the carbon dioxid is set free and returns to the air (Fig. 2). Starch, sugar, oil, and woody fiber are examples of substances thus formed. Can you think of any fuel not due to plants?