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 carbon dioxid which is thus absorbed may be used in making an organic food is a complex question, and need not be studied here; but it may be stated that carbon dioxid and water are the constituents. Complex compounds are built up out of simpler ones.

''Chlorophyll absorbs certain light rays, and the energy thus directly or indirectly obtained is used by the living matter in uniting the carbon dioxid absorbed from the air with some of the water brought up from the roots. The ultimate result usually is starch.'' The process is obscure, but sugar is generally one step; and our first definite knowledge of the product begins when starch is deposited in the leaves. The process of using the carbon dioxid of the air has been known as carbon assimilation, but the term now most used is photosynthesis (from two Greek words, meaning light and to put together).

Starch and Sugar.—All starch is composed of carbon, hydrogen, and oxygen (C_{6}H_{10}O_{5})_{n}. The sugars and the substance of cell walls are very similar to it in composition. All these substances are called carbohydrates. In making fruit sugar from the carbon and oxygen of carbon dioxid and from the hydrogen and oxygen of the water, there is a surplus of oxygen (6 parts CO_{2} + 6 parts H_{2}O = C_{6}H_{12}O_{6} + 6 O_{2}). It is this oxygen that is given off into the air during sunlight.

Digestion.—''Starch is in the form of insoluble granules. When such food material is carried from one part of the plant to another for purposes of growth or storage, it is made soluble before it can be transported.'' When this starchy material is transferred from place to place, it is usually changed into sugar by the action of a diastase. This is a process of digestion. It is much like the change of starchy foodstuffs to sugary foods by the saliva.