_Chlorophyll absorbs certain light rays, and the energy thus directly or indirectly obtained is used by the living matter in uniting the carbon dioxide 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 dioxide of the air has been known as carbon assimilation, but the term now most used is =photosynthesis= (from two Greek words meaning _light_ and _placing 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 dioxide 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 effected by the saliva. [Illustration: FIG. 118.—TRUNK GIRDLED BY A WIRE. See Fig. 85.] =Distribution of the Digested Food.=—After being changed to the soluble form, _this material is ready to be used in growth_, either in the leaf, in the stem, or in the roots. With other more complex products it is then _distributed throughout all the growing parts of the plant_; and when passing down to the root, it seems to pass more readily through the _inner bark_, in plants which have a definite bark. This gradual downward diffusion through the inner bark of materials suitable for growth is the process referred to when the “descent of sap” is mentioned. Starch and other products are often _stored in one growing season to be used in the next season_. If a tree is constricted or strangled by a wire around its trunk (Fig. 118), the digested food cannot readily pass down and it is stored above the girdle, causing an enlargement.