_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_ (C₆H₁₀O₅)_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 CO₂ + 6 parts H₂O = C₆H₁₂O₆ + 6 O₂). 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. =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 of 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. [Illustration: FIG. 118.--TRUNK GIRDLED BY A WIRE. See Fig. 85.] =Assimilation.=--_The food from the air and that from the soil unite in the living tissues._ The “sap” that passes upwards from the roots in the growing season is made up largely of the soil water and the salts which have been absorbed in the diluted solutions (p. 67). This upward-moving water is conducted largely through certain tubular canals of the _young wood_. These cells are never continuous tubes from root to leaf; but the water passes readily from one cell or canal to another in its upward course.