LEAVES— FUNCTION OB WORK 



95 



the carbon dioxide which is thus absorbed may be used in 

 making an organic food is a complex question, and need not 

 be studied liere; but it may be stated that carbon dioxide 

 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 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 carlion 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 (CoH^oOj)^. 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^ -|- 6 parts H.O 

 = CgH^oOe + 60.). 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. Wlien 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. Tt is much like the change 

 of starchy foodstuffs to sugary foods effected by the saliva. 



