LEAVES— FUNCTION OR WORK 95 



carbon dioxid which is thus absorbed may be used in mak- 

 ing 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, atid the energy 

 thus directly or indirectly obtained is used by the living 

 ■matter in uttiting the carbon dioxid absorbed from the air 

 with some of the water brought ttp from the roots. The 

 ultimate result itsually 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 (CgHigOg)^. 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 

 = CgHj.^Og + 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 atiother 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 ^digestion. It is much like the change 

 of starchy foodstuffs to sugary foods by the saliva. 



