STAKCH 



77 



of plants during the hours of sunlight. How the car- 

 bon dioxid which is thus absorbed may be used as food 

 is a complex question, and need not be studied here. 



163. Chlorophyll absorbs the energy of the sun's rays, and 

 the energy thus obtained is used by the living matter in unit- 

 ing the carbon dioxid absorbed from the air with some of the 

 water brought up by the roots. The process by which these 

 compounds are united is a complex one, but the ultimate result 

 usually is starch. No one knows all the details of this 

 process; and our first definite 



knowledge of the product be- 

 gins 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 photo- 

 synthesis (from Greek words, 

 meaning light and to put to- 

 gether) . 



164. STARCH. All starch 

 is composed of carbon, hydro- 

 gen, and oxygen (CeHioOs). 

 The sugars and the woody 

 substances are very similar to 

 it in composition. All these 

 substances are called carbo- 

 hydrates. In making this 

 starch from the carbon and 

 oxygen of carbon dioxid and 

 from the hydrogen and oxygen 

 of the water, there is a sur- 

 plus of oxygen. It is this oxygen which is given off into 

 the air. To test the giving off of oxygen by day, make the 

 experiment illustrated in Fig. 111. Under a funnel in a 



111. To show the escape of 

 oxygen. 



