PHOTOSYNTHESIS 85 



178. Chlorophyll absorbs certain of the sun's rays and 

 the energy thus derived is used in uniting the carbon dioxid 

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

 process is complex, with some kind of sugar or starch as 

 the ultimate product. Glucose is probably the first carbo- 

 hydrate formed. In most plants, the first visible product is 

 starch. Certain plants do not produce starch. The common 

 onion, amaryllis and iris are of this class. The process of 

 using the carbon dioxid of the air has been known as carbon- 

 assimilation, but the term now commonly used is photo- 

 synthesis (from Greek words, meaning "light" and "put 

 together"). 



179. Glucose or grape sugar is composed of carbon, hydro- 

 gen, and oxygen (C e Hi 2 6 ). Starch is likewise composed 

 of carbon, hydrogen, and oxygen, but differs in the percent- 

 ages. Its chemical formula is generally given (C 6 Hio0 5 ). 

 Cane sugar, malt sugar, woody substances are very similar 

 in composition. They are called carbohydrates. In making 

 the glucose sugar from the carbon dioxid and water, the 

 oxygen gas is given off by the plant as a waste product. 

 The general chemical formula for the process is: 6C0 2 + 

 6H 2 = C 6 Hi,0 6 +60 2 . 



180. In the daytime the plant, therefore, takes in carbon 

 dioxid and gives off oxygen. It is not so easy to demonstrate 

 this fact. Chemical analysis is the only way of proving it. 

 The escape of oxygen can best be demonstrated by employ- 

 ing water plants. Make an experiment as illustrated in Fig. 

 130. Under a funnel in a deep glass jar containing fresh 

 spring or stream water, place fresh pieces of the common 

 water-weed, elodea (or anacharis). In sunlight, bubbles of 

 oxygen will arise and collect in the test-tube. Some of the 

 bubbles may be only air, particularly if marked changes in 

 the temperature of the water occur. A simple experiment is 

 to immerse a stem of elodea in a test-tube of water and hold 

 the tube in bright sunlight. Bubbles of gas will arise from 



