42 TEXTBOOK OF BOTANY 



stances within their own bodies into more complex foods. 

 In this respect the green plants have a great advantage over 

 plants without chlorophyl (like the bacteria and the yeasts), 

 and over the animals (including ourselves) as well. Green 

 plants can live and thrive on a food supply that would be 

 quite useless to an organism without chlorophyl. Among 

 the simple substances absorbed by Spirogyra are water (which 

 is made up of hydrogen and oxygen), and carbon dioxid 

 (composed of carbon and oxygen). Carbon dioxid is a gas 

 which makes up about three-hundredths of one per cent of the 

 air ; it is also present in solution in the water in which Spi- 

 rogyra lives. Cells containing chlorophyl can manufacture 

 out of carbon dioxid and water certain substances known as 

 carbohydrates (composed of carbon, hydrogen, and oxygen). 

 Starch and sugars are among the commonest carbohydrates. 

 Probably the first carbohydrate formed in the Spirogyra cell 

 is glucose or a similar sugar. When a sugar is built up out 

 of carbon dioxid and water, some of the oxygen which these 

 substances contain is given off. This oxygen appears as 

 bubbles of gas in the water, and if we examine a mass of 

 vigorous Spirogyra that is floating on the surface of a pond 

 on a bright day, we shall find many of these bubbles tangled 

 among the Spirogyra threads. Carbohydrates are manufac- 

 tured only by cells that contain chlorophyl, and only in the 

 presence of sunlight. 



64. Starch. — The first carbohydrates manufactured by 

 the Spirogyra cell, as we have seen, are probably sugars. 

 They are at once dissolved in the cell sap. As time goes on, 

 the sap contains more and more sugar. But when the sugar 

 in the sap reaches a certain joroportion, some of the sugar 

 is changed into another carbohydrate called starch. Starch 

 is a solid, insoluble substance and, being compact, it is a con- 

 venient form in which to store carbohydrates when more are 

 manufactured than can be used at once. The pyrenoids are 

 the parts of the cell that actually do the work of changing 



