390 GENERAL SCIENCE 



light and chlorophyll, plants make glucose from the 

 carbon dioxide and water which are taken in through 

 their leaves and roots. This form of carbohydrate the 

 plant uses as food. But the plant makes much more 

 food than it needs, and this is changed into more compact 

 forms of sugar and starch and stored in its roots and seeds. 



Sugar and starch are very similar substances. In fact 

 nature is constantly at work making sugar from starch. 

 The reason that ripe fruit is sweeter than green fruit is 

 because in ripening much of the starch is changed to 

 sugar. Similar changes occur in the process of digestion. 

 When starchy foods are eaten, the starch is changed V 

 a usable form of sugar by the action of the digestive 

 fluids. 



The normal functioning of the body requires so much 

 power that much more fuel food is required than tissue- 

 building food ; that is, more carbohydrates and fats are 

 needed than proteins. Protein is both a tissue builder 

 and a fuel food, while carbohydrates when taken into the 

 body unmixed with other foods are usable only as fuel 

 foods ; however, they may combine with nitrogen-bearing 

 foods and aid in the repair of tissues. 



Starch. Starch is found in all green plants, and in 

 tubers, seeds, and tapioca (Figure 349). Its presence may 

 be detected by the action of iodine. A solution of iodine, 

 in potassium iodide and water, colors raw or cooked starch 

 blue. The starch to be tested should be cool and either 

 neutral or acid to litmus. 



Test a freshly cut piece of potato for the presence of 

 starch by putting a drop of iodine solution on it. Other 

 vegetables and grains may be tested in the same way. 

 Celluloses and glycogen are related to starch. Cellulose 

 is found in the cell walls of plants and is almost insoluble ; 



