228 GENERAL SCIENCE 



Starch is lacking in solubility in cold water, while the 

 sugars are easily dissolved. It would seem probable that 

 whichever carbohydrate may be the direct product of the 

 building-up process under the influence of light, photo- 

 synthesis as it is called, the distribution of this product through 

 the tissues of the plant for use as food or for storage involves 

 changes from starch to sugar and then again to starch. 

 These changes, apparently, are easily accomplished in the 

 plant economy. 



FIG. 74. Starch cells of potato showing granules within cellulose. When 

 cooked these granules swell and burst open, a condition favorable for 

 digestion. 



There are very many examples in Chemistry of substances 

 entirely unlike one another, but having the same elements 

 in them united in the same proportion by weight. The most 

 satisfactory explanation of their differences in properties 

 is a supposed difference in the arrangement of atoms in the 

 make-up of their molecules. 



In the processes of digestion, too, while starch and sugar 

 are foods of greatest value and readily digested ordinarily, 

 the cellulose is very largely indigestible. Long-continued 

 cooking softens and to some extent makes available small 

 portions of the cellulose of vegetables and grains. A certain 

 amount of cellulose in our foods is of great value, however, 

 in giving bulk to it, i.e., in making it less concentrated. 



