CARBOHYDRATES BY NIGHT AND BY DAY 



In daylight, photosynthesis normally produces sugar faster than it can diffuse out 

 of the cells and move into growing tissues or into underground ports. Surplus sugar 

 in the leaves, and the sugar brought from the leaves into underground structures, 

 become converted into starch. In the dark the starch that has accumulated in the 

 leaves becomes transformed into sugar, which is carried into tubers or other under- 

 ground "storage" structures 



How Is Food Digested in Man? 



The Human Food Tube^ The mouth is the beginning of a long tube 

 inside of which all the digestion takes place. This tube is called the ali- 

 mentary canal, or food tube. It consists of several fairly distinct regions. It 

 is ten or eleven yards long and is coiled or twisted in parts (see illustra- 

 tion, p. 167). 



In the mouth, food is crushed and ground by the teeth. The taste of the 

 food, the movement of the jaws, and the rubbing of the food against the 

 inside of the mouth stimulate the saliva glands. As a result, a quantity of 

 saliva flows into the mouth and becomes mixed with the food. An enzyme 



1 See No. 4, p. 183. 

 165 



