NUTRITION 



77 



in the presence of sunlight, into a carbohydrate — probably 

 some form of sugar. // is this series of steps that is called 

 photosynthesis.'^ Not all of the oxygen contained in the 

 water (H2O) and carbon dioxide (CO2) used, enters into 

 the composition of the carbohydrate. The unused por- 

 tion is set free, and is either utilized in other processes, 



Fig. 57. — A tree (hornbeam), seen from among the branches (as the 

 squirrel sees it), showing absence of leaves except at or near the tips of the 

 branches. The same tree as in Fig. 56. 



or diffuses out through the stomata to the surrounding 

 air. Thus, the taking in of carbon dioxide and the 

 giving off of oxygen are outward indications that photo- 

 synthesis is going on inside the green cells. 



77. Starch -making. — The sugar made by photosynthe- 

 sis is soluble in the cell-sap, and if it were not removed 



^ The word means combining (synthesis) in the presence of, or by means 

 of, light (photos). 



