NUTRITION 
77 
in the presence of sunlight, into a carbohydrate probably 
some form of sugar. It is this series of steps that is called 
photosynthesis. 1 Not all of the oxygen contained in the 
water (H 2 O) and carbon dioxide (CO 2 ) 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 
1 The word means combining (synthesis) in the presence of, or by means 
of, light (photos'). 
