LEAF FUNCTIONS 



97 



tions are found for exposing leaves to light, since, without light, 

 starch-making cannot go on, and without starch the plant cannot 

 survive. The chlorophyll is placed in the long palisade cells so 

 that, if the light be weak, the chlorophyll bodies may move to the 

 upper ends of the cells and get better illumination; or if the light 

 is too bright, they line up along the sides and so escape the direct 

 rays. In the deeper tissue of the spongy parenchyma of the leaf, 



Light 



Courtesy of American Museum of Natural History 



FIG. 25. Activities going on in the "cells" and. air spaces of a leaf. 



the chlorophyll is sufficiently protected and does not need to move 

 in this way; here we find the cells irregular in shape. 



Materials used in Photosynthesis. The water for starch mak- 

 ing is supplied from the soil by means of the absorption of the 

 roots. It rises to the leaves by way of the ducts and veins. Any 

 excess is disposed of through the stomata. The carbon dioxide 

 is supplied from the air, where oxidation, respiration, combustion, 

 fermentation, and decay are constantly producing it. As fast as 

 the plants remove it they return the oxygen. As a result the 

 composition of the air remains practically constant. 



