THE EVOLUTION OF THE PLANT BODY 



35 



absorptive duties are assumed by the underground portion of the 

 maple tree which forms a network ramifying for hundreds of feet 

 in all directions underneath the surface of the soil; the ultimate 

 branches of this root system act as millions of tiny hairs taking in 

 the water needed for plant maintenance. At the same time, the 

 root system anchors in place the otherwise top-heavy mass of 

 foliage and branches. Thus there is the fundamental division of 

 the tree into leaves (photosynthetic region), stems (supporting 



Fig. 14. — ^A surface view of a leaf epidermis shows a firmly packed layer of 



colorless cells, interrupted by the air pores, or stomates. 



and conducting region), and roots (absorptive and anchorage 

 region). In each of these, the cells differ considerably in structure 

 and function depending upon the special activities resulting from 

 this division of labor. 



The LEAVES of the maple, like those of most flowering plants, 

 are broad and thin in order to expose a maximum amount of 

 surface to the light. Each leaf is a compact mass of green cells 

 protected above and below by an epidermis and traversed by a 

 network of supporting veins. The epidermis (fig. 14) is made up 

 of colorless cells which do not interfere with the transmission of 

 light to the green cells beneath; their chief function is to keep the 



