92 



ELEMENTARY STUDIES IN BOTANY 



(as the green expansion is called), while in many Ferns the 

 large leaf may be broken up into many blades (Figs. 70 

 and 72). 



Since the largest amount of working tissue is exposed to 

 the air by leaves, it follows that the leaves lose most water. 



(FIG. 72. Shield ferns (Aspidium), showing the large leaves broken into many blades. 



They must be kept full of water and they must be exposed 

 to the air, so that great loss is inevitable, and the vascular 

 system must make it good. The escape of water from plants, 

 chiefly by way of the leaves, is called transpiration, but it 

 might just as well be called plant-evaporation, for the water 

 evaporates from the moist cells of the plant just as it does 

 from any moist surface exposed to the air. How the leaves 



