STRUCTURE 237 



does little to save it. As to their use, then, stomates 

 may be well defined as " organs which facilitate gas ex- 

 changes." 



An advantage to the plant in having its stomates 

 generally much more abundant on under rather than on 

 upper surfaces of leaves is evident in connection with rain. 

 The work of leaves depends completely upon the free 

 access of air into the mesophyll. Both photosynthesis 

 and respiration would be seriously impeded if the stomates 

 became choked by raindrops, or if the intercellular spaces 

 were filled with water. Even in the absence of stomates, 

 the upper surfaces of leaves are usually so constructed as 

 to cause water to run off rapidly. Depressed veins and 

 ribs often form a sort of drainage system for the upper 

 surface of the leaf, while hairs and waxy coatings prevent 

 water from soaking in as well as interfering with its 

 evaporating out. 



Another advantage in having stomates on under rather 

 than on upper surfaces is that the loss of water by transpi- 

 ration is less under the former condition. Plants which 

 grow in moist places commonly have stomates almost 

 equally abundant on both sides of the leaves, while plants 

 which frequent dry places have their stomates almost ex- 

 clusively on under surfaces. 



C. Mesophyll. — This term refers to all the tissues of 

 the leaf except the veins and the epidermis ; the word 

 means in the midst of the leaf. Another useful word 

 in connection with this subject is chlorenchyma. All tissue 

 which contains chlorophyll, whether it is in the leaf or 

 elsewhere, is called chlorenchyma; the word means green 

 tissue. Why is it more accurate to refer to photosynthesis 



