Leaves and Their Structures 



27 



oilcloth, and when thick it is quite as impervious to water. The 

 cuticle is useful to the plant also because it serves as a first line of 

 defense against disease germs. The importance of the epidermis 

 as a protective covering for the delicate inner tissues of the plant 

 may be judged from the drying and decay that follow the breaking 

 of the thin epidermal coat of an apple or a pear. 



Scattered among the colorless cells of the epidermis are pairs of 

 small, crescent-shaped green cells, the guard cells. Each pair 

 of these surrounds a small opening or pore, the stoma (Greek : 

 stoma, mouth ; plural, stomata) ,^ which is opened or closed by the 

 expansion or contraction of the guard cells. The stomata are 

 very important, for they connect the air spaces among the cells 

 inside the leaf with the external atmosphere. When open, they 

 allow the exchange of water vapor and other gases through the 

 epidermis ; and when closed, they complete the barrier to gas 

 movements in either direction (Fig. 11). 



In most of our trees and in many other plants the stomata 

 occur only in the epidermis on the lower surfaces of the leaves. 

 In some plants, especially in those growing in shaded situations, 

 they are found in the epidermis on both the upper and lower leaf 

 surfaces. In such leaves the number of stomata is always greater 

 on the lower side. 



^ Stomata are so small that 2500 of them have an area about equivalent to that 

 of an ordinary pinhole. They are so numerous, however, that they occupy about 

 Ywo of the area of the average leaf. On a square centimeter of the lower surface of 

 a sunflower leaf there are about 15,000 of them. 



Illustrating terms used in describing leaf margins : A , entire ; 

 D, dentate ; E, crenate ; F, undulate ; G, pinnately lobed ; 



B, serrate ; C, doubly 

 H, palmately lobed. 



