FOODS IN PLANTS 



35 



From the upper and the lower surfaces of leaves such as 

 those of live-forever, Wandering Jew, Easter lily, corn, and 

 spiderwort one may peel a thin, almost colorless layer which 

 is known as the epidermis (fig. 25). The epidermis is com- 

 posed of cells more or less compactly arranged. Different 

 plants show much variation in the way in which these 

 epidermal cells fit together. 



One or both epidermal 

 layers may include special 

 structures known as stomata 

 (singular, stoma*) (fig. 25). 

 Usually, when viewed from 

 the surface, the stoma is 

 readily seen to consist of 

 two crescentic or kidney- 

 shaped cells with their 

 concave sides facing one 

 another, so as to leave an 

 opening between the two 

 cells. The opening is really 

 the mouth of a larger space 

 extending within the leaf. 

 It is known as the stomatal 

 opening, and the two cells 

 that are about the mouth 

 are known as the guard cells. Unlike other epidermal cells the 

 guard cells are green. The stomatal opening serves as a place 

 of entrance for the carbon dioxide used by the plant, though 

 carbon dioxide may doubtless enter the plant with the water 

 from the soil. The guard cells may also press closely together 

 or may separate until a wide circular opening is formed, and in 

 thus closing and opening they influence the interchange of air 

 between the interior and the exterior of the leaf. Obviously 

 this opening and closing also affects the interchange of such 

 gases as carbon dioxide and oxygen, as well as the outgo of 

 moisture from the leaf. 



FIG. 25. A surface view of leaf epider- 

 mis from the geranium (Pelargonium) 



Among the ordinary epidermal cells (c) are 

 four stomata, each with two guard cells (gc) 

 and the mouth of an air cavity (p). Con- 

 siderably magnified 



