FOODS IN PLANTS 85 
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. Among the ordinary epidermal cells (c) are 
It is known as the stomatal four stomata, each with two guard cells (gc) 
opening, and the two cells #4 ‘é Oe meee oe 
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. 
Fic. 25. A surface view of leaf epider- 
mis from the geranium (Pelargonium) 
