118 ELEMENTARY BOTANY 
The cobalt paper over the leaf with the under 
side uppermost turns red rapidly ; that over the 
upper leaf remains blue, or, at any rate, turns red far more 
slowly. : 
Result. 
More water is given off from the lower than 
from the upper surface of the leat. 
To see if there is any difference in. structure in the two 
surfaces of the leaf, strip off the skin from the upper and 
lower surfaces and examine under a microscope. Little open- 
_ ings are found in the skin of-the lower surface which are 
absent from the upper (cf. Figs. 138, 139). ‘These openings 
or mouths are called stomata (Gk. stoma =a mouth). 
Conclusion. 

Fic. 188.—LoweEr EPIDERMIS Fic. 189.—Upper ErIpERMIS 
WITH STOMATA. WITHOUT STOMATA., 
We may therefore conclude that the water passes off from 
the leaf by these stomata. Some leaves have the stomata 
scattered equally over both surfaces, but those, which have the 
stomata only on the lower surface will give off water chiefly 
from the under side. This giving out of water hardly occurs 
in water plants; it is owing to the fact that land plants are 
in contact with an atmosphere that water is constantly pass- 
ing from them, not only from the leaves, but also from the 
surface of the stem, which is also provided with stomata. 
The amount of water given off varies with the conditions of 
the atmosphere, just as the perspiration of animals does. 
When it is dry and hot more water is given off than when 
the temperature is low and the air moist. When the plant is 
