132 ESSENTIALS OF BOTANY 
the meadow rue and the nasturtium, possess this power to 
shed water to such a degree that the under surface of the 
leaf is hardly wet at all when immersed in water ; the air- 
bubbles on the leaves then give them a silvery appearance. 
157. Hairs on Leaves. — Many kinds of leaves are more 
or less hairy or downy, as those of the mullein, the “ mul- 
lein pink,” many cinquefoils, and other common plants. 
In some instances this hairiness may be a protection 
against snails or other small leaf-eating animals, but in 
other cases it seems to be pretty clear that the woolliness 
(so often confined to the under surface) is to lessen the 
loss of water through the stomata. 
158. Operation of the Stomata.—— During the early 
morning the stomata usually continue to open gradually 
until they reach their maximum size for the day and 
then gradually lessen in size until the minimum width is 
reached. In some plants the greatest stomatal opening 
occurs at about 8 A.M. and the least at about 5 P.M. 
When leaves begin to wilt the stomata soon close tightly. 
In many plants, especially those with a very thick leaf- 
epidermis, the process of giving off water from the leaves, 
transpiration, is almost entirely stopped when the stomata 
are wholly closed. The under side of the leaf, free from 
palisade-cells and abounding in intercellular spaces, is especi- 
ally adapted for the working of the stomata, and accordingly 
we find them in much greater numbers on the lower than 
on the upper surface. On the other hand, the little flower- 
less plants known as liverworts, which lie prostrate on the 
ground, have their air-pores on the upper surface, and 
here also occur the stomata of the leaves of pond-lilies. 
In those leaves which stand with their edges nearly ver- 
tical, the stomata are distributed somewhat equally on both 
