158 FOUNDATIONS OF BOTANY 
169. Hairs on Leaves. — Many kinds of leaves are more 
or less hairy or downy, as those of the mullein, the 
‘“mullein pink,” many cinquefoils, and other common 
plants. In some instances this hairiness may be a protec- 
tion against snails or other small leaf-eating animals, but 
in other cases it seems to be pretty clear that the woolli- 
ness (so often confined to the under surface) is to lessen 
the loss of water through the stomata. The Labrador 
tea is an excellent example of a plant, with a densely 
woolly coating on the lower surface of the leaf. The 
leaves, too, are partly rolled up (see Fig. 224), with the 
upper surface outward, so as to give the lower surface 
a sort of deeply grooved form, and on the lower surface 
all of the stomata are placed. This plant, like some 
others with the same characteristics, ranges far north into 
regions where the temperature, even during summer, 
often falls so low that absorption of water by the roots 
ceases, since it has been shown that this nearly stops a 
little above the freezing point of water (see Exp. X VII). 
Exposed to cold, dry winds, the plant would then often 
be killed by complete drying if it were not for the pro- 
tection afforded by the woolly, channeled under surfaces 
of the leaves.? 
170. Operation of the Stomata. — The stomata serve to 
admit air to the interior of the leaf, and to allow moisture, 
in the form of vapor, to pass out of it. They do this not 
in a passive way, as so many mere holes in the epidermis 
might, but to a considerable extent they regulate the 
rapidity of transpiration, opening more widely in damp 
weather and closing in dry weather. The opening is 
' 1 This adaptation is sufficiently interesting for class study. 
