158 FOITNDATIONS 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 wooUi- 

 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. XVII). 

 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. 



