MINUTE STRUCTURE OF LEAVES. 113 



covered with a waxy coating, which doubtless increases the 

 power of the leaf to retain needed moisture, and which cer- 

 tainly prevents rain or dew from covering the leaf-surfaces, 

 especially the lower surfaces, so as to prevent the operation 

 of the stomata. Many common plants, like 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, and the air-bubbles on the leaves 

 give them a silvery appearance when held under water. 



142. 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 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. 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, with the upper 

 surface outward, so as to give the lower surface a sort of 

 deeply-grooved form, and on this 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 stops a little above the freezing point of 

 water. Exposed to cold, dry winds, the plant would then 

 often be killed by complete drying up if it were not for the 

 protection afforded by the woolly, channeled under surfaces of 

 the leaves.^ 



143. 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 



1 This adaptation is sufficiently interesting for class study. 



