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, hke 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. 
