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 wooUiness 

 (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, 

 trajispiration, 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 irpper 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 



