118 STRUCTURE AND FUNCTIONS OF LEAVES 



which the outer parts of the leaf and the stem are at times 

 supplied. 



In many cases, noticeably in the cabbage, the epidermis is 

 covered with a waxy coating, which doubtless increases the 

 power of the leaf to retain needed moisture, and which certainly 

 prevents rain or dew from covering the leaf surfaces, especially 

 the lower surfaces, so as to hinder the operation of the stornata. 

 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. 

 The air bubbles on such leaves give them a silvery appearance 

 when held under water. 



141. 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 con- 

 siderable extent they regulate the rapidity of transpiration, open- 

 ing more widely in damp weather and in sunlight, and closing 

 in very dry weather. The opening is caused by each of the 

 guard cells bending into a more kidney-like form than usual, 

 and the closing by a diminution of twrgor and straightening out 

 of the guard cells. 



The details of the mechanical explanation of stoma move- 

 ments are complicated, and it is difficult fully to account for 

 their irritability in response to light, heat, and moisture stimuli, 

 and to variations in the amount of salts in the water absorbed 

 by the roots. 



142. Location of the stomata. The under side of the leaf, 

 free from palisade cells, abounding in intercellular spaces, and 

 pretty well protected from becoming covered with rain or dew, 

 is especially adapted for the working of the stomata, and accord- 

 ingly we usually find them in much greater numbers on the 

 lower surface. On the other hand, stomata occur only on 

 the upper surface of the leaves of pond lilies, which lie flat 

 on the water. In those leaves which stand with their edges 



