106 Field, Forest, and Wayside Flowers 



In tropical plants it is comparatively thick ; for 

 were it not so, the ardent sun would soon parch 

 all the juices out of the foliage. The oleander, in 

 its native soil, has to endure long droughts, and its 

 leaves are provided with skins four times as thick 

 as those of some leaves which grow in moist cli- 

 mates. But, thick or thin, the leaf-skin must not 

 keep the air away from the green cells, or the 

 little chlorophyll-grains would get no carbonic acid 

 to digest, and the luckless vegetable would die 

 of starvation. Neither must it totally check the 

 evaporation of the water which has ascended 

 from the roots. So the leaf-skin is full of pores, 

 through which air and vapor can pass freely. 

 To these openings botanists give the name of 

 "stomata" or ''mouths" (Fig. 20). They open 

 into passages which are channelled out, as it were, 

 in the fleshy part of the leaf, and their office is 

 best described by the term transpiration. They 

 enable the leaves to breathe out any moisture 

 which may be contained in them over and above 

 the plant's immediate needs. Thus the " transpira- 

 tion" of a plant -body is comparable to the per- 

 spiration of an animal body. 



During rainy or misty weather, when leaves nat- 

 urally contain more fluid than they need, these 



