142 THE BEAUTIES OE NATUEE chap. 



Sycamore, space would be wasted, and it is 

 better that they should expand at once, so 

 soon as their stalks have carried them free 

 from the upper and inner leaves. 



In the Black Poplar the arrangement of 

 the leaves is again quite different. The leaf 

 stalk is flattened, so that the leaves hang 



Fig. 16. — Acer platanoides. 



vertically. In connection with this it will 

 be observed that while in most leaves the 

 upper and under surfaces are quite unlike, in 

 the Black Poplar on the contrary they are 

 very similar. The stomata or breathing holes, 

 moreover, which in the leaves of most trees 

 are confined to the under surface, are in this 

 species nearly equally numerous on both. 



