142 THE BEAUTIES OF NATURE 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. 
