LEAVES 137 
narrow that no veins of any considerable length could 
exist except in a position lengthwise of the leaf. 
The fact that a certain plan of venation is found mainly 
in plants with a particular mode of germination, of stem 
structure, and of arrangement of floral parts, is but one 
of the frequent | 
eases in botany 
in which the 
structures of 
plants are corre- 
lated in a way 
which it is not. 
easy to explain. 
No one knows 
why plants with 
two cotyledons 
should have 
netted-veined 
leaves, but many 
such facts as this 
are familiar to 
every botanist. 
147. Simple 
and Compound 
Leaves. — The 
leaves so far studied are s¢mple leaves, that is, leaves of which 
the blades are more or less entirely united into one piece. 
But while in the elm the margin is cut in only a little 
way, in some maples it is deeply cut in toward the bases 
of the veins. In some leaves the gaps between the 
adjacent portions extend all the way down to the petiole 
Fic. 102. — The Fal! of the Horse-Chestnut Leaf. 
