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 

 cases 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 

 n e 1 1 e d-v e i n e d 

 leaves, but many 

 such facts as this 

 are familiar to 

 every botanist. 



147. Simple 

 and Compound 

 Leaves. — The 

 leaves so far studied are simple 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 



Fig. 102.— The Fall of the Horse-Chestnut Leaf. 



