72 



ELEMENTS OF STRUCTURAL, BOTANY. 



of the leif. The leaf of the Mallow is simple ; that of 



the Clover is compound. A 



Between the simple and 



the compound form there / ^ '\j / ' 



is every possible shade of •'c:::;;^^ — .^ \ 



gradation. In the Mallow ^ -^^ i ^ 



leaf the lobes are not very ~" ^ ^ 

 clearly defined. In the 

 Maple (Fig. 108) they are 

 well-marked. In other 

 cases, again, the lobes are 

 so nearly separate, that 



Fig. 108. 



the leaves appear at first sight to be really compound. 



101. You will remember that in our examinations of 

 dicotyledonous plants, we found the leaves to be in- 

 variably net-veined. But, though they have this gener- 

 al character in common, they differ considerably in the 

 details of their veining, or venation, as it is called. 

 The two leaves employed as illustrations in the last 

 section will serve to illustrate our meaning here. In 

 the Tklallow, there are several ribs of about the same 

 size, radiating from the end of the petiole, something 

 like the spread-out fingers of a hand. The veining in 

 this case is therefore described as dirjitalc, or radiate, or 

 palmate. The leajlet of the clover, on the other hand, 

 is divided exactly in the middle by a single rib (the 

 midrib), and from this the veins are given off on each 

 side, so that the veining, on the whole, presents the 

 appearance of a feather, and is therefore described as 

 pinnate [penna, a feather). 



102. Both simple and compound leaves exhibit these 

 two modes of venation. Of simple pinnately-veined 



