LEAVES 



189 



out not only by the framework of the water-conducting 

 vessels, but also by the turgor of its cells. 



The general shape of a leaf is largely determined by the 

 character of its vein system, and although the kinds of vein 

 systems are numerous, the three most common ones may be 

 noted. In one kind, a large main vein runs through the center 

 of the leaf, and smaller veins arise from it on either side, 

 branching in turn (Fig. 154, A). For convenience, the large 

 main veins are called ribs, and 

 the solitary central rib in the 

 case just described is called a 

 midrib. Leaves with midribs are 

 called pinnate, because the vein 

 system is like a feather, with its 

 central shaft and branches, and 

 such leaves are apt to be com- 

 paratively narrow and elongated. 

 In another kind of vein system 

 several ribs of equal prominence 

 start out at the base of the leaf 

 and diverge more or less widely, 

 each giving rise to branches (Fig. 

 154, B). Such leaves are called 

 palmate, because the ribs suggest 

 the spread fingers arising from the 



palm of the hand, and they are apt to be broad and com- 

 paratively short. In a third kind of vein system there 

 are no especially prominent ribs, but the veins run ap- 

 proximately parallel through the leaf from base to apex 

 (Fig. 154, C). These " parallel " veins are not the only 

 veins, but the branches (" veinlets ") that arise from 

 them are so small that they are not visible to ordinary 

 observation. Such leaves are said to be parallel-veined, 

 and they are always comparatively narrow and elongated, 

 as in lilies and grasses. 



FIG. 155. Portion of a skeletonized 

 leaf (Ficus), showing the network 

 of veins, and also the free end- 

 ings of veins (open system). 



