378 



PI. A NT STi:i>IES 



Fk;. 334. Section across a young twig of 

 bos elder, showing the four stem regions: 

 €, epidermis, represented by the heavy 

 bounding line; c, cortex: w, vascular cyl- 

 inder; p, pith. — From "Plant Relations.'' 



eter of woody stems (Fig. 

 .')o5, «'), and a possiblo 

 increase of the braneli 

 sy.stem and foliage dis- 

 jilay eacli year. 



(:)) Leaf veins foriu- 

 ing an open system (Fig. 

 333, figure to riglit). 

 The network of smaller 

 veinlets lietween the 

 larger veins is usually 

 very evident, esjieeially 

 on the under surface of 

 the leaf, suggesting the 

 name " n e t - v e i n e d ' 

 leaves, in contrast to the " parallel-veined " leaves of Mono- 

 cotyledons. The vein system ends freely in the margin of 

 tlie leaf, forming an " open venation." In consequence of 

 this, altliough the leaf 

 may remain entire, it 

 very commonly be- 

 comes toothed, lobed, 

 anil divided in various 

 ways. Two main types 

 of venation may be 

 noted, wliieli influence 

 the form of leaves. In 

 one case a siii,2;le very 

 prominent vein (///') 

 runs through the mid- 

 dle of the blade, and 

 is called tlie niidriJi. 

 From tliis all the mi- 

 nor veins arise as 

 branches ( l''ig. ■>,!(;), 

 adid such a k'af is said 



