234 



PLANT STRUCTURES 



eter of woody stems (Fig. 

 217, w), and a possible 

 increase of the branch 

 system and foliage dis- 

 play each year. 



(3) Leaf veins form- 

 ing an open system (Fig. 

 215, figure to right). 

 The network of smaller 

 veinlets between the 

 larger veins is usually 

 very evident, especially 

 on the under surface of 

 the leaf, suggesting the 

 name "net- veined" 

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

 cotyledons. The vein system ends freely in the margin of 

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

 this, although the leaf _____ 



C 



rrj 



FIG. 216. Section across a young twig of 

 box elder, showing the four stem regions: 

 e, epidermis, represented by the heavy 

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

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



may remain entire, it 

 very commonly be- 

 comes toothed, lobed, 

 and divided in various 

 ways. Two main types 

 of venation may be 

 noted, which influence 

 the form of leaves. In 

 one case a single very 

 prominent vein (rib} 

 runs through the mid- 

 dle of the blade, and 

 is called the midrib. 

 From this all the mi- 

 nor veins arise as 

 branches (Figs. 218, 

 210), and such a leaf 



US 



FIG. 217. Section across a twig of box elder 

 three years old, showing three annual rings, 

 or growth rings, in the vascular cylinder; the 

 radiating lines (m) which cross the vascular 

 region (w) represent the pith rays, the princi- 

 pal ones extending from the pith to the cor- 

 tex (c). From " Plant Relations." 



