234 



PLANT STRUCTUKES 



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, esj)ecially 

 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 



^:^,^.^\ \ iT:i :/y:T?^:-cK ^W 



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; ;?, pith.— From "Plant Relations." 



w 



may remain entire, it 

 very commonly be- 

 comes toothed, lobed, 

 and divided in various 

 ways. Two main types 

 of venation may be 

 noted, Avhich influence 

 the form of leaves. In 

 one case a single very 

 prominent vein {ril)) 

 runs through the mid- 

 dle of the blade, and 



is called the midrih. Fig. 217. section across a twig of box elder 



From this all the mi- 

 nor veins arise as 

 branches (Figs. 218, 

 219), and such a leaf 



three years old, showing three annual rings, 

 or growth rings, in the vascular cylinder; the 

 radiating lines (771) 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." 



