3Y8 
PLANT STUDIES 
FIG. 334. Section across a young twig of 
box 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. 
335, w), and a possible 
increase of the branch 
system and foliage dis- 
play each year. 
(3) Leaf veins form- 
ing an open system (Fig. 
333, 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 
may remain entire, it ^^^^^^^^^^^ 
very commonly be- S&^\ ^^^/ff 
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 (Fig. 336), 
and such a leaf is said 
FIG. 335. Section across a twig of box elder 
three years old, showing three annual rings, 
or growth rings, in the vascular cylinder; the 
radiating lines (rri) 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." 
