36 
PLANT STUDIES 
framework of veins is variously arranged. The large veins 
which enter the blade send off smaller branches, and these 
send off still smaller ones, until the smallest veinlets are 
invisible, and the 
framework is a 
close network of 
branching veins. 
This is plainly 
shown by a "skel- 
eton" leaf, one 
which has been so 
treated that all 
the green sub- 
stance has disap- 
peared, and only 
the network of 
veins remains. It 
will be noticed 
that in some 
leaves the veins 
and veinlets are 
very prominent, 
in others only 
the main veins 
are prominent, 
while in some it 
is hard to detect 
any veins (see 
Figs. 25, 2G). 
29. Significance 
of baf veins. It 
is clear that the 
framework of veins is doing at least two things for the 
blade: (1) it mechanically supports the spread out green sub- 
stance ; and (2) it conducts material to and from the green 
substance. So complete is the network of veins that this 
FIG. 26. A leaf of hawthorn, showing a short petiole, and 
a broad toothed blade with a conspicuous network of 
veins. Note the relation between the veins end the 
teeth. After STRASBURGER. 
