288 
THE LEAF 
or flaccid. The turgid guard cells leave the stomata wide 
open and allow free passage of air into the cell, and of water 
and gas out of it. Flaccid guard cells, on the other hand, 
make the opening small, decrease 
the amount of air that enters, 
and prevent undue evaporation 
from the inside of the leaf. Al¬ 
though each stoma is very small, 
they are so numerous that their 
combined action accomplishes a 
great deal. A square millimeter 
of the under surface of a lilac 
leaf contains 330 stomata, that 
of white birch, 237. 
FipuRE 266 .-—Peltate Leaf of 
Nasturtium. 
The petiole is attached at the 
middle of the back of the blade. 
The blade is orbicular, with en- 
Figure 265.— Leaves of Barberry. 
These leaves, all taken from the same tire margin. Note the twining 
bush, show the transition from normal petiole, a device for holding up 
leaves to thorns for protection. a weak stem. 
Most of the grasses show adaptations (1) in having very 
narrow leaves, fitted to grow close together; (2) in wavy 
edges if they are long, an adaptation which prevents their 
being torn by the wind; (3) in a clasping base, helping to 
strengthen the stem, and (4) in a collar /which prevents 
water from running down between the clasping base and the 
stem. 
