TRANSPIRATION 
283 
3. The stomata contain guard cells which regulate the 
size of the opening by absorbing moisture or losing it. When 
the guard cells are full of water, they are plump or turgid, 
leaving the stoma wide open. When they are soft or 
flaccid from lack of water, they collapse, partially closing 
the stoma. 
4. In addition to this regulating device the stomata may 
be under coverings of hair or wax to make evaporation less 
rapid, or they may be at the bottom of a very thick layer of 
cutin. 
Figure 263 . — Bit of Epidermis 
of Leaf. 
The dark structures are stomata. 
An area as large as a period on 
this page contains hundreds of 
stomata. 
5. A leaf may still further 
check transpiration by changing 
its position or by rolling its 
edges together. Corn leaves 
during a very dry period illus¬ 
trate this. 
6. Another device is seen in 
cactus plants which have a 
greatly reduced surface. The 
stem in these plants is green 
and it contains stomata, thus 
enabling it to do the work of 
leaves. 
7. The shedding of leaves in the fall is a device used by 
deciduous (de-sid'u-us: Latin, deciduus, falling off) trees to 
prevent undue transpiration. When the ground is frozen 
the roots are unable to absorb much water. If the leaves 
remained on the tree, water would be given off more rapidly 
than it could be gathered from the soil, resulting in damage 
to the plant. 
When the moisture in the air exceeds a certain quantity, 
evaporation does not take place readily, causing too much 
water to accumulate in the plant. To prevent damage 
from this condition, plants have modified stomata, called 
water pores, at the ends of the veins. These contain cells 
