XEROPHYTE SOCIETIES 
201 
the cell, so that when 
the light is very intense 
they move to the more 
shaded depths of the 
cell, and when it be- 
comes less intense they 
move to the more exter- 
nal regions of the cell 
(see Fig. 182). The 
stomata, or breathing 
pores, which are devel- 
oped in the epidermis, 
are also great regulators 
of transpiration, as has 
been mentioned already 
(see 31). 
143. Water reservoirs. 
In xero- 
phytes at- 
t e n t i o n 
must be 
given not 
only to the 
regulation of transpiration, but also to the 
storage of water, as it is received at rare inter- 
vals. It is very common to find a certain re- 
gion of the plant body given over to this work, 
forming what is known as water tissue. In 
many leaves this water tissue may be distin- 
guished from the ordinary working cells by 
being a group of colorless cells (see Fig. 183). 
In plants of the drier regions leaves may 
become thick and fleshy through acting as 
water reservoirs, as in the case of the agave, 
sedums, etc. Fleshy or " succulent " leaves 
are regarded as adaptations of prime impor- 
FIG. 178. A branch of Cytisus, showing the 
reduced leaves and thorny branches. After 
KERNER. 
179. A 
leaf of traga- 
canth, show- 
ing the re- 
duced leaf- 
lets and the 
thorn-like 
tip. After 
KERNEB. 
