XEEUl'HYTE SOCIETIES 



201 



the cell, so that when 

 the light is very intense 

 tliey move to tlie more 

 sliaded deptlis of the 

 cell, and when it be- 

 comes less intense they 

 move to tlie more exter- 

 nal regions of the cell 

 (see Fig. 182). The 

 stomata, or breathing 

 pores, wliich are devel- 

 oped in the epidernds, 

 are also great regulators 

 of transpiration, as has 

 been mentioned already 

 (see gyi)- 



11.3. Water reservoirs, 

 — In xero- 



FlG. 178. A branch of Cijtlsus, showing the 

 reduced leaves and thorny branches. — After 

 Kerner. 



phytes at- 

 t e n t i o n 

 must be 

 given not 

 01 dy 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 tliis work, 

 forming what is known as water tissue. In 

 many leaves this water tissue may Ije distin- 

 guislied from the ordinary working cells by 

 being a group of colorless cells (see Fig. 18-3). 

 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- 



^i^ 



^ 



Fig. 179. A 

 leaf of traga- 

 canth, show- 

 ing the re. 

 duced leaf- 

 lets and the 

 thorn - 1 ike 

 tip. — After 

 Keener. 



