206 



PLANT RELATIONS. 



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. 177). 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). 



151. 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 Figs. 178, 

 179, 180). 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. 173. A branch of Cytisus, showing the 

 reduced leaves and thorny branches. After 

 KERNER. 



174. A 

 leaf of traga- 

 canth, show- 

 ing the re- 

 duced leaf- 

 lets and the 

 thorn-like 

 tip. After 

 KERNER. 



