XEROPHYTE SOCIETIES. 



209 



Fig. 179. A section through a fleshy leaf (C/in-ia), show- 

 ing the chlorophyll region on the outside (shaded and 

 marked as), and the large interior water-storage region 



inent illustra- 

 tions will be 

 given. 



153. Eock 

 societies. — Vari- 

 ous plants are 

 able to live up- 

 on exposed rock 

 surfaces, untl 

 therefore form 

 distinct associa- 

 tions of xero- 

 pliytes. In gen- 

 eral they are lichens, mosses, and crevice plants (see Fig. 

 181). The crevice plant.s are those 

 which send their roots into the rock 

 crevices and so gain a foothold. 

 The crevice plants also commonly 

 show a rosette habit, the rosette of 

 overlapping leaves being against the 

 rock face, and therefore in the most 

 favorable position for checking loss 

 of water. 



154. Sand societies. — In general 

 sand societies may be roughly grouped 

 as beach societies, dune societies, and 

 sandy iield societies. These three 

 hold a certain definite relation to 

 one another. This natural relation- 

 ship appears on the borders of the 

 large lakes, and on seacoasts. The 

 beach is nearest the water, the dunes 

 are next, and behind them stretch 

 the sandy fields. AVhen the three 

 types are thus associated, the plants 

 of the different areas pass gradually 



Fig. 180. A section through 

 a leaf of an epiphyte, 

 showing a very large de- 

 velopment of water tissue 

 bet^'een the upper epi- 

 dermis and the chloro- 

 phyll region, which is 

 restricted to near the 

 under surface of the leaf. 

 —After ScHiMPER. 



