XEKOPHYTE ASSOCIATIONS. 



209 



Fig. lg4. A section through a fleshy leaf (Clirda\ show, 

 ing the chlorophyll region on the outside (shaded and 

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



Op- 



tions, but a few 

 prominent illus- 

 trations vrill be 

 given. 



156. Rock as- 

 sociations. — Vari- 

 ous plants are 

 able to live upon 

 exposed rock sur- 

 faces, and there- 

 fore form distinct 

 associations of xe- 

 ropbytes. In gen- 

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

 186). The crevice plants 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. 



157. Sand associations. — In gen- 

 eral, sand associations maybe roughly 

 grouped as beach associations, dune 

 associations, and sandy field associa- 

 tions. These three hold a certain 

 definite relation to one another. 

 This natural relationship appears on 

 the borders of the large lakes, and on 

 seacoasts. The beach is nearest the 

 water, the dunes are next, and be- 

 hind them stretch the sandy fields. 

 When the three types are thus asso- 

 ciated, the plants of the different 



Pig. 185. A section through 

 a leaf of an epiphyte, 

 showing a very large de- 

 velopment of water tissue 

 between the upper epi- 

 dermis and the chloro- 

 phyll region, which is 

 restricted to near the 

 under snrf ace of the leaf. 

 —After Schimpkr. 



