V] ASSOCIATIONS OF ROCKS AND SCREES 143 



PETROPHYTES 



(Rock plants generally-) 



LITHOPHYTES CHOMOPHYTES 



(Plants growing 

 on the bare surface | ' 1 



of rocks) EXOCHOMOPHYTES CHASMOCHOMOPHYTE8 



(Plants growing on ( = Chasmophytes) 



the accumulated (Plants growing in 



detritus of rocks) the crevices of rocks) 



Of lithophytes (using the term in the strict sense) there 

 are probably only certain Algae, lichens, liverworts, and mosses, 

 i.e., plants which are able to absorb atmospheric moisture by 

 means of their general superficial tissues. It is doubtful if 

 those plants on rocks, even including Algae and lichens, which 

 absorb moisture by means of roots or root-like structures, 

 should be placed in a single plant formation. At all events, no 

 such "formation" is recognized in the present book, although 

 a subdivision of rock plants, like that of Ottli's, is very useful 

 from many points of view. As regards this district, it seems 

 sufficient to regard the vegetation of the limestone rocks and 

 screes as belonging to the plant formation of calcareous soils, 

 and the vegetation of the sandstone rocks and screes as be- 

 longing to the plant formation of siliceous soils except where 

 the plants occur on the acidic humus of the rock ledges (see 

 figure 19), when the vegetation would appear to be best placed 

 in the moorland formation. 



