v] ASSOCIATIONS OF ROCKS AND SCREES 143 
PETROPHYTES 
(Rock plants generally) 
ARTE SE OEMS DI Tt 
LITHOPHYTES CHOMOPHYTES 
(Plants growing 
on the bare surface re 
of rocks) EXOCHOMOPHYTES CHASMOCHOMOPHYTES 
(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. 
