240 FORMATIONS ON ROCKS SECT, vui 



gradual intermediate stages between the two present themselves. For 

 the oecologist a rocky and mountainous piece of country is the most 

 difficult of all objects to investigate. 



In this Section we deal solely with vegetation on true rock, not with 

 vegetation on the loose soil covering rock, even though this may entertain 

 species that are very intimately associated with the rock. Still, to this 

 limitation an exception must be made in favour of the vegetation growing 

 in clefts and niches, because in such habitats it is characteristic alike in 

 species and in growth-forms. 



We shall therefore distinguish two formations : 



1. Lithophytes. 



2. Chasmophytes 



in accordance with the suggestion made by Schimper, 1 who wrote : 

 ' The vegetation on the surface of rocks or stones may be termed that of 

 lithophytes. Crevices in rocks, in which more finely grained components 

 and more water accumulate than on the surface, produce a somewhat 

 more copious vegetation, that of chasmophytes.' 



Ottli 2 defines as ' rock-plants (petrophytes) ' ' all those plants growing on sides 

 of rock or blocks which are able, as the first of their kind, to colonize the rock 

 permanently, and which display in distribution or structure a more or less pro- 

 nounced dependence upon rock as a substratum. Within this definition are 

 included both lithophytes and chasmophytes. But it is not a natural scheme to 

 co-ordinate chasmophytes and lithophytes ; in opposition to the latter should be 

 placed those Cryptogamia and Phanerogamia which only colonize rock where 

 detritus has accumulated, whether this be in crevices or on the general surface 

 of the rock. Plants of this latter type we term chomophytes (TO x&pa, an aggre- 

 gation) '. He suggests the subjoined scheme : 



Petrophytes = Rock-plants 



Lithophytes Chomophytes 



Exochomophytes Chasmochomophytes 



(Surface-plants). [= Chasmophytes] 



(Crevice-plants). 



The ' exochomophytes ' are merely immigrants from other formations such as 

 sward, meadow, bush, heath (callunetum), forest, and the like. 



Following our account of Lithophytes and Chasmophytes we shall 

 consider in Chapter LXIV the closely allied formations on Shingle and 

 Rubble. 



CHAPTER LXII. LITHOPHYTES 



LITHOPHYTES are those plants that can colonize steeply inclined and 

 bare rock. They are exclusively cryptogamic, and include algae, lichens, 

 and mosses. Among these, algae and lichens are perhaps in general 

 the first colonists, and are the growth-forms that can settle upon the 

 steepest rocks. 



Algae may give the colour even to vertical rocks over large tracts ; 



1 Schimper, 1898 ; Eng. Edit. p. 178. * Ottli, 1903. 



