991 



gravel. On the other hånd, it happens often enough, that the wind 

 gets hold in some break in the moss-carpet, and using this as a 

 point of attack, rolls up the carpet and carries it away, thus actu- 

 ally peeling the earth and laying it bare (see Figs. 170 and 189); 

 the whole process of colonisation must then begin anew. Tufts of 

 higher piants may also be observed which have been uprooted and 

 carried away by the wind. 



This continual struggle and the precarious conditions resulting 

 from it, must greatly limit the number of piants capable of existing 



Fig. 189. Mountain-plaleau above Velbestad on Strcimo. The surface has bccn niuch denuded leaving 



a gravelly expanse with stones scattered about. A moss-carpet can be seen to the left. 



(From photo. by F. Borgesen.) 



in such an environment, and only the hardiest and most accom- 

 modating species can succeed in the contest. 



It is necessary here, as in the case of cliff-vegetation, to di- 

 stinguish between the lithophyte formation on the surface of the solid 

 rock or the larger stones, and the vegetation which appears where 

 loose soil has gathered. Lichens cover all the rocks and constitute 

 a lithophyte formation which is most strongly developed where the 

 solid rock, as distinct from gravel, forms the plateau (compare Fig. 

 182); there is, however, no essential difTercnce from the lithophyte 

 formation already described. 



The gravel-flats are too unstable to carry any plant-growth. 

 The reproductive bodies (soredia, etc.) of lichens will try no doubt 

 to obtain a hold on the stony gravel; but every storm will set the 

 small stones rolling and the friction will rub the piants off. In 

 the case of larger stones which are not moved by the wind, plant- 



