THE BRYOPHYTA OF ICELAND 623 



II. THE BRYOPHYTE VEGETATION OF MOUNTAIN HEIGHTS. 



In "Vegetal! onen paa Snaefellsnes" (Helgi Jonsson, 1900) the 

 plant formations of mountain heights are classified into three groups, 

 viz., Rocky flat, Grimmia heath and Mountain bogs. Of these the 

 Grimmia heath (Rhacomitrium heath) has already been described in 

 a previous section. 



Rocky Flat. 



By Rocky Flat Jonsson understands that part of the country 

 which is situated above the upper limit of the heather moor (and 

 the birch), with the exception of the mountain bogs and the Rha- 

 comitrium heath. Its surface varies greatly, and consists sometimes 

 of gravelly or clayey flats, sometimes of horizontal or sloping stony 

 tracts, bare rocks or talus of debris, all of which, however, have 

 one characteristic in common, viz., that higher plants do not form 

 any continuous carpet, but grow scattered singly or in small societies 

 in the most sheltered localities. The lower limit of the rocky flat 

 varies greatly according to the local conditions. It generally begins 

 at an altitude of about 300 400 metres, but often typical rocky-flat 

 formations are met with at far lower levels, in NW. Iceland, for 

 instance, as far down as to the sea-level. 



Bryophyta play a very prominent part in the vegetation of the 

 rocky flat, and are often dominant there, or form special Bryophyte 

 associations. 



The dry, gravelly flats are poor in plants, and are often quite 

 devoid of vegetation. Of mosses, as a rule, only Rhacomitrium hyp- 

 noides is met with in scattered cushions, especially around some- 

 what large stones, where it can hold fast, and find some shelter. 

 In somewhat damper and less exposed localities Rhacomitrium heaths 

 are developed as in the lava-fields of the low land, but whilst the 

 mosses in the low land gradually make room for other plant asso- 

 ciations by accumulating the blowing sand around them, and by 

 forming humus, this is not the case on the rocky flat. Here flowering 

 plants occur extremely scantily in the Rhacomitrium heath, which 

 therefore forms the final stage in the development of the plant- 

 covering, and is not replaced by other formations. 



On large stones on the rocky flat there grew jet-black cushions 

 of Andrecea petrophila, Dicranoweisia crispula, Rhacomitrium fascicu- 

 lare, R. sudeticum and Schistidium apocarpum. 



