THE BRYOPHYTA OF ICELAND 



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swamp vegetation may occur. The Rhacomitrium heaths are gra- 

 dually transformed into heather moors and birch coppices through 

 the decaying of the mosses and their forming humus. But all these 



Fig. 36. The surface of a lava block with small moss-cushions (Andrcira. 

 Grinnnict spp. and Gymnomitrium coralloides). 



formations will not be treated of more fully here as the mosses 

 occurring in them have been mentioned in previous sections. Very 

 extensively distributed and very characteristic is the Rhacomitrium- 

 canescens formation which often replaces R. hypnoides in flat depres- 

 sions where the ground is somewhat damp, and where some sand 

 has accumulated. It is often abundantly mixed with other Bryophyta 

 such as Hylocomium spp., Hypniun nncinatiim, Dicranum scoparium, 

 Polytrichum alpinum and species of Lophozia. 



