II. THE BRYOPHYTE COMMUNITIES. 



A in the other Arctic and Sub-arctic countries, the Bryophyta play a 

 very important part in the plant-covering of Iceland. They occur 

 either as an essential component of, so to speak, all plant associations, 

 and often in far greater numbers as regards species and individuals, 

 than do the higher plants, or as distinct Bryophyte associations from 

 which other plants are entirely absent, or in which they occur only as 

 a subordinate component. 



As a basis of classification it comes natural to distinguish 

 between (1) Lowland Formations and (2) Highland Formations. 

 Within these two main groups the formations are again arranged 

 according to their water-requirements, commencing with those which 

 are the most water-loving. Naturally, no sharp limit can be drawn 

 between these formations; on the contrary, all possible transitions 

 between them are constantly met with, which makes a comprehen- 

 sive survev extremelv difficult. In addition to this, the invesliea- 



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tions are in many respects very incomplete, especially with regard 

 to the history of development of the Bryophyte associations. Also, 

 several of the formations occur both in the Highlands and in the 

 Lowlands. This is especially the case with decidedly xerophilous 

 formations like the Rhacomitrinm heath and, partly, also other 

 moss-heaths. 



Mountain plateaus and slopes situated above the growth-limit of 

 the birch and the heather moor are in this paper considered as be- 

 longing to the Highlands (the Alpine Region). The limit in question 

 may lie at various levels and can, of course, never be drawn 

 sharply, but is in many cases dependent on the structure of the 

 mountains. The flat land along the coasts and at the bottom of 

 the vallevs rises, as a rule, in terraces inwards towards the high 



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