LICHENOLOGY OF ICELAND 203 



I shall describe one more specimen of a Dryas-heaili which I 

 investigated near EinarstaQir. The ground was slightly inclined and 

 partially bared in many places. The plant-covering was 8 10 cm. 

 high and consisted of Dryas (F % 100), Empelrum (F /o 100), grasses 

 (F /o 100), dwarf-birch (F % 64), Azalea procumbens (F % 24), Vac- 

 cinnm uliginosum (F % 16), Polygonum inmpamm (F % 12) and Tha- 

 lictrnm alpiiinm (F % 4). In this low-growing, open vegetation a 

 quantity of lichens was growing (F ,o 100), frulicose and foliaceous 

 lichens and Lecanora tartarea. The species were: 



Alectoria ochroleuca (fruticose lichen). 



nigricans 



Cetraria nivalis (foliaceous lichen), 

 aculeata (fruticose lichen). 

 Thamnolia vermicularis (fruticose lichen). 

 Lecanora tartarea (crustaceous lichen). 



The types of heath described above are characterized by their 

 level, partially sloping substratum, their open and low-growing ve- 

 getation, and chamsephytes and hemicryptophytes with slight leaf- 

 fall which dominate, both physiognomically and ecologically. Con- 

 sequently, the conditions are favourable to the lichens, and their 

 frequency-percentage is everywhere 100 or thereabout, sometimes 

 crustaceous lichens (mostly Lecanora tartarea}, sometimes fruticose 

 lichens dominating. 



Type II. Dry, knolly heaths with phanerogams on the hori- 

 zontal surface of the knolls, lichens on the sides of the knolls, and 

 mosses, etc., in the narrow depressions or ruts between the knolls. 



A third type of heath which is common in Iceland is the 

 Knolly heath; it has fewer lichens than has the low-lying, level 

 Dryas-Empetrum-grass-heaih. 



I noted some examples of this type of heath from different 

 areas in North Iceland between Einarsta9ir (in ASalreykjadalur) and 

 Myvatn, on Reykjahei5i (south of Axarfjor5ur, between the Jokulsa 

 and the Laxa), along the left bank of the Laxa (which runs out 

 into Skjalfandi) and in a few other places. 



As already mentioned it is peculiar to these heaths that the 

 ground is very knolly, i. e. it consists of mounds with deep inter- 

 vening depressions. The heaths appear usually or perhaps exclu- 

 sively to develop on level (not sloping) ground. 



Between Einarstadir and Myvatn (in the valley of the Laxa) 

 heaths were found composed of Empetrum (F % 100), grasses (F % 

 100), dwarf-birch (F % 80), and a few other phanerogams with a 



