LICHENOLOGY OF ICELAND 199 



of leaves which cover and choke such lichen-germs as might pos- 

 sibly fall on the plant-carpet and try to hold their own there. 



The frequency-percentage (100 %>) of the dwarf-birch in this 

 association does not, as a matter of course, give us any idea of the 

 fact that it dominates the area to such a high degree and has such 

 an exclusive influence as regards the lichens. It has not for instance 

 a higher frequency-number than have the Empetrnm and Gramineie 

 in the same association. Yet we shall see further on that both Em- 

 petrnm and Graminece in purer growths i. e. not at all or only 

 slightly mixed with birches - - are far more hospitable towards the 

 lichens than is the association described here, whose want of lichens 

 must therefore undoubtedly be attributed to the presence of the 

 dwarf birch. 



In itself it is a drawback of the method in question, that this 

 quality cannot be deduced from the frequency-number - that the 

 latter expresses so imperfectly the area covered by the species pre- 

 sent; but I fear that this drawback will ultimately be found to be 

 insurmountable, whatever method should be adopted. The word- 

 description of the association must here supplement the statistical 

 figures. 



I found heaths of this kind or of very much the same com- 

 position on extensive tracts between the farms of Hals and Einar- 

 staSir, lower down on the mountains; especially in Fljotshei5i, a 

 locality near the latter farm, I noted down a vegetation of dwarf- 

 birches (F % 95), i. e. frequency-number 95, Empetrum (F % 90), 

 Glumiflorce (F % 85), Vacdmum uliginosum (F % 65), Dryas (F % 45), 

 'Salix lanata (F %> 30) and Calhuia (F % 10). The dwarf-birch was 

 consequently somewhat less frequent and a little less dominant 

 there. A few other species grew scattered in the plant-carpet, and 

 there occurred also a small quantity of lichens, F % 20 (10 /o cru- 

 staceous lichens, /o foliaceous lichens, 10 % fruticose lichens) and 

 a small quantity of mosses (F /o 5). 



The lichens in question were Alecloria ochrolenca and Tham- 

 nolia vermicularis, both podetia-wandering fruticose lichens, and a 

 few crustaceous lichens which were not determined more closely. 



In large, extensive tracts of land along the left bank of the 

 Jokulsa, and between the farms of Svinadalur and As, I observed 

 heaths somewhat more luxuriant in composition and characterized 

 by an abundant mixture of Salix lanata. The other species were 

 Betula nana (F /0 100), Glumiflora* (F /o 100), Empetrum (F , o 95), 



