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 °/o) 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 Empetrum and Gramineæ 
in the same association. Yet we shall see further on that both Em- 
petrum and Graminee 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. À 
I found heaths of this kind or of very much the same com- 
position on extensive tracts between the farms of Häls and Einar- 
stadir, lower down on the mountains; especially in Fljötsheidi, a 
‘locality near the latter farm, I noted down a vegetation of dwarf- 
birches (F °/o 95), i. e. frequency-number 95, Empetrum (F °/o 90), 
Glumiflore (F °/o 85), Vaccinium uliginosum (F °/o 65), Dryas (F °/o 45), 
Salix lanata (F °/o 30) and Calluna (F °/o 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 °/o 20 (10 °/o cru- 
staceous lichens, 0 °/o foliaceous lichens, 10 °/o fruticose lichens) and 
a small quantity of mosses (F °/o 5). 
The lichens in question were Alectoria ochroleuca 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 
Jökulsä, 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 °/o 100), Glumifloræ (F °/o 100), Empetrum (F °/o 95), 
EEE 
