STUDIES ON THE VEGETATION OF ICELAND 103 
Of plants which are characteristic, that is to say, which either 
occur here exclusively or occur here with the greatest F.- percentage, 
or which are comparatively frequent on melar, we may mention 
the following chamaephytes: Thymus serpyllum, Cerastium alpinum, 
Arabis petrea, Minuartia verna, Arenaria ciliata, Saxifraga cespitosa, 
and S. oppositifolia, Dryas octopetala, Empetrum nigrum, Salix her- 
bacea, and Silene acaulis. Of hypogeophytes we mostly meet with 
grasses and cyperaceous plants. The following species occur: Poa 
glauca, Festuca ovina, Juncus trifidus, Luzula arcuata, and L. spicata, 
further Polygonum viviparum. Of species found exclusively on melar 
and thus characteristic of this type, we may mention Arabis petrea, 
Luzula arcuata, Saxifraga cespitosa and S. oppositifolia. Of thero- 
phytes only Euphrasia latifolia occurs. 
The Betula nana mo. Cf. fig. 18 and table 22 A, 7—11. 
This type of vegetation, provisionally named after its dominant 
chamaephyte, Betula nana, and referred to the mo on account of its 
comparatively close carpet of vegetation, occurs especially in large 
flat stretches among tracts of melar. The surface is not knolly as 
in the typical mo, nor does solifluction occur in any appreciable 
degree, as in melar. In small hollows in the Betula nana mo (the 
level mo), we meet with the knolly mo, which would seem to in- 
dicate a comparatively low degree of moisture in the soil of the Betula 
nana mo. Its distribution points to a snow-covering intermediate 
between the two types melar and knolly mo. 
The vegetation is continuous, a feature which renders the Betula 
nana mo physiognomically very different from melar. The number 
and density of species are, however, not very much higher than in 
melar, the number of species being 32, the density 11.3, varying 
from 10.4 to 13.0. In the biological spectrum Ch are still dominant, 
even though the Ch percentage is reduced from 52 to 47. The re- 
duction of the Ch percentage has resulted in an increase in the G 
percentage from 9.9 to 15.2. The species group spectrum shows a 
similar change; the A percentage has been reduced from 81 to 70, 
the A3 percentage from 55 to 43, while, on the other hand, the A 2 
percentage has risen. The increase of the E percentage falls prac- 
tically only to E 4, which shows a percentage of 29 against 19 in the 
melar vegetation. The E 3 group only occurs with a percentage of 0.8. 
The difference between melar and the Betula nana mo is most 
striking in a floristic respect, as even a hasty glance at table 22 A 
