STUDIES ON THE VEGETATION OF ICELAND 141 
Equisetum variegatum and very high for Polygonum viviparum, corre- 
sponding to a dense growth of Polygonum viviparum, a somewhat 
more scattered growth of Equisetum variegatum, and a very scattered 
growth of Poa alpina. 
C. A third series isrepresented by Cardamine pratensis, Deschampsia 
ceespitosa, and Carex capitata. These species all have their maximum 
frequency percentage in class IV; they differ from each other in the 
magnitude of the F.-percentage. For Cardamine pratensis the F 
percentage is 59, for Deschampsia cespitosa it is 41, and for Carex 
capitata 8. They also differ in the number of classes over which 
they are distributed: Cardamine pratensis occurs in all classes of 
moisture, I—VII, Deschampsia cespitosa in classes II—V, and Carex 
capitata only in class IV. 
D. Most Icelandic species show the above-mentioned regular dis- 
tribution in the scale of moisture. A small number of species deviate in 
that they have two maxima with an intermediate relative minimum. 
These species are represented by Empetrum nigrum, Vaccinium 
uliginosum, and Betula nana. In a previous section on the distribu- 
tion of the chamaephytes in the scale of moisture we attempted to 
explain these facts. The relative minimum is due to annual varia- 
tions in the water level. 
Scale of Snow-Covering. Distribution of Species, 
Species-Groups, and Life-Forms in the Same. 
For the present it is not possible to set up a scale of snow- 
covering as minutely graduated as the scale of moisture. The reason 
is that so far the depth and duration of the snow-covering have 
not been investigated. In the individual localities it is easy enough 
to observe the effect on the vegetation of the different depths of the 
snow-covering, but comparisons between the various localities are 
rendered difficult by the fact that we have no exact particulars on 
which to base a comparison between the scales of snow-covering 
of the different localities. According to Thoroddsen (1914) the 
snow lies longer and is of greater depth in the north country than 
in the south-west, and in the south country there may be no snow 
at all during a long period. In the highland tracts the depth and 
duration of the snow-covering is greater than in the lowlands. These 
facts must be taken into consideration in a comparison between 
the differences in vegetation at the various stations. 
