84 H. MØLHOLM HANSEN 
of the knolls, especially on the side facing the sea (north), had 
sometimes been deprived of its carpet of vegetation, exposing the 
loose interior of the knoll to the desiccating and eroding action of 
the wind. Hence large areas of the valley bottom have been trans- 
formed into barren stretches of gravel covered with the melar ve- 
gatation described above. 
The number and density of species — especially the latter — 
are comparatively high in the mo both in comparison with the sur- 
rounding vegetations and with the mo vegetation in other parts of 
Iceland. The greatest density hitherto demonstrated, viz. 17.8, was 
found on the mountain slopes of Vididalsfjall. H play a very pro- 
minent part in the composition of the vegetation, while Ch have de- 
creased. The G percentage has increased while the Th percentage 
is unaltered, c. 3. The species group spectrum shows a change in 
the same direction. The A and especially the A 3 species have de- 
creased in quantity, whereas the E species, especially the lower sub- 
groups, have increased. 
Even if the chamaephytes are of subordinate importance in 
respect of species and points, species such as Dryas octopetala, 
Empetrum nigrum, Vaccinium uliginosum, and Thymus serpyllum are 
physiognomic. Of other dominant chamaephytes we may mention 
Cerastium alpinum, Silene acaulis, Armeria vulgaris, and Salix herbacea. 
Grasses and cyperaceous plants play a very prominent part, especially 
Elyna Bellardi; further Luzula spicata, L. multiflora, Juncus trifidus, 
Carex capillaris, C. rigida, and C. sparsiflora, Festuca ovina and F. 
rubra, Poa glauca and P. alpina, Agrostis canina, and Trisetum spicatum. 
Of herbaceous plants we find especially Polygonum viviparum, and 
Thalictrum alpinum, further Rumex acetosa, Tofieldia palustris, Ping- 
vicula vulgaris, and Galium Normanni; of pteridophytes Selaginella 
selaginoides and Equisetum spp., thus Equisetum arvense, E. pratense, 
and E. variegatum. The Th are Euphrasia latifolia, Rhinanthus minor, 
and Gentiana spp. 
Some floristic differences may be shown to exist between the 
different types of mo. In the low mo Deschampsia cespitosa will 
always be met with; even though the F.- percentage is not very high, 
the species is physiognomically fairly predominant. Carex sparsiflora 
is also more frequently met with in the low mo than at higher levels. 
In the mo of the valley slopes the following species not found 
at the bottom are met with. Equisetum variegatum, Rhinanthus 
minor, Habenaria viridis, Parnassia palustris, and Bartschia alpina. 
