50 H. MOLHOLM HANSEN 
and finally, $—9 are the mo vegetation at a somewhat higher level 
above the sea. 
The mo vegetation is very rich in species, in 2.5 sq. m. there 
occur on an average 35 species of phanerogams or c. 10 p.c. of all 
Icelandic phanerogams. For the typical mo on Lyngdalsheidi the 
density of species is 12—15. The biological spectrum shows that 
H forms the bulk of the vegetation, constituting 50 —60 p.c. of all 
the species noted. The Ch percentage is 25—30, the G percentage 
15—2. Th occurs very sparingly. The proportion of the two spe- 
cies groups A and E is as 1 to 1. 
As far as the floristic composition is concerned it is difficult to 
point out one or more species that are physiognomically dominant, 
and in that respect the knoll formation is most conspicuous. 
Of chamaephytes Salix herbacea and Empetrum nigrum are most 
important, species such as Thymus serpyllum, Silene acaulis, and 
Cerastium alpinum being less conspicuous. The other Salix species, 
such as S. glauca, S. lanala, and S. phylicifolia are practically of no 
consequence, and the same is the case with Vaccinium uliginosum 
— this species occurs principally in mo which adjoins geiri, as 
shown in table 11, 5. Calluna vulgaris and Arctostaphylos uva ursi 
only occur in some few specimens in the mo round Lyngdalur; 
further down, at the foot of Lyngdalsheidi, both species were phy- 
siognomically predominant in the mo, whereas they were only found 
in the geiri in Lyngdalur. Loiseleuria procumbens only occurred 
sporadically; higher up, at Thrasaborgir, it was considerably more 
frequent. Of herbaceous plants Polygonum viviparum, Galium bo- 
reale, G. Normanni and Thalictrum alpinum are most important, a 
few other species occur more sporadically, particularly Cardamine 
pratensis. 
It is, however, grasses or cyperaceous plants that dominate, 
such as Festuca rubra, F. ovina, Agrostis canina, Carex rigida, Juncus 
trifidus, Elyna Bellardi, Luzula spicata, further Deschampsia flexuosa, 
Trisetum spicatum, Anthoxanthum odoratum, and Poa glauca. Of 
vascular cryptogams Selaginella selaginoides, Equiselum pratense, and 
E. variegatum are most frequently met with, while Botrychium Lu- 
naria and a few other Equisetum species occur now and again. 
The typical mo on Lyngdalsheidi is thus characterised by a 
long series of species, each species occurring with a mean frequency 
characteristic of the species in question (mean F.-percentage) from 
which value the individual occurrences deviate but little. In table 11, 
