The Botany of Iceland - 01.12.1930, Side 156

146
H. M0LHOLM HANSEN
There are, however, a few interesting deviations from the rule-
Thus Calluna vulgaris is of common occurrence in the mo at
Bjork. At Lýngdalur it is absent, or practically absent from the
mo, wliereas it occurs very abundantly in the snow patches. At
Thrasaborgir it occurs neither in the mo nor the geiri.
Deschampsia flexuosa occurs both in the mo and the geiri in
the south country; in the south-west, the north, and the highland
tracts it is either entirely absent or occurs only in scattered growth
in the mo, whereas it is very abundant in the geiri (and forest).
Thalictrum alpinum is a mo plant in the south country but
shuns geiri; in the south-west and north country it is still a mo
plant, but here it is also met with in the more snow-covered types
of vegetation as forest and geiri.
It seems natural to suppose that these deviations are due to
differences in temperature.
Vaccinium uliginosum is another interesting example. Its F.-
percentage varies as follows in the scale of snow-covering. In I a
(mosathembur) it has an average F.-percentage of 0.5, in I b (melar)
4. Class II, i. e. the mo, has an average F.-percentage of 54 and
cláss III 75. The species is Lhus a pronounced geiri plant though
with strong tendencies towards the mo. At Bjork it plays a promi-
nent part in the mo, especially the Arctostaphylos mo, while the
Elyna mo is less favourable. In the mo at Lýngdalur it is but
sparsely represented, while it is dominant in geiri. The same
applies at Thrasaborgir. In the highland tracts, at Lækjamót and
Norðtunga, it is not only peculiar to the areas with a deep snow-
covering, it also occurs with a high F. -percentage in the mo. On
melar it does not occur, however.
These peculiarities must no doubt be put down to differences
in the scale of snow-covering beUveen the south country on the
one hand and the rest of the country on the other. In the high-
land tracts and the north country the snow-covering is more constant
than in the south country, the mo of which is sometimes covered
with snow, sometimes bare.
Table 30 shows the distribution of the species groups
in the scale of snow-covering. The signatures are the same
as in table 29. The average values are given at the end of the
table. It appears from the table with all desirable plainness that
the A percentage is highest in the class most bare of snow, i. e. in