The Botany of Iceland - 01.12.1930, Side 144

134
H. M0LHOLM HANSEN
in summer, though in less degree than Zone I. Hence the same
types of plants occur here.
C. Jaðar (Zone IV). As previously indicated, this zone must
be regarded as relatively warm in winter and warm in summer,
owing to the variations in the level of ground-water. Hence the
result is that the vegetation consists in marked degree of southern
types, H, Th, and E species.
D. Mýri (Zones V—VI) is warm in the winter but cold in
the summer. Hence southern piants requiring much heat (E2+1
species) and northern plants requiring much cold (A 3 species) thrive
badly or are unable to thrive here. As a matter of fact the vegeta-
tion consists of southern plants requiring little heat (E 4+3 species)
and northern plants requiring little cold (A2+1 species).
E. Flói (Zone VII). Here the vegetation is covered by so deep
a layer of water that the frost hardly reaches it in the winter.
Hence it is never exposed to the conditions required by arctic plants;
consequently these are absent at any rate in the lowlands and as
compared with the mjrri formations. In the summer, too, the vegeta-
tion is covered by water. The heat which benefits the plants on
drier soils is latent in the water here. The result is a relatively
low temperature which excludes the southern plants requiring more
lieat. Hence the species group spectrum is compressed to the central
parts of the spectrum.
Between halla mýri and fór mýri there is a peculiar difference
in regard to the species group spectrum.
As previously mentioned, the difference betwreen the halla mýri
and the fór mýri is this, that halla mýri appears where the ground
w'ater cotnes to the surface, while the fór mýri is dependent for
its moisture on the surface precipitation water. While the tem-
perature of the water is to a certain extent dependent on the tem-
perature of the air in the latter case, the temperature of the water
in the halla mýri is dependent on that of the ground-water, which
again is e(jual to the annual mean temperature of the locality in
question. In the winter there will be a constantly varying amount
of relatively warm wrater in the halla mýri to be cooled, whereas,
in the fór mýri, there w'ill be a constant amount of water to be
cooled, and the result must be that the temperature of the
h a 11 a m ý r i i n t h e w i n t e r m u s t b e h i g h e r t h a n t h a t o f t h e
fór mýri. In the summer the reverse must be the case. Tlie
heat in the halla mýri will have a constantly varying amount ot