The Botany of Iceland - 01.12.1930, Blaðsíða 187

STUDIES ON THE VEGETATION OF ICELAND
177
jaðar, and mýri, handsomely illustrates the influence of the degree
of moisture on the temperature of the surface. — The snow-covering
is the same for the three types of vegetation, viz. the normal snow-
covering of the country.
In winter the dry soil is exposed to the hardest frost, while the
moderately moist and especially the moist soil are protected by the
moisture of the earth. In the summer, tlie moist soil cannot, on
account of its water content, attain as high a temperature as the
dry and moderately moist soils. The last-mentioned is dry in sum-
mer, like the mo.
Since the moderately moist soil has the advantages of moist
soil in the winter, and of dry soil in the summer, it must be the
most favourable of these three types for southern plants. This will
be confirmed by an examination of table 38.
Between dry and moist soils there is a peculiar difference. The
dry soil, the mo, is relatively cold in winter but dry in summer,
’while the moist soil, the mýri, is relatively warm in winter and
cold in summer. And, as a matter of fact, the result is that the
mo has more high-arctic species requiring a low temperature and
more species requiring higher temperature than the mýri. Conditions
m still moister and still drier vegetations than mýri and mo, re-
spectively, further confirm this difference.
The halla mýri is another case in point. Owing to the constanl
supply 0f ground-water this mýri becomes still warmer in winter
and still colder in summer tlian the usual lype, the fór mýri. And
the result is a further reduction of the number of species requiring
eold and, since the winter lasts longer than the summer, an in-
crease of the species requiring warmth. The effect of the cold water
m the summer on the composition of the vegetation is likewise
appreciable.
Since in Iceland it is the temperature in winter and in Den-
mark the temperature in summer which determines the formation
°I the types of vegetalion, the halla mj'ri vegetations of the two
countries form a peculiar contrast. In Denmark the halla mýri is
characterised by its high content of arctic plants, but in Iceland by
ds high content of southern plants. Even in Iceland, however, the
cold water in the summer tends to give the vegelation an artic
charaeter.
Around the hot springs the vegetation consists exclusively of
the species requiring most heat.
The Botany of' Iccland. Vol. III.
12