STUDIES ON THE VEGETATION OF ICELAND 177 
jadar, and myri, 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, the 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 myri, 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 myri. Conditions 
in still moister and still drier vegetations than myri and mo, re- 
spectively, further confirm this difference. 
The halla myri is another case in point. Owing to the constant 
supply of ground-water this myri becomes still warmer in winter 
and still colder in summer than the usual type, the för myri. And 
the result is a further reduction of the number of species requiring 
cold and, since the winter lasts longer than the summer, an in- 
crease of the species requiring warmth. The effect of the cold water 
in 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 
of the types of vegetation, the halla myri vegetations of the two 
countries form a peculiar contrast. In Denmark the halla myri is 
characterised by its high content of arctic plants, but in Iceland by 
its high content of southern plants. Even in Iceland, however, the 
cold water in the summer tends to give the vegetation an artic 
character. 
Around the hot springs the vegetation consists exclusively of 
the species requiring most heat. 
The Botany of Iceland. Vol, 111, 12 
