STUDIES ON THE VEGETATION OF ICELAND 135 
now relatively cold water to heat, while in the för myri there will 
be a constant amount of water to be heated, and the result will 
be that in the summer the temperature ofthe för myri will 
be higher than that of the halla myri. Which will be best 
for the vegetation must depend on the relative lengths of summer 
and winter. Where summer is the longer season, the result will 
be a relative cooling of the locality in question and a stronger 
cooling than that which is conditioned by stagnant water. Where 
winter is the longer season, the locality in question will offer favour- 
able temperature conditions for the vegetation, even though the 
summer, short as it is, must also exert its influence. 
The former conditions prevail in Denmark, the latter in Iceland. 
For Denmark A. Mentz (1912) has shown that the Paludella bog 
is tenanted by a series of northern-alpine species not found elsewhere 
in this country. Thus the Paludella bog offers more favourable 
growth conditions to arctic plants than other types of bogs. The 
same is the case in Iceland. In the halla myri at Lekjamot the 
quantity of A 2 species is considerably higher than in the corre- 
sponding för myri zones in the south country, while the quantity of 
A 2 + A 1 species is higher in the halla myri of the valley slopes 
than in the för myri of the valley bottom. | 
Hence the cold water peculiar to the halla myri in 
the summer has even in Iceland a noticeable influence 
on the vegetation and gives it an arctic character. 
The effect on the vegetation of the warm water in winter is, 
however, much stronger. 
If, in a för myri series, we pass from the drier to the more 
moist zones, the E 3 percentage has practically the same value 
throughout the zones until we reach the very wettest, when it shows 
a very great rise. In the halla myri series the E 3 percentage has 
its lowest value in the drier zones, whence it rises steadily until 
it attains its highest value in the dampest zone. 
In the för myri series E 2 attains its highest value in the driest 
to moderately moist zones (mo and jadar), whereas, in the dampest 
zones (myri and flôi), it has decreased much or is entirely absent. 
In the halla myri, on the other haud, the E 2 percentage rises on 
the passage from dry to moist soil; where there is the highest degree 
of moisture, the E 2 percentage is highest. 
This difference between the halla myri and the för 
mvriis most naturally explained if we assume that it is 
