STUDIES ON THE VEGETATION OF ICELAND 133 
any rate the case within the sub-groups of each of the two larger 
groups, and this justifies a linear grouping like the one employed. 
If now we regard the spectra of the scale of moisture as a whole, 
as a series-spectrum, it shows a pronounced tendency to form a 
wedge downwards, produced by a decrease in quantity of the sub- 
groups of both main groups from without inwards. This wedge 
shape appears in all the series. 
The wedge shape of the series-spectrum must be put down to 
the different temperature conditions in the different sections of 
the scale. 
Thus the variations in temperature in the upper part of the 
scale will be greater than in the lower part since the specific heat 
of the water will act as a buffer here against changes of tempera- 
ture. To this must be added the fact that considerably more water 
will evaporate from the damper than from the drier areas, and 
since the temperature at which water evaporates, as well as its 
specific heat, is very high, this will in practice mean a slower and 
slighter heating of a moist than of a dry area. In the summer, 
therefore, the temperature will be lower in the former than in the 
latter area. In the winter the reverse will be the case. If, in addi- 
tion, the temperature drops below zero, considerable amounts of 
heat will be liberated within the moist areas, viz. the heat which 
has become latent by the thawing of the ice, the effect of which 
will be that a damp soil will freeze slower and not to such depths 
as a dry soil. 
In the case of moderately moist soil, one more circumstance 
must be noted. This zone is relatively dry in the summer, and 
relatively moist in the winter, which gives it more or less the 
advantages of dry soil in the summer and of wet soil in the winter. 
All in all this area will have more favourable temperature conditions 
than the areas above and below. 
The interaction of temperature conditions and vegetation in the 
various zones of moisture will thus be as follows: — 
A. Melar (Zone I) is in pronounced degree cold in the winter 
as well as warm in the summer. Hence plants which require much 
cold (Ch and A 3 species) and much heat (E 2+1 species) thrive 
well here. In contrast to the other zones of moisture it is, however, 
bare of snow in the winter. This will further encourage the A 3 
species, while the E 2+1 species will decrease in quantity. 
B. The Mo (Zones II—III) is likewise cold in winter and warm 
