STUDIES ON THE VEGETATION OF ICELAND 127 
Possibly the depression of the Ch percentage in Zone IV may 
be referred to similar circumstances. 
If we compare the A percentage and the Ch percentage in the 
scale of moisture, both series are seen to take a corresponding 
course with a relative minimum in Zone IV. Thus the depression 
may perhaps also be due to the temperature conditions which are 
more favourable to the vegetation in this zone than in those above 
and below. 
The hemicryptophytes are peculiar to the upper, drier part 
of the scale of moisture with perhaps a slight indication of a rise 
from Zone I to Zone IV. From this zone onward there is, at any 
rate, a strong decrease. The maximum of the hemicryptophytes in 
the moderately moist area which coincides with the relative mini- 
mum of the chamaephytes is most pronounced in the highland 
tracts. The hemicryptophytes dominate more in the lowland than 
in the highland formations, and of the lowland formations those of 
the south country are more abundantly provided with H than those 
of the north country. Thus, in contrast to the chamaephytes, the 
hemicryptophytes seem to thrive best in a moderately moist envi- 
ronment of favourable temperature. This appears with even greater 
distinctness in the distribution of the species, as is sufficiently 
evident from the biological spectra in table 8 for the heath and mo 
vegetation, in contrast to the vegetation of the littoral meadows and 
the grassland. 
The therophytes play no very great part in the composition 
of the vegetation in Iceland. They occur most abundantly in the 
relatively continental parts of Iceland, the highlands and the north 
country; in the rainy south country they hardly occur at all in the 
typical formations. In the north country they occur more abun- 
dantly in the lowlands than in the highlands. 
In the scale of moisture Th attain two maxima, one in Zone I 
(melar), and one in Zone IV (jadar). In the part showing the greatest 
moisture they do not occur at all. The rise in Zone IV is of spe- 
cial interest: for the individuals it is most marked in the highland 
series, for the species it is even more marked than for the indivi- 
duals, as shown by the biological spectra of the vegetation of the 
littoral meadows and the grassland in Vestfirdir. The Th per- 
centage for these two types of vegetation which correspond to the 
jadar vegetation, is 15—16, for the heath and mo vegetation it is 
only 1—8. 
