STUDIES ON THE VEGETATION OF ICELAND 131 
temperate zones, to the south of, or below, the 20 per cent Ch 
biochore. 
The A group was again divided into 3 minor groups according 
to the temperature requirements of the groups. 
The Al group requires the highest temperature and is only 
found in subarctic regions. 66° N, lat. in West Greenland was chosen 
as a practical northern limit. 
The A2 group does not require so high a temperature, yet 
it does not occur in the most pronouncedly arctic regions. The 
northern limit of the group in West Greenland lies south of 76° N. lat. 
The A3 group is of common occurrence as far north as 
northern Greenland, hence it is the group that thrives best in the 
most extreme cold. 
While cold, i. e. a low temperature, together with a varying 
amount of heat is indispensable for the A groups, heat is indispen- 
sable to the E group. In the latter group we may likewise distinguish 
a series of types according to their temperature requirements. Hence 
group E was divided into 4 minor groups of which E 1 required 
most heat, E 4 least. 
The species of the E1 group have their northern limit in 
Scandinavia that is to say, they belong to southern Scandinavia. 
The E 2 group has no northern limit in Scandinavia, but 
does not occur in Greenland. 
The E3 group is composed of Icelandic species which occur 
in Greenland, but south of 66° N. lat. 
The E 4 group occurs in Greenland north of this line. 
The distribution of the species groups in the various parts of 
Iceland as well as in the Icelandic altitudinal zones fully confirms 
the above described distribution of the groups, both as regards quantity 
and as regards mere presence. Thus the A group occurs most 
abundantly and with the greatest number of species in the north 
and in the highland tracts, whereas the E group is the dominant 
group in the lowlands and the south country. 
Of the A sub-groups A 1 prefers the lowland, A 2 the lower 
tracts of the highland, and A 3 the upper tracts of the highland. 
Of the E sub-groups E 4 is of common occurrence everywhere, 
though there is an appreciable decrease in the upper tracts of the 
highlands, E 8 occurs most frequently in the lowlands. E 2 and 
especially E 1 occurs solely, and only in scattered specimens, in the 
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