150 H. MØLHOLM HANSEN 
the same if the species numbers alone are employed instead of the 
frequency numbers. 
The occurrence and quantitative distribution of the species 
groups in the Icelandic types of vegetation is determined throughout 
by the temperature conditions prevalent in the locality. The pro- 
longed low temperature prevalent in the highlands and 
the north country but especially in localilies where the 
snow is blown away in the winter, promotes the growth 
of northern but restricts the growth of southern species, 
while a prolonged high temperature, as it occurs in the 
lowlands and the south but especially wherever the 
ground is covered witha deep and constant layer of snow, 
restricts the growth of northern but promotes the growth 
of southern species. This is abundantly confirmed by 
the vegetation around the hot springs. 
We are thus fully justified in regarding the species 
groups asi indicators of: environment, and. the specnes 
group spectra will then prove an important guide in a 
more precise analysis of environment. Under the treatment 
of the distribution of the species groups in the Icelandic scale of 
moisture, the spectra furnished important holds for an examination 
of the physical conditions. The legitimacy of the above-stated con- 
siderations is further confirmed by the distribution of the species 
groups in the scale of snow-covering, in the types of vegetation of 
the different parts of the country and the altitudinal zones, and by 
the vegetation around the hot springs. 
In two areas, partly in the geiri and partly in the flöi, on soil 
covered respectively with snow and with water, the E species are 
unusually abundant. In both places the winter temperature must be 
supposed to be almost the same, at or below zero. When the snow 
has melted in the geiri, the heat that is left will directly benefit the 
plants. In the flöi, on the other hand, a great deal of the heat is 
latent in the water which still covers the vegetation, consequently 
the result will be a relatively low summer temperature. The flöi 
is thus warm in the winter but cold in the summer, while 
the geiri is warm both in the winter and the summer. 
The result will be that in both places the species group spectrum 
is characterised by a high E percentage; on the water-covered soil 
it is 72, on the snow-covered soil 66. The difference in the summer 
