STUDIES ON THE VEGETATION OF ICELAND 85 
It is chiefly these species which cause the greater density of species 
on the valley slopes. 
Biological differences also occur. Thus the Ch percentage is 
higher in the high than in the low mo, whereas the reverse is the 
case with the H percentage. The high E percentage of locality No. 7 
is due to a deeper and more constant snow-covering in the winter. 
The Myri Vegetation. Cf. table 20 A, 1—7. 
Owing to the short time at disposal only a very few of the myri 
formations were investigated; these were a series of moist formations 
in the halla myri (1—5), and a couple of formations in the för- 
myri (6—7). 
The halla myri is only found along the foot of mountains, 
and it is essential to its occurrence that the ground-water comes to 
the surface. This causes a peculiar difference between the halla 
myri (well myri) and the förmyri (swampy myri). In the förmyri 
the amount of moisture is determined by the precipitation on and 
around the depressions in the myri; the quantity of nutrition sup- 
plied by the precipitation is comparatively small or nil, just as also 
the temperature conditions are relatively closely dependent on the 
temperature of the air. In the halla myri the nutrient salts are 
constantly renewed by the ground water, the temperature of which 
will more or less influence the temperature of the soil according 
to its amount. The temperature of the ground water is constant 
throughout the year, that is to say, it is equal to the mean annual 
temperature in the locality in question. Thus the halla myri will 
be warmer in the winter but cooler in the summer than the swampy 
myri. As a result the species group spectra differ widely. The lower 
E sub-groups (E 3 and E 2) dominate in the halla myri owing to 
the favourable temperature conditions in the winter (I) and A 2 and 
A3 species owing to the cool summer soil (!), while the A 1 species 
are peculiar to the flöi of the formyri. 
Owing to the larger amount of nutrition the number and density 
of the species is greater in the halla myri than in the förmyri, 
especially in the dampest areas. The biological spectra agree in 
regard to the preponderance of the geophytes, while there is an 
essential difference in the chamaephytes, the Ch percentage being 
highest in the förmyri. 
In the halla myri the physiognomical dominant is Equisetum 
dalustre, as in the förmyri it is Carex Goodenoughii besides Erio- 
