STUDIES ON THE VEGETATION OF ICELAND 63 
The situation of these two formations in relation to each other 
affords an excellent illustration of the relation to moisture of Arcto- 
staphylos uva ursi and Calluna vulgaris. On the heath of Jutland 
we may similarly distinguish between a higher tract with Arcstosta- 
phylos and Calluna and a lower tract where Arctostaphylos is absent. 
Though the environment differs widely in the Icelandic mo and 
the heath of Jutland, it is worth noting that species which they 
have in common react similarly to the same change of environment. 
It is not the sum of environmental factors but the indi- 
vidual factors of the environment which determine the 
distribution of the species. 
The Jadar and Myri Vegetations: Fig. 11 and table 15 A—B. 
In areas where the degree of moisture is determined both by 
the precipitation and the ground water we meet with those stretches 
which are called “myrar’ in Icelandic. Owing to the abundant 
precipitation the myri is very extensively distributed throughout the 
Icelandic lowlands. Several types of myri are found. Of most com- 
mon occurrence is the »förmyri« or swampy myri, formed in 
cup-shaped depressions on level or slightly inclined ground. Its for- 
mation and peculiarities are due to the sour stagnant ground water. 
Where the soil grows very damp, that is to say, where the ground 
water covers the bottom all the year round, swampy stretches, 
“floar” are formed. The second type of myri is the »hallamyri” or 
well myri. This is formed where the ground water is pressed up 
out of the soil, hence it is often seen at the foot of mountains. 
Where the water is pressed up with such force that springs are 
formed and where the bottom is therefore swampy and damp all 
the year round the “dy” vegetation is formed. 
The third type of myri is the “fetmyri’” or irrorated myri formed 
on tracts inundated by water for shorter or longer periods of the 
year. Natural fetinyrar are thus formed on the banks of rivers but 
most extensively in deltas near the sea. The characteristic plant 
in this type of myri is Carex Lyngbyei, and since this plant is an 
important forage plant, successful damming experiments have lately 
been made with a view to creating conditions for an inundation 
myri in places where it was not formerly found. 
Three different types of vegetation are associated with these 
three types of myri. I was afforded most opportunity for a thorough- 
going study of the swampy myri which I investigated both in Lyng- 
