The Botany of Iceland - 01.12.1912, Side 114

100
H. JÓNSSON
brown algæ are found most abundantly, the green algæ come next,
red algæ occur less frequently, and blue-green algæ are found in
the smallest quantity. The substratum is either rock -— solid rock
or else talus of debris — or pebbles, or in many places consists
of gravel, clay or mud. The perennial species prefer almost
exclusively the rock-substratum; but they occur, though
very seldom, on other substrata, and are then, as a rule, dispersed
here and there on small stones, viz. on a gravel-clay soil at the
head of the fjords. The annual, short-lived species also occur most
frequently on a rock-substratum, but they also occur fairljr frequently
on the pebbles of the littoral zone.
1. The Prasiola stipitata-association.
This association extends furthest upwards and is found well
developed on the rocks in several places round the coasts. It is
usually almost on a level with the Verrucaria-maura belt and, up-
wards, sometimes approaches the outposts of the land-vegetation.
At the highest water-level it is covered by the sea for a short period,
or is, at least, washed by the breaking waves; but such high water
occurs only rarely, and in normal conditions this association must,
without doubt, be content with the spray from the waves during
spring and summer. On less exposed coasts this association is, as
a rule, sharply defined from the community of filiform algæ which
exists below, but on a very exposed coast the boundary is more
variable.
The dominant species in this association is the small, leaf-like
Prasiola stipitata, which grows very socially upon the tops of flat
rocks. As it has its distribution almost exclusively in this belt it
seems natural to designate the association by its name. It is essen-
tially adapted to live in the air, and is capable of withstanding
desiccation well, whicli may be perceived, inter atia, by the fact
that it does not seek clefts and crevices but grows on surfaces
which are exposed to light, wind and weather. The density of the
vegetation must also afford each individual some protection against
desiccation. In spring and summer long periods must occur during
. which this association is not wetted by the sea, and, during summer
drought, I have often seen Prasiola stipitata as dry as a bone upon
the rocks. Prasiola furfuracea also occurs side by side with this
species, but is rarer.
Species and varieties of species, such as Enteromorpha intesti-