The Botany of Iceland - 01.12.1912, Side 115

MARINE ALGALVEGETATION
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nalis f. micrococca and Rhizocloninm riparium, which essentially be-
long to the lower belts, but can accommodate themselves to life in
the air, are also included as members of this association. They are
species which, owing to their structure or their manner of growth,
are capable of enduring desiccation. Enteromorpha intestinalis f.
micrococca has, as is well known, small cells, the walls of w'hich
are very thick, the inner walls, in particular, being highly thickened,
serving possibly as reservoirs for water. It prefers fissures in the
rocks, w'here, as a rule, it is less exposed than the species charac-
teiástic of the association. Sometimes, how'ever, I have come across
f. micrococca grow'ing, like Prasiota stipitata, on flat rocks manured
by birds, but then it had a different aspect and, at first sight, some-
what resembled Prasiola. Rhizoctonium also occurs in fissures, where,
owing to its manner of growth — that is to say its pulvinate form
— it is protected from desiccation during the long periods of drought.
Catothrix scopulorum also occurs as a memher of the Prasiola-
association and forms Calothriceta of limited dimensions; the indi-
viduals are procumbent and are placed so closely togetlier that tlie
rock is completely covered, and thus they protect each other from
desiccation. From a biological point of view, the manner of growth
of this species, in the dry condition, is similar to that of the cru-
staceous algæ. Enteromorpha intestinalis f. minima also occurs in
this coinmunity.
The species in this community grow, as a rule, in small, pure
societies which form a nairow, though not a continuous belt along
the coast. This belt is situated higher on exposed coasts than it
is on tliose which are less exposed.
The Prasiola-association is undoubtedlv commonly distributed
in neighbouring’ countries, but the constituting species may be dif-
ferent. In Greenland (Rosenvinge, 63, p. 200) Calothrix scopulorum,
Ectocarpus maritimus, and Rhizoclonium riparium occur only in the
uppermost part of the littoral zone. This vegetation, howæver, scarcely
corresponds with tlie Prasiola -association, but rather W'ith tliat
occurring below. In the Færöes, how'ever, an exactly corresponding
association is found, which Börgesen calls the Chlorophijceœ-
formation (12, p. 712). The Færöese Chlorophijceœ-formation, how-
ever, appears to be more luxuriant and is composed, in part, of
other species. A Prasiola-associaúon (Foslie, 18, p. 127) similar to
that of Iceland is evidently found in Finmark.