The Botany of Iceland - 01.12.1912, Síða 147

MARINE ALGAL VEGETATION
133
The Halosaccion-association also commences below the
lower boundar}r of the Fucus-belt, and extends from there to a
depth of about 5 metres below low-water mark. The species grows
very socialljr, entirely covering the rockv substratum upon which
it grows; it is very variable, and, while f. densa seems to extend
rather high up, in pools in the littoral zone, it is f. robusta which,
as a rule, reaches to the greatest depth. F. subsimplex is very com-
mon just below the Fucus-belt, where it grows so densely that it
covers the substratuin completely. It is very often interwoven witli
byssal-threads of Mytilus edutis. In this form, as in the laid-bare f.
robusta, inflated shoots occur fairly frequently. As is the case with
Rhodymenia palmata, this species is very frequently decoloured in
the lower littoral zone; yet in both species the lower part of the
frond is, as a rule, reddish in colour.
Of the epiphjdes, Elachista fucicola is especially frequent, and
various other species may also occur closely applied to Halosaccion,
such as Porphijra miniata, Monostroma fnscum, M. Grevillei and others.
At times the Monostroma species may be so abundant that they
may be said to play a distinct rðle as epiphytic vegetation. Among
the Halosaccion individuals, Dumontia fdiformis, Chœtomorpha Mela-
gonium and others often occur growing scattered at the very limit
of low-tide.
A similar Halosaccion-vegetation occurs in northern Nor-
way (Foslie, 18), and probably on the arctic coasts as well (Kjell-
man, 36; Rosenvinge, 63). In the Færöes the association is
poorly represented (Börgesen, 12, 13).
11. The Polysiphonia urceolata-association.
At and about low-water mark in the lower littoral zone, an
association is found consisting principally of excessivel}r branching,
red algæ whicli I will name after the species Polgsiphonia urceolata,
which is dominant in tlie association. This vegetation frequently
occurs on a flat or convex substratum of rock (lava-substratum)
and often forms, especially in SW. Iceland, for example at Grotta
near Reykjavík, a broad belt around Corallina-pools, near low-
water mark.
The species which occur in greatest abundance in tliis asso-
ciation are Polysiphonia urceolala, Cystoclonium purpurascens and
Rhodomela lycopodioides.
The Polysiphonia urceolata-association is very luxuriant