The Botany of Iceland - 01.12.1912, Blaðsíða 135

MARINE ALGAL VEGETATION
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metum of Monostroma groenlandicnm was here found in abundance,
often entirely covering the boulders and occurring on them, like
U. flacca, closely pressed to the stone-surface with a radiating ar-
rangement. In addition, an abundance of E. intestinalis f. compressa
and a few Ulothrix ftacca were found. A little lower down, a Uro-
sporetum (U. Wormskioldii) occurred, whieh can scarcely be separated
from this association; it seemed to form a connecting link with
the semi-Iittoral communities.
Cladophora gracilis occurred intermingled in the E. clathrata-
association, yet without playing anv important rðle in the vegetation.
A similar association, especially an Enteromorphetum of E. in-
lestinalis, is, beyond doubt, very common in other countries. It
seems to be more poorly developed in Greenland (Rosenvinge,
63, p. 205) than in Iceland; in the Færöes (Börgesen, 12, pp. 714,
715), on the other liand, the Enteromorpha m/es/ina/is-vegetation seems
lo be more luxuriant than in Iceland.
5. The Acrosiphonia-association.
This association consists of decidedly filiform, much branched
algæ, which grow very socially. During the period of desiccation
they retain the water as a sponge retains it. This circumstance is
mentioned with regard to some of these species hy Rosenvinge
(63, p. 202), who says of Cladophora arcta (= Acrosiphonia incurva,
cf. Jónsson, 32, p. 43) that in Greenland it behaves during low-
tide like a sponge saturated with water.1
The principal species in this association are Acrosiphonia alhe-
scens and A. incurva. They form individually, pure Acrosiphonieta
in the upper and lower littoral zones, where they cover flat stones
with a densely matted vegetation. On flat rocks the association may
be somewhat widely distributed, in otlier places it has more the
character of scattered Acrosiphoniela. The density of llie vegetation
is due to the structure of these species. As the basal part of the
prineipal axis is too weak to carry the plant when its branch-
system has developed, lateral rhizoids, which slope downwards, are
developed at an early stage from the principal axis itself and from
the lower branches. These rhizoids often form creeping filaments .
from which arise erect shoots, which can be detached from the
parent-plant and thus become independent individuals. The principal
1 Possibly this is the case in an even higher degree with Callithamnion Ar-
buscula (cf. Börgesen, 12, p. 726).