The Botany of Iceland - 01.12.1912, Page 116

102
H. JÓNSSON
2. The Community of Filiform Algæ.
It is not easy to find a suitable name for this community. It
is composed of several species which are all equally common and
are all dominant to almost the same degree. As almost all the
species are flliform and non-branching, it seems to me that the
community may fitly be named in accordance with the form of
the frond.
The community of filiform algæ forms a narrow belt, which
is often continuous along fairly considerable stretches of the coast,
at about the average limit of high water. The vertical height of
the belt is inconsiderable, about one foot, but the breadth conforms
somewhat to the slope of the coast, and may attain to 3—4 feet,
or even more. This vegetation is very well developed on the face
of vertical rocks, and the various associations of the community
can be distinctly seen, one above the other, as parallel bands of
varying colour. The species whicli occur most frequently are the
following: —
Ulothrix flacca. Bangia fuscopurpurea.
Urospora mirabilis. Porphyra umbilicalis f. typica.
Monostroma groenlandicum.
These are all dominant species, and form extensive associations,
of which some are pure and others mixed. Other species may also
occur, but only in lesser quantities.
The Ulothrix-association, as a rule, reaches highest up
the cliff. The principal species is Ulothrix flacca, which forms a
distinct belt, extending rather far in a horizontal direction. On
rock-walls, the fllaments are often comparatively long, and are
moved to and fro over the entire belt by the beat of the waves
or the ripple of the sea at flood-tide; during the period of exposure
they hang down, pressed closely against the face of the rocks. The
outer filaments protect the underlying ones from desiccation during
low-tide, and thus it happens rather frequently that the outer layer
is dry while the protected layer — that nearest to the rocks — is
moist. In this way the social growth of the plants protects them
against desiccation (cf. Rosenvinge, 63, p. 201), at any rate under
normal conditions, and so long as no exceptionally long periods
of drought occur. It happens rather frequently, however, that the
Ulothrix-xegetation becomes quite dry during low-tide. This is espe-
ciallj' the case when the vegetation occurs on boulders in the lit-
toral zone where, when the water subsides, the filaments radiate