The Botany of Iceland - 01.12.1912, Side 117

MARINE ALGAL VEGETATION
103
from the highest point of the stone. Such a stone-surface is smooth
and dries more quickly and completely tlian the uneven face of
the rock. Tliis vegetation appears to maintain itself well in spite
of being completely dried up, day after day, during low-tide, —
during the nocturnal Iow-tides, naturally, it is dried up to a mucli
lesser extent. As a rule, such a dried-up Ulothrix-xegeiaúon is so
tightly adpressed to the surface of the stone that it can only be
removed by being scraped away with a knife. Thus, owing to de-
siccation, it appears, from a biological point of view, to have as-
sumed a crustaceous form, which evidently diminishes the evapora-
tion from its surface.
The otlier species which occur in great quantities in this as-
sociation, sucli as Urospora mirabilis and Monostroma groenlandi-
cnm, act biologically in a similar manner to Ulothrix flacca. Both
these species are found fairly frequently growing among Utothrix
flacca; and as I think that these species may in several respects
be comprehended in one biological unity, I consider them members
of the same association, although tliey both occur in pure growths
(Urosporetum, Monostrometum).
While Ulothrix flacca and Urospora mirabilis are commonly
distributed, the distribution of Monostroma groenlandicum is more
restricted, for this species can be reckoned as commonly distributed
only in E. Iceland.
Of tlie species which are rare or of Iocal occurrence, and which
are reckoned in this association, the following maj^ be mentioned:
Ulothrix pseudoflacca, Ulothrix consociata var. islandica and Urospora
Hartzii. In addition, Enteromorpha intestinalis f. micrococca and
Rhizoclonium occur here as they do in the Prasio/a-community.
Of these species Ulothrix consociata var. islandica requires to
be described most fully. It is very social, and grows in cushion-
like masses liigher up, as a rule, than the otlier species of the as-
sociation. It appears to be protected from desiccation both by its
manner of growth and by the thickness of its cell-walls.
Codiotum gregarium I have found to be of very social growth,
covering comparatively large stones as pure Codioleta. It is most
nearly related to this association.
The structure of the frond in this association displays various
peculiarities, which must be considered as being beneficial to the
plant during tlie period of desiccation; for instance, the interior
of the frond of Monostroma groenlandicum is filled with a gelatinous