The Botany of Iceland - 01.12.1912, Side 166

152
H. JÓNSSON
where there are openings in the Laminaria-vegetation, and beyond
the Laminaria-belt it is very frequently found on a sandy or pebbly
substratum, at any rate at inconsiderable depths. In the Laminaria-
belt proper it plays the röle of a kind of “underwood,” but beyond
the belt, at greater depths, it protrudes far above the associations
of red algæ.
Desmarestia viridis occurs in a similar manner, very often witli
the other species, both inside and outside the Laminaria-belt. It is
of less importance, however, as it is much less common. At depths
of between 20—30 metres it may also occur dominantly in patches.
In E. Iceland, at a depth of 20—30 metres, Chorda tomentosa
occurs growing very socially with tlie Desmarestia species.
Of the epiphytes on Desmarestia aculeata the small Porphyropsis
coccinea is of most importanee (in S. and SW. Iceland).
In several respects this association recalls the semi-littoral
Chorda-association.
Similar Desmarestia-vegetation occurs in the Færöes (Börge-
sen, 14), in Greenland (Rosenvinge, 63) and in northern Norway
(Foslie, 18, p. 100).
16. The Deep-water Community of Florideæ.
A mixed society, which consists mainly of a few species of red
algæ, generally occurs at a depth of about 15—40 metres (over 50
metres in E. Iceland). To judge from the dredgings, the plants some-
times seem to grow socially — at any rate over small areas — and
sometimes to growr scattered, then, as a rule, much intermingled
with one another.
The species wdiich seem to have an abundant local distribution
in fairly many places are the followring: Delesseria sinuosa at depths
of from 14 to nearly 40 metres and somewrhat deeper in E. Iceland;
Ptitota plumosa from 16 metres to about 40 metres; Odonthalia
dentata from 15—30 metres, and Polysiphonia arctica from 16—40
metres (10 to about 60 metres in E. Iceland). Thus each of these
species forms associations, but these are often of inferior extent,
with the exception, however, of that formed by the last mentioned
species.
The Polysiphonia arctica-association. Polysiphonia arc-
tica is of extremely social growrth in E. Iceland, in Seyðisfjörður
and in Reyðarfjörður. It grows most luxuriantly at depths of from
16—40 metres and forms an enormous, continuous belt along large