The Botany of Iceland - 01.12.1912, Side 133

MARINE ALGAL VEGETATION
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The following species of Ihe under-vegetation occur in such
quanlities lliat they form associalions of considerable extent which
are usually pure, but may somelimes be mixed: —
Hildenbrandia rosea.1 Gigartina mamillosa.
Rhodochorton Rothii. Ceramium rubrum.
Sphacelaria britannica. Callithamnion Arbuscula.
Polysiphonia urceolata.
The Hildenbrandia-associaíion is most extensively distributed,
and is found everywhere along the coasts. It usually occurs as a
pure association, though, in many places, Verrucaria mucosa is inter-
mingled with it. Hildenbrandia may also occur at the same height
outside the íueus-covering, but is then — if the light is not sub-
dued by other plants or projecting rocks — usually of a darker
colour.
The R h o d o c h o r t o n - a s s o c i a t i o n ol'ten covers extensive areas,
like a dense, red carpet of felt. Most often it is pure, and appears,
as far as its littoral distribution is concerned, to be coniined inainly
to the shady parts of the littoral zone. A few of the other shade-
species may be found intermingled with it in small numbers, which
fact is of minor importance, whereas the occurrence of the epiphyte
Pleurocapsa amethijstea is of great importance, as this species is
found only on Rhodochorton Rotliii.
R. Rothii does not occur lielow the Fucus-belt of the littoral
zone unlil it occurs again at a considerable depth on Laminaria
stems; thus it is found in two distant belts: the shade-belt of the
littoral zone and the Laminaria-belt.
A Sphacelarietum (S. britannica) occurs almost quite pure
in many places over rather large surfaces; it forms a dense covering
on the rock as does R. Rothii. Sphacelaria radicans, Pohjsiphonia
nrceolata and Rhodochorton Rothii occur intermingled.
Polysiplionietum. P. urceolata occurs in a similar manner,
covering rather large surfaces of rock. In places where the species
forms extensive mats, it is usually very low in growth, yet always
considerably higher than Rhodochorton Rothii. A thin layer of tine
clay may often be seen to have accumulated between its basal parts.
A Gigartinetum occurs as under-vegetation, especially in more
exposed places, and is then generally found in the lowest part of
the Fucus-belt or in the Fucus /u/íafus-association; it is a direct
conlinuation upwards of the Giqar/iuu-association below.
1 Arranged in accordanee with the frequency, the most i'requent coming first.