The Botany of Iceland - 01.12.1912, Síða 146

132
H.JÓNSSON
10. The Community of Rhodymenia.
The species are Rhodymenia palmata and Halosaccion ramen-
taceum, both of which occur along the coasts in great abundance
as pure associations. These two associations most frequently ac-
company each other, and seem to demand somewliat similar con-
ditions of life. Halosaccion, however, extends the higher up in the
littoral zone, bul is then almost always submerged in water-filled
depressions in that zone, while Rhodymenia extends the further
down below the limit of low-tide. In Rhodymenia the form of the
thallus is leaf-like and branching; in Halosaccion it is round and
excessively or slightly branching.
The Rhodymenia-association. On regarding a steep rocky
coast, where the succession of the associations is always most easily
observed, it is seen that a luxuriant Rhodymenia -vegetation coin-
mences even at the lower boundary of the Fucns-be\t. In many
places the species occurs so socially that it predominates as a
characterizing plant over relatively large stretches, both in the lower
littoral zone and far below the limit of low-tide, to a depth of
about 12 metres. On a gently sloping rocky coast, the upper boun-
dary of the association is not so sharply defined, but if proper
allowance is made for the depressions and elevations in such a
littoral zone the boundary is fairly easily ascertained although it
may then lie in very many curves. On a gently sloping pebbly
coast, the same regular boundary is found as on a steep rocky
coast and at about the same height, although the Fucaceœ-community
is not present there.
Various algæ may occur on stones under the fronds of the
Rhodymenia, as, for instance, Hildenbrandia rosea, Ralfsia clavata,
Sphacelaria radicans and others; also a number of epiphytes, espe-
cially Myrionemaceœ and Ectocarpaceœ, often grow on old individuals
of Rhodymenia palmata.
Rhodymenia occurs also epiphytically on the stems of Laminaria
in the upper part of the Laminaria-belt.
The Rhodymenia-\egetation does not seem to be so luxuriant
in Greenland (Rosenvinge, 63) as in Iceland. Possibly this is true
also of the nortli of Norway (Foslie, 18). In the Færöes the
Rhodymenia-xegetation is luxuriant, but seems to differ somewhat
from that of Iceland, as it consists of a form with lower and more
dense growth and narrower branches; this form also extends further
up (Börgesen, 12, p. 727).