The Botany of Iceland - 01.12.1912, Side 169

MARINE ALGAL VEGETATION
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forms of Æcjagropila, and in my opinion they are outgrowths upon
old “blocks” of calcareous algæ. The action of the current probably
loosens them, and they then roll about on the bottom. Undoubtedly
they can live fairly long in that condition, but if they roll about
much, they will surely by degrees go to pieces.
This association recalls the semi-littoral Corallina-community,
particularly the Corallina-associaúon.
The Lithothamnion-associaúon occurs in Greenland (Rosen-
vinge, 63) but not at the Færöes (Börgesen, 12).
18. The Community of Crustaceous Algæ.
The characteristic life-form of the crustaceous algæ which is
so essentially different from that of tlie rest of the marine algæ
seems to justify the idea that they all belong to one community.
With all of them the thallus is flat and, like the crustaceous lichens
on the rocks, adheres by llie whole of its lower surface to the sub-
stratum. As the form and the manner of development of the thallus
in the different species are identical in their main features, I think
that the community may appropriateljr be named after the crusta-
ceous growth.
The subslratum of the community consists of rocks, pebbles,
mussel-shells and the like; also of other algæ, especially species of
Laminaria.
The community lias a very large distribution both in a hori-
zontal and in a vertical direction, and possibly it is more particu-
larly members of this community which we may expect to find in
the vicinity of the absolute depth-limit of growth of algal vegeta-
tion. The community lias already been mentioned as the under-
vegetation in the Laminariaceœ-community; it occurs also as an
undergrowth in communities which extend to a greater depth and
thus, partly as a dominant growtli on the bottom and partly as
undergrowth, it reaches from the great depths right up to the limit
of low-tide. The semi-littoral and the littoral crustaceous alga asso-
ciations should also be regarded as part of this community although,
for practical reasons, they have been dealt with earlier in this paper.
The community is pure, tliat is to say it is composed of only cru-
staceous algæ; there occur, it is true, various intermingled species, of
which the majority are Florideœ, though some are Phœophijceæ, but these
I consider unessential and almost irrelevant to the crustaceous alga-
community proper. Thejr liave their homes in other communities,