The Botany of Iceland - 01.12.1912, Side 170

156
II. JÓNSSON
and sometimes, perhaps, are in the act of forming an upper vege-
tation; sometimes they may be individuals which have “strayed”
beyond the real limits of their community.
The community is divided into various associations, according
as to whether the one or the other of the species is dominant over
considerable areas. A widely distributed and typical Phyinato-
lithon-polymorphum-association occurs, thus, in S. Iceland, and
Lithothamnion Lenormandi has a fairly social growth at Reykjavík.
L. lœve and Clathromorphum compactum also form associations in
several places. Judging from the dredgings, Lithothamnion flavescens
and L. foecundum have a more scattered growth while, on the other
hand, L. glaciale often occurs abundantly. Otherwise, it is very
common for tlie Lithothamnion species to grow intermingled with,
and at times upon, one another. The crustaceous, calcareous algæ
form the greater part of this community in Iceland. The remaining
crustaceous species, such as Peyssoncllia, Cruoria arctica and Litho-
derma fatiscens, are found more scattered, although the last-named
species forms associations in shallow water right up to the limit
of low-tide. These species must, however, be much more common
on the sea-bottom than is shown by the dredgings. It may be taken
for granted, also, that Petrocelis Hennedyi and Rhododermis parasitica
occur on a stony substratum in deeper water, seeing that they are
so common on the stems of Laminaria hyperborea at considerable
depths. Both Hildenbrandia and Petrocelis occur on a stonv sub-
stratum at shallower depths, and also Ralfsia ovala.
The species which has, with absolute certainly, been found
growing deepest is Lithothamnion lceve (see Part V), and it occurs
in masses at a depth of 88 inetres, that is, it forms a Lithotham-
nionetum at this depth. Thus, of all the marine algal communities
in Iceland, this community extends deepest.
A similar vegetation occurs in Greenland (Rosenvinge, 63,
p. 223), and in other places in the Arctic Sea (Kjellman, 36), but
in both these places it differs in the wider distribution and greater
luxuriancy of the Lithoderma /ah'scens-association. In the Færöes
(Börgesen, 12) the conditions seem to be somewhat similar, simi-
lar in any case to the conditions in S. and SW. Iceland, although
Lithoderma fatiscens seems to occur there more sparsely than in
Iceland.