The Botany of Iceland - 01.12.1912, Side 148

134
H.JÓNSSON
in many places at about low-water mark, and often covers rather
large stretches of rock with a dense but, at times, low vegetation.
As previously mentioned, it grows luxuriantly on flat rocks, without
any protection whatever during the period of exposure, which is cer-
tainly short, and indeed exceedingly short during a heavy sea. Here,
as in the upper liltoral zone, the association occurs also on the face
of the rocks, and the species seems as a rule to be larger than
when on flat rocks. The association appears to have an extended
vertical distribution since, as previously mentioned, it occurs as
shade-vegetation or undergrowth in the upper littoral zone; further-
more it must be assumed that it has a Iarger sublittoral distribu-
tion than is shown by the dredgings, as it has been found outside
the 10-metre contour (see below). From what I have seen, the
stretch of sliore just above and somewhat below low-water mark
is essentially the home of this association. It usually grows on a
rocky substratum and on Laminaria stems at no great depth.
Cystoclonium purpurascens grows socially in many places
both on a rocky substratum and on Gigartina. Sometimes it occurs
in sucli abundance as an epiphytic vegetation in the Gigartina-
Corattina-belt, that it is the alga which characterizes the vegetation,
and not until the plants are moved aside is it seen tliat they are
attached to a living substratum. On Cystoclonium various epiphytes
occur, for example, Chantransia, Monostroma Grevillei and others.
Rhodomela lycopodioides very frequently grows scattered,
and then plays only an insignificant röle in the vegetation; but
Rhodomeleta of limited extent also occur, often in contact with the.
Polysiphonia urceolata-associations.
Ceramium rubrum, C. acanthonotum and Callitham-
nion Arbuscula reallj7 belong to this association. In many places
these species grow fairly socially and Ceramieta of C. rubrum and
C. acanthonotum and Callithamnioneta of limited extent oceur both
on rocky substrata and on one of Gigartina; however, I think that
these species occur too sparsely to be termed association-formers.
Sphacelaria radicans also belongs to this association. Often,
even at low-water mark, it covers flat stones with a dense vegeta-
tion, but though these small Sphacelarieta are considerably distri-
buted, yet they cannot be called associations.
In the Færöes a similar Polysiphonia urceolata-vegetation occurs
(Börgesen, 12, p. 731).