The Botany of Iceland - 01.12.1912, Blaðsíða 118

104
H.JÓNSSON
mass. Rosenvinge (63, p. 201) has pointed out that this mass
must be of importance as a reservoir for water during low-tide.
Ulothrix flacca also has very thick cell-walls, especially in the fruit-
bearing filaments (cf. Rosenvingel. c.). The remaining species also
are rather thick-walled.
Although the Ulothrix-association is distributed to an extra-
ordinary extent, yet it cannot be expected to be found everywhere.
It prefers that part of the littoral zone which is rocky and stony,
and grows luxuriantly on a somewhat exposed coast, and even,
indeed, on one which is very exposed. It also extends right into
the fjords, if the nature of the shore is favourable to it.
The succession of the associations of filiform algæ is seen most
distinctly on vertical rocks on somewhat exposed coasts, where the
Ulothrix-association appears uppermost; but where the substratum
is uneven — a talus of debris or irregularly heaped-up fragments
of rock — the zonal division of the community becomes less ap-
parent, and it may then well liappen that the Bangia-association
extends above the Ulothrix-association.
Tlie Rangia-association prefers the rocky part of the litto-
ral zone, and vertical rock-walls in particular, and is only rarely
found in the stony part of that zone; it occurs normally below the
Ulothrix-association, but where the latter is absent the Bangia-
association is not infrequently the algal vegetation which reachest
highest. In many places it has an extremely wide horizontal dis-
tribution, and is often perceived at a considerable distance as a
reddish-brown band in, and at the edge of, the water. This Bangia-
belt may attain a considerable breadth, as much as three feet, and
it often displays several shades of colour. Thus, in a less exposed
spot facing the south, I have seen the uppermost part coloured
green, the middle part brownisli and the lowest part a fresh reddisli-
brown. This is certainly connected in some way witli the long
period of drought which, when the vveather is calm or the wind
blowing ofF the land, may well continue fi’om neap-tide till about
spring-tide. During the period of drought Bangia behaves somewhat
similai’ly to Ulothrix flacca. The filaments are rather long, as long
as 10 cm., and are very closely packed together; on vertical rocks
they hang straight down during low-tide, and are, on the whole,
protected by their structure and manner of growth from too
severe desiccation, in the same way as has been mentioned in
connection with the Ulothrix-association. The Bangia-heit is, as a