The Botany of Iceland - 01.12.1912, Síða 100

86
H. JÓNSSON
in all. Of tliese species some are common and others rare; some
are found exclusively in the upper littoral zone and otliers extend
further downwards. In order to show this more distinctly, each
group will here be dealt with separately and in detail.
Rhodophyceæ. Of the 18 species mentioned 5 are common,
and the remaining 13 rarer. The following are the commonly dis-
tributed species: Bangia fuscopurpurea, which is found exclusively
in the upper littoral zone; Porplujra umbilicalis which is found also
in the lower littoral zone; Polgsiphonia fastigiata wliich occurs ex-
clusively on Ascophyllum nodosum in the upper littoral zone; Rho-
dochorton Rothii which extends to a depth of 10 metres, and Hilden-
brandia rosea which extends to a depth of 5 metres. The three
first-named species are common in places exposed to the light in
the littoral zone; Rhodochorton and Hildenbrandia, on the other
hand, occur most frequently as an undergrowth, or in shady clefts
(Rhodochorton), and at the bottom of pools (Hildenbrandia). These
two species are met with more rarely in places exposed to the
light. The 13 species which occur in the upper littoral zone, but
which must be called somewliat rare, are there shade-loving, and
are tlien found either as an undergrowth or in shady clefts and
depressions. Some are epiphytes, and are then protected against de-
siccation by the host-plant. The greater number of these 13 species
are common in the lower littoral zone, and the 3 Ceramúim-species
which are recorded exclusively from the upper littoral zone may
doubtless be expected to occur also in the lower littoral zone.
Conchocelis does not occur in the lower littoral zone but is found
at a depth of 6—35 metres.
Phæophyceæ. Of brown algæ the Fucaceœ play the most
imporlant róle. The species are few in number, but are of social
growth, and occur in such quantities that they comprise by far tbe
greater portion of the bulk of tlie plants in the upper littoral zone.
Of the 18 species mentioned above, 12 are commonly distributed,
and 6 are more rare, in tbe upper littoral zone. Of the 12 common
species, 9 are found exclusively in the upper littoral zone, viz., 6
species of Fucaceœ, Sphacelaria britannica, Ectocarpus tomentosus
and Ralfsia clavata; Pylaiella littoralis, Elachista fucicola and Isth-
moplea, on the other hand, range to a depth of about 10 meti’es.
Of the rarer species, Petroderma is found exclusively in the upper
littoral zone, Ralfsia verrucosa is most frequent in the upper littoral
zone, but is also met witli in the lower littoral zone, as an epi-