The Botany of Iceland - 01.12.1912, Side 138

124
H. JÓNSSON
sociations, and are then dominant here and there in patches, or
the species may be found intermingled with each other. Besides
these associations a Plumarietum, consisting exclusively of Pln-
maria elegans, is found in many places. This association is darker
in colour than the other red-algæ associations of shadv places; it
occurs frequently in S. and SW. Iceland. Intermixed in il occur
Callithamnion scopulorum and Chantransia virgatula.
Rhodochorton Rothii is sometimes found high up in the littoral
zone on flat rock-surfaces exposed to the light and to the beat of
the waves, but then it grows in small globular cushions (f. globosa).
Consequently, this globetum of Rhodochorton Rothii does not be-
long exclusively to the shade-vegetation.
On ílat surfaces in the shade the usual arrangement is that
the decided shade-associations occur at the bottom, where the light
is feeblest, e. g. the Sphacelarietum, Rhochortonetum, Polysiphonietum,
Plnmarietum; at the top, where the illumination is stronger, light-
loving species occur, e. g. Pylaiella, Ulothrix, or others. Near Rejr-
kjavík a vertical section of sucli a surface showed uppermost, at
the edge, Pylaiella littoralis, next Rhodochorton Rothii f. ylobosa, and
lowest of all Plumaria elegans.
In a grotto in the Vestmannaeyjar, where the illumination was
very feeble, Enteromorpha intestinalis f. micrococca occurred on the
roof, Ceramium acanthonotum grew rather liigh up on the walls,
and Plumaria elegans, together with Delesseria alata, formed a belt
lower down the walls.
A vegetation corresponding to the shade-vegetation appears to
occur in Greenland wliere Hildenbrandia rosea, Ralfsia clavata and
Verrucaria mucosa form the undergrowth in the Fucus -belt and
in other places (Rosenvinge, 63, pp. 198 and 203). Rhodochorton
Rothii and Sphacelaria britannica also appear to grow in a similar
manner in Greenland (Rosenvinge, 63, p. 205).
A comparison with the Færöes sliows some difference. The
vegetation in the grottoes in the Færöes (Börgesen, 12, p. 739),
however, resembles in its main features the Iceland shade-vegetation,
and, in addition, a similar vegetation is beyond doubt to be found
in fissures and clefts of the rocks in the Færöes, but the mode of
occurrence of the species is not the same. Thus, in the Færöes (Bör-
gesen, 12, p. 711), the Hildenbrandia-association appeai's to have a
much wider distribution upwards and to occur in fully illuminated
localities. Rhodochorton Rothii seems to occur in a similar manner