The Botany of Iceland - 01.12.1912, Síða 137

MARINE ALGAL VEGETATION
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doubt, very common in adjacenl lands. During low-tide the upper-
most free branches appear lo be more exposed to desiccation and
possible death in Greenland than in Iceland; in the latter place it
occurs ratlier frequenlly. Both in Greenland (Rosenvinge, 63) and
in the Færöes (Börgesen, 11, 12) the Acrosiplwnia-associaiion is
well represented.
bb. The Shade-vegetation.
To this vegetation are referred certain littoral associations which
occur exclusively in shady places. The shade is produced by the
light being subdued both bv a covering of living plants and by
projecting blocks of stones. Tlius the greater part of the under-
vegetation-associations of the Fucus -belt mentioned above belong
to the shade-vegetation. In addition, the shade-vegetation grows,
as already mentioned, on the under side of overhanging rocks —
on their in-sloping sides. The vegetation is divided into several
associations which have already been mentioned in connection
with the under-vegetation of the Fucus-belt (cf. p. 118).
The lrond differs in form and structure in tlie various species
of this vegetation. It is a feature eommon to the whole of the
shade-vegetation to be low and dense in growth. The frond of
Hildenbrandia is a crust which covers the rocks; the others are
tinely branching; some, as Sphacelaria and Polysiphonia are
bush-like and are richly and finely branched; Rhodochorton lias
a similar mode of branching, but to a considerably less degree.
These three species often propagate vegetatively by means of run-
ners, whicli increases the density of the vegetation. Plumaria is
closely and distichously branched; it is tightly adpressed to the
surface of the stones during the time of low-tide and retains much
water between its branches, considered from a biological point of
view it might almost lie regarded as leaf-like during the period of
desiccation.
6. The Hildenbrandia-association belongs almost exclu-
sively to the under-vegetation of the Fucus-belt, and of the shady
localities in the littoral zone (see above, p. 118).
7. In shady places the Rhodochorton-association, the
Sphacelarietum britannici, and the Polysiphonietum ur-
ceolatæ behave in a similar manner outside the Fucus-belt as
they do within it (see above). They may occur either as pure a$-