The Botany of Iceland - 01.12.1912, Side 132

118
H. JÓNSSON
Monostroma Grevillei.
Ulothrix flacca.
Monostroma groenlandicum.
Pylaiella littoralis.
Chordaria flagelliformis.
Cladophora rupestris.
Acrosiphonia.
Enteromorpha intestinalis.
Porphyra umbilicalis.
Chætomorpha tortuosa.
The Under-vegelation. This is found commonly distributed,
and varies considerably, both as regards luxuriance and the species
composing it. The luxuriance seems to increase with Ihe degree of
exposure (except perhaps in the most exposed places), and then the
composition of the species is also changed, as species which pri-
marily belong to a lower belt extend higher up, probably on ac-
count of the frequent movements of the sea which cause the de-
siccation-period to last but a short time. The undergrowth must
be regarded as a kind of shade-vegetation; during low-tide it is
completely covered by the Fucaceœ, and at high-water the intensity
of the light is also subdued by them, as they float on, or rise with,
the water, and are moved backwards and forwards by its ripples.
The greater part of the species of the undergrowth do not occur
at the height of the Uucus-belt in places open to the light, but
occur frequently and abundantly in shaded localities, altliough these
may be found at tlie upper boundary of the Fucus-belt as, for in-
stance, in depressions in tlie talus of debris, upon the under side
of overhanging blocks of stone. This vegetation may therefore be
justly termed the shade-vegetation of the littoral zone. The species
are normally developed and cannot be compared with the shade-
forms of light-plants belonging to the land-vegetation. On the other
hand, the shade-vegetation of the littoral zone actually corresponds
with the shade-vegetation of the land, such as the fern-vegetation
and the liverwort-vegetation in the lava-fissures.
The under-vegetation belongs in part to the shade-vegetation
(see p. 123) and is divided into several associations; here it is dealt
with under the Fucus-belt as its strictly littoral distribution almost
coincides with that of the latter, and it is just as dependent on
the upper-vegetation, or even more so, as is the undergrowth in a
coppice. As regards the relation between the upper-vegetation and
the undergrowth in the Fucus-belt the main advantage is, I think
with the undergrowth; although during low-tide, the upper-vegeta-
tion may gain some advantage from the fact that the layer of
living plants under it retains more water than does the bare sur-
íace of the rock.