The Botany of Iceland - 01.12.1912, Blaðsíða 119

MARINE ALGAL VEGETATION
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rule, distinctly separated from the Porp/n/ra-associalion which exists
below, but sometimes the two belts are concurrent, even over rather
large areas.
The Porphyra-association is formed by Porphyra umbili-
calis f. typica. It might be justifiable to incorporate this association
in the Bangia-association, but I prefer to reckon it as a distinct
association; partly on account of the difference in the form of its
thallus, and partly because Bangia is confined to its narrow belt,
while Porphyra occurs also in other associations in and below the
Fucus-belt.
The Porp/iyra-association occurs on a rocky substratum. On
vertical rock-faces it is very luxuriant, and then forms a continuous
belt below the Bangia-associalion. Where the littoral zone has large
fraginents of rock, that is to say consists of a talus ol' debris, this
species grows higher than any others upon the fragments at the
water’s edge. In such places no continuous belt is formed, but the
species occurs in small scattered patches on the upper part of the
blocks of stone. Tbis patchy distribution of tbe vegetation may
extend horizontally over either a small or a wide area, according
to the form of the coast. The vertical height of the belt is ahvays
inconsiderable.
During the period of drought Porphyra liangs down on the
vertical rock-faces, the thallus being repeatedly folded, according
to its length; the individuals in the upper rows of the association
often, to some extent, covering those below and thus, in some wrays,
recalling the behaviour of the above-mentioned filiform algæ; on
the blocks of stone, also, the thallus is folded up in accordance
with its length, and rests on the surface of the stone. The folding
apparently serves to reduce evaporation during low-tide, as the
evaporating surface is thereby diminished; the folding may xvell be
of importance also in counteracting the effect of the light. As the
folds of the thallus are produced while the water is subsiding, it
is probable that some of tliis is retained between them, especially
on a flat substratum; on a sloping or vertical substratum, however,
the quantity of included water would be very small. This circum-
stance is probably of some importance, and has also been empha-
sized by Börgesen (12).
The Porphyra-association seems to maintain its freshness for
a considerable time, and I saw it with its usual dark purple colour
in several places in E. Iceland in the summer; but round about