The Botany of Iceland - 01.12.1912, Blaðsíða 152

138
H.JÓNSSON
ing scattered in the Corallina-Gigartina-belt, without playing any
further part in the vegetation.
The Leathesia-association is here mentioned in connection
with the Corallina-Gigartina-belt. The brown, spherical, cartilaginous
Leathesia difformis is found in abundance in this belt only, and I
prefer to mention it here rather than to place it in the Chorda-
association. The species occurs only as an epiphyte, not only on
Ahnfeltia but also on Corallina, Gigartina and Chondrus, and is often
wonderfully numerous. The size varies greatly, often the globules
mav be ralher large and, by their yellowish colour greatly alter
tlre appearance of the vegetation. It often almost entirely covers
rather extensive Corallineta. The species is a summer-form, which
does not occur in any quantity worthy of mention until the month
of June, flourishes during July and August, and disappears in Sep-
tember, at any rate in SW. Iceland.
Small specimens of Leathesia occur also on Rhodymenia palmata.
13. The Crustaceous alga-association.
At low-water inark and in the lower littoral zone, there occurs
a crustaceous algal vegetation which may be considered a direct
continuation of the sublittoral crustaceous algal vegetation (see be-
low, p. 148). Here, the species are, for the most part, the same, e. g.
IJthoderma fatiscens and Lithothamnion compactum. Both these
species often have a somewhat considerable distribution in the lower
littoral zone. The Hildenhrandia-association mentioned as existing
in the upper littoral zone also occurs here. Lithothamnion lœve and
Phymatotithon polymorphum also occur in the lower littoral zone.
In addition, we have Ralfsia deusta which, in many places, forms
patches upon the bottom of pools in the lower littoral zone, and
plays rather an important róle in the vegetation.
Tide-Pools of the Lower Littoral Zone are most fre-
quently depressions which are either directly connected witli the
part which is constantly submerged or separated from it by a ridge;
which may happen to be tlie case is uniinportant as the period of
exposure in this part of the littoral zone is extremely short, as is
also seen frorn the fact that the vegetation in these depressions
consists of sublittoral associations, or more correctly, of stragglers
from them. Thus, Laminaria and Alaria species frequently occur in