The Botany of Iceland - 01.12.1912, Side 136

122
H. JONSSON
axis dies away from below and in (his way branches, or systems
of branches, with well developed rhizoids are detached from the
parent-plant and become independent individuals (cf. Jónsson, 32).
The rliizoids of one individual hecome entangled with those of
others, and in this way the basal parts of the entire vegetation be-
come matted together. In addition to this, the lower branches are
hook-shaped, or bent downwards and outwards; thus it easily hap-
pens that they are entangled between the branches of other indi-
viduals, which further increases the matted condition. The upper
branches are directed obliquely upwards, without being entangled
with the branches of other individuals.
An A. albescens-association occurs everywhere along the coasts,
but on very exposed coasts the species generally grows dispersed.
Here it occurs in separate tufts which are, no doubt, composed of
several individuals, densely matted together below, but branching
upwards in various clusters, which are themselves densely matted
together by their hook-shaped branches; the branch-systems above
are, as usual, free. These characteristically matted branch-clusters
are probably an effect of the heavy beat of the waves; the matted
Acrosiphonia “tufts” will not he able to resist the force of the waves
and, even at an early stage, will become divided into very densely
matted branch-clusters.
Acrosiphonia albescens occurs both in the upper and the lower
litloral zone, while A. incurva belongs to the lower littoral zone,
but yet also extends upwards into the pools of the upper littoral
zone, and helow low-water mark.
Spongomorphetum. Spongomorpha vernalis grows very so-
cially, but never forms such a dense vegetation as does Acrosiphonia,
although it is sufficiently dense to characterize tlie spot. The species
has been found in only a few places; it occurs in abundance at
Grótta, where it grows both on a stony substratum and also as
epiphytic vegetation in the Corallina-Gigartina-bell and in the Polg-
siphonia urceolata-association of the lower littoral zone.
A Cladophoretum formed of Cladophora sericea f. (see under
Tide-Pools) also belongs to this association. It forms a densely
matted belt in pools high up in the upper littoral zone. As a rule
it foliows the edge of the water round the entire pool.
Cladophora rupestris ought also to be eonsidered to belong lo
the Acrosiphonia-associaúon.
An associatiou similar lo the Acrosiphonia-assoc'wÚQn is, no