The Botany of Iceland - 01.12.1912, Side 172

158
H. JÓNSSON
nities occur intermingled in the Zosfera-belt; these tlien grow in
small depressions, where the subsoil, usually a solid, clayey and
pebbly substratum, appears. Such species are: Chorda Filum, Chor-
daria flagelliformis, Castagnea virescens, Dictgosiphon foenicnlaceus,
Pylaiella littoralis, Ceramium rubrum, Cystoctonium purpurascens and
others. They should not be classed in the Zosíera-association, and
are mentioned only to explain the appearance of the Zosfera-meadow.
The Zos/era-association is sublittoral, but hardly extends as far
down as do the semi-littoral communities. It cannot endure pro-
tracted exposure, and thus a substratum which would adapt itself
well to Zostera may be found completely devoid of plants in places
which are exposed for a long time during low-tide. From time to
time, during extreme ebb-tides, the upper part ofthe Zosfera-meadow
may, however, be seen quite dry. But here two points have to be
taken into consideration: the íirst being that the period of exposure
is extremely short, and the second being that the extreme ebb-tides
occur so seldom that they ought not to be taken into calculation.
As a rule, the Zostera substratum is always submerged during low-
tide. The water is so low, liowever, that the leaves float on the
surface of the water, giving it a greenish tinge. When wading in
a Zostera-meadow during low-tide the water reaches to about the
knees.
In SW. Iceland the time of fruiting is during August—October.
On Zosterci-soil there is in most places a very rich animal-life,
but whether this has any iníluence upon the vegetation or on the
nutrient substratum needs further investigation.
In connection with the Zosfero-association I will just mention
the Brackislr-water-vegetation. It is so little known that there
is nothing to be said about it, except that I have found Ruppia
maritima in one solitary spot, wliere it grew so luxuriantly that,
although scattered, it characterized the bottom.