The Botany of Iceland - 01.12.1928, Qupperneq 38

362
JOHS. BOYE PETERSEN
on the floor of the cave. The algal vegetation here consisted of
Bumilleria exilis, BumiIIeriopsis brevis, Hormidium flaccidum, and
several Diatoms, some of the usual nitrophilous species such as
Navicula Atomus and forms of N. mutica, and a few of those usually
growing on rocks such as Fragilaria lœvissima, Melosira Boeseana,
Navicula contenta. In the other cave (184, Skútustaðir) there were
no distinct traces of sheep, but it is probable that they used this
cave as a shelter, too. In the loose soil here there occurred moss-
protonema and some Diatoms, among which forms of Navicula
mutica and Pinnularia intermedia were the commonest.
Essentially different was the vegetation in a small (basall?) cave
near the farm Mjóanes (E. Iceland). Here the water dripped con-
tinually from the roof of the cave and trickled down the walls.
Hence the algal vegetation here bore a strongly hydrophilous cha-
racter. Of green algæ I found Hormidium flaccidum, Ulothrix zonata,
and Tribonema bombycinum besides numerous Diatoms, among
which Meridion circulare was the commonest.
By the shore north-east of Húsavík I found a cave in palagonite-
breccia, evidently formed by the action of the waves (243). At the
time of hiy visit it was quile dry, both the floor and the walls
of it. Here I found Stichococcus bacillaris which usually thrives well
in a dim light, and Navicula mutica f. minima in a peculiar form
with pointed apices.
At Heimaey (Vestinannaeyjar) I had the opportunity of examining
a couple of rocky caves (399, 401, 403) in the western wall of Heima-
klettur at a height of about 20 m. They were- old caves washed
out by the waves but could not now be reached by the salt water
on account of the elevation of the land that has taken place in
recent times. (Helgi Jónsson 1911, p. 122). It is possible, however,
tliat very fine salt water spray may reach tliem in stormy weather.
The vegetation had partly a halophilous character. On the floor of
one cave was a green carpet consisting of Bhizoclonium lapponicum
and Vaucheria synandra among which there were numerous Dia-
toms. We shall mention Achnanthes delicatula, Caloneis fasciala,
Navicula peregrina v. Meniscus. On the hard rocky wall of the same
cave there was likewise an abundant vegetation of Diatoms con-
sisting partly of the usual petrophilous species such as Achnanthes
coarctata, Hantzschia amphioxys, Navicula cincta v. Heufleri, N.con-
tenta, Navicula perpusilla, Pinnularia íntermedia, and P. parva v. Lager-
stedtii f. interrupta, partly halophilous species such as Navicula cinda,