The Botany of Iceland - 01.12.1928, Side 36

360
JOHS. BOYE PETERSEN
a. Rockij Walls.
Dry rocks in Iceland are for ihe most part covered with lichens,
only the most prominent and thus exposed peaks are devoid of
visible vegetation. In places where water often trickles over the
rocks, either when the snow is thawing or after rain, an algal vege-
tation is met with. Where the moisture is so slight that the rock
seems quite dry in periods of desiccation, we meet with the algal
vegetation which has been called »Tintenstriche«, often mentioned
because it is so conspicuous in the landscape. In Iceland I observed
such an algal vegetation on the steep basalt wall that bounds the
Fljótsdalur (E. Iceland) on the north-west. Sample 89 is derived
from the driest locality there. The algæ formed a thin black lajær
on the rock and could only be scraped off in the shape of a powder
in which I found the following species: Calothrix parietina, Gloeo-
capsa alpina, Nostoc sp., Schizothrix Heufleri, Scytonema crustaceum,
that is to say, all Cyanophyceæ, and all with well developed sheaths,
browm or violet in colour. Under almost similar conditions I found
a growth of Stigonema minutum and S. panniforme on a wall formed
of old lava-streams in Almannagjá (see Thoroddsen 1914, íig. 9).
In somewhat moister places the algal vegetation grows richer
in species, and the individuals reach a far better development than
in the driest localities. In the summer the moisture will no doubt
be slight in periods of dry weather, and the algæ growing here may,
I think, be classed as aérial algæ. These localities correspond in
the main to Strom’s »moist rocks« (Strom 1926, p. 18). Among
these I class samples 85, 87, 99, 101, 127, 128, 129. In addition to
Cyanophyceæ, numerous Diatoms occur here. The chief Cyano-
phyceæ are Scytonema Myochrous, which may form large continuous
felted layers (87), Calothrix parietina (101, 127), besides many Chroo-
coccaceæ (Chroococcus turgidus, Gloeocapsa alpina, G.Magma, G. ru-
pestris, Gloeothece rupestris). The most characteristic Diatonis are
Caloneis fasciata, Diatomella Balfouriana, Fragilaria lœvissima, Navi-
cnla contenta, N. perpusilla, Nitzschia Denticula, and N. sinuata.
Over some parts of the rocky walls there is a continuous flow
of water, and these places answer in the main to Strom’s »inun-
dated rocks« (1926, p. 18). The algæ growing here are mostly hydro-
philous species which are submerged most of the time even though
the water layer is only tliin (samples 83, 84, 86, 88, 90, 91, 100,
102, 122). In addition to some of the species that inhabit moist
rocks such as Scytonema Myochrous, Calothrix parietina, Gloeo-