The Botany of Iceland - 01.12.1928, Side 37

THE AERIAL ALGÆ OE ICELAND
361
capsa alpina, G.rupestris, some species are met with here, both
Cyanophyceæ, Chlorophyceæ, and Diatoms, which will hardly sur-
vive complete desiccation. Among the first-mentioned I shall call
attention to Anabœna Catenula, A. osciltarioides, Plectonema roseolum,
Desmonema Wrangelii. Of Chlorophyceæ several species of Desmi-
diaceæ occur, Tribonema bombgcinum, Vaucheria borealis, and Hor-
midium flaccidum. The Diatoms are often abundantly represented,
both as regards species and individuals. Often the species usually
occurring in streams are dominant. Diatomella Balfouriana, Rhopa-
lodia gibba, Diatoma hiematis, Meridion circulare, Synedra Ulna, Epi-
themia Zebra are frequently occurring species.
b. Shady Mountain Clefts.
It must be assumed that life conditions in mountain clefts
shaded e. g. by low trees differ essentially from conditions on ex-
posed rocky walls. Evaporation and, consequentty, the risk of
strong desiccation must especially be supposed to be far less, as
also the amount of light. I have therefore preferred to discuss these
localities separately though they have many species in common with
the rocky walls. The samples which I will include in this category
are Nos. 24, 25, 68, 69, 70. Of Chlorophyceæ only Desmidiaceæ
occur here, while the Cyanophyceæ are represented by Schizothrix
lardacea and Tolypothrix tenuis v. terrestris. Diatoms are numerous,
the commonest are Diatomella Balfouriana, Eunotia prœrnpta f. curta,
Navicula contenta, N. perpusilla, and Pinnularia lata v. Rabenhorstii.
c. Caves.
In the lava of Iceland there often occur caves, sometimes of
considerable size. Helgi Jónsson writes (1900, p. 93): »TrentepohIia
aurea is quite common in the dimly lighted grottos of the lava
fields; it grew there in all the small lava holes and was often the
sole plant in the grottos«. In addition he found in the lava caves
of Snæfellsnes mosses and Lepraria latebrarum. He also mentions
this Trentepohlia vegetation on other occasions, but as I have stated
elsewhere, we have no collected material of this species. I myself
have examined a couple of caves in N. Iceland, but found no algal
vegetation on the rocky walls, tliough I found some on the ground
on the íloor of the caves. In one of these caves (179), located near
Skjálvandiíljót, the sheep evidently sought shelter in bad weather, as
might be seen from their excrements i'ound in considerable amounts