The Botany of Iceland - 01.12.1928, Side 39

THE AERIAL ALGÆ OF ICELAND
363
N. peregrina v. Meniscus, and Nitzschia vitrea v. salinarum. In another
cave (402, 403) of the same kind the floor was formed of calcareous
sand consisting of fragments of mollusc shells, Bryozoa etc. on which
grew a sterile Vaucheria. The walls of the cave were covered with
a Diatom vegetation of the same character as that of tlie first-
mentioned cave. Of more or less halophilous species there occurred
liere Navicula pusilla, N.cinda and Nitzschia vitrea v. salinarum.
In a cave in the northern part of Heimaey Helgi Jónsson
found a vegetation of a land Floridea in 1897. This was described
hy Rosenvinge (1900, p. 61) under the name of Rhodochorion is-
tandicum. Later Helgi Jónsson (1911, p. 119) gave further parti-
culars of its habitat to which 1 refer the reader.
d. Loosc Stones.
On stones not connected with the firm rock, algæ are often
found, but at the outset it is probable that the algal vegetation of
sucli stones will vary greatly according to their situation.
On llie Icelandic farms there are occasionally stone fences, for
instance round the tún (homefield). On the stones of a couple of
such fences (113, 353) I have found Prasiola furfuracea together witli
a number of Diatoms, common to botli were Pinnularia borealis and
P. intermedia. In the town of Reykjavík the vegetation on the stones
was of quite a ditferent character, there were several Chlorophyceæ
and no Diatoms. In samples 270 and 275 were found Desmococcus
vulgaris, Pleurococcus vulgaris, Prasiola crispa, and Trochiscia hirta
(compare remarks above on the algal vegetation on woodwork at
Reykjavík (p. 347)).
On a large stone in the cleft »Stóra Gjá« near Mývatn there
grew mosses, Prasiola crispa, Pleurococcus vulgaris, Trochiscia hirta,
besides numerous Diatoms. About the conditions in Stora Gjá I
refer the reader to my remarks above (p. 349). A hot spring has
its outlet at the bottom of the cleft. Its vapours will no doubt keep
the air in a constant state of moisture and this in connection with
the sheltered position will favour the development of aerial algæ.
On gravel and loose stones of volcanic origin I have found algal
associations in two places where their occurrence seemed to me
very peculiar. I shall describe these occurrences in more detail
without attempting to explain them.
On the edge of a small volcanic crater (185) I found Prasiola
crispa, Diatomella Balfouriana, Hantzschia amphioxys, Pinnularia