The Botany of Iceland - 01.12.1928, Side 104

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JOHS. BOYE PETERSEN
solution for 5 minutes pyrenoids could be plainly seen (cf. Geitler
1922). While in M. globosa all cells show a thickening of the wall, this
is not the case in M. pyriformis, the young cells, which are spherical
or oval, having rather thin walls uniform on all sides. It is only in the
older cells, which I have repeatedly observed attached by their broad
end to the wood (Fig. 34 a), that a thickening of the wall appears at that
end of the cell which is turned away from the substratum. Reproduc-
tion seems to take place in the same way as in M. globosa. First the
cell contents divide in two, then in four (Fig. 34 i), etc., i. e. successively
in a greater number of spores; whether these are zoospores or aplano-
spores it has been impossible for me to decide. The basal, thin part
of the wall of the mother cell bursts whereby the spores are liberated.
The chief characters by which M. pyriformis differs from M. globosa are,
then, briefly the following:
1. Young cells show no thickening of the cell wall.
2. Pyrenoids are present.
3. The cells are attached.
4. The cell lumen is pear-shaped, not spherical.
It formed a green layer on a horizontal beam near the ground on
a farm in E. Iceland.
Trebouxia arboricola Puymaly. Puymaly 1924, p. 200.
Cystococcus humicola Boye Petersen 1915, p. 327, Tavle II, fig. 26.
— — Tréboux 1912, p. 69.
Chlorococcum sociabile Brand 1925, p. 331.
N. Icel. 261.
It is only in a single sample from woodwork in Isafjörður that I
think I have seen this species which is extremely widespread in Europe
on the trunks and young branches of trees, as well as on thatched
roofs and other substrata. Lichens containing this species being very
common in Iceland, it might have been expected that the algæ would
be found verv frequently free living. I have, however, sought it in vain
in localities corresponding to those in which I have often found it in
Denmark, viz. especially tree-trunks and the lenticels of young branches.
Thus I have collected samples of bark and branches of Betula from the
following localities: E. Iceland: Egilstaðir, Hallormstaðir — N. Iceland:
Háls — W. Iceland: Norðtunga, but in none of these places have I been
able to show with certainty the presence of free living Trebouxia arbori-
cola. Tréboux found (1. c.) that in Southern Russia the green coatings
of algæ on tree-trunks and woodwork consist predominantly of this
species, while Pleurococcus vulgaris Nág. (= P. Nágelii Ghod.) is predomi-
nant in Northern Russia. It would seem, then, that life conditions for
this plant are quite different in Iceland and Southern Russia, and the
effect mentioned can only be supposed to be produced bjr diíferences
in climate, while Denmark seems to occupy an intermediate position.
In Denmark Trebouxia is not uncommon on tree-trunks, but it is far
from predominant in the vegetation, and according to Puymaly similar
conditions seem to prevail in France.