The Botany of Iceland - 01.12.1928, Side 110

434
JOHS. BOYE PETERSEN
he called Schizogonium crispum and S. murale. The former was stated
to be recognisable by its softer, as a rule somewhat thinner fllaments,
which showed a greater tendency to form a thallus of a band-like or
flat expanded form, and by its often being furnished with rhizoids.
S.murale, on the other hand, was supposed to be recognisable by its
stifler filaments, as a rule somewhat thicker and destitute of rhizoids,
but occasionally furnished with pluricellular side-branches, and seldom
attaining a further development than the formation of tlie band-like form.
Brand, however, has shown (1. c.) that these characters do not always
occur together. Thus he finds that those Hormidium-forms that show
the greatest tendency to form a Prasiola thallus are not the thinnest
ones, but on the contrary as a rule the thickest. I have arrived at the
same result on the basis of the present material. In each of the samples
at hand I have measured the thickness of the thickest and the thinnest
Hormidium-íi\amen[ I could find and taken the average. All the averages
for the samples in which there occurred longitudinai divisions, i. e. for-
mation of Schizogonium- or Prasiola thallus, were added and divided by
the number, and the averages from the samples in which longitudinal
divisions did not occur were treated in a similar way. It then proved
that the y>Hormidium« filaments in samples without longitudinal divisions
of the filaments showed an average thickness of 12,6,«, while in the
samples with longitudinal divisions they had a thickness of 14,9,«. Nor
do I think that the presence or absence of rhizoids can be used as a
distinguishing character. In this respect tlie difference between the
various growths is more probably due to external conditions. Hence I
follow Brand and unite the two species under one.
Brand (1914, p. 305) states that he has not in any case been able
to show the presence of starch with iodine in this species. I have
treated most of my samples with chlor-zinc-iodine, and after this treat-
ment I found a distinct layer of small starch grains surrounding the
pyrenoid in almost all specimens (Cf. also Wille 1901, p. 15).
Prasiola crispa is presumably in the main a terrestrial alga but
while it can grow at considerable heights above the ground in Denmark
when the light conditions etc. are favourable, thus e. g. on tree-trunks,
it never reaches such heights in Iceland. There it is predominantly
found on the ground, but thence it spreads up the lower part of the
turf walls of the houses, especially in places where the water runs
down in rainy weather. (Cf. Helgi Jónsson 1895, p. 54, 1900, p. 58,
59). The species is also very common on bird-cliffs, a fact which is
well known from other parts too. I myself have only had the oppor-
tunity of examining the bird-cliffs of the Vestmannaeyjar.
It rarely occurs on woodwork in Iceland; I have only found it
sparingly developed on a beam in Isafjörður, and in a Pleurococcus
form on a fence pole at Möðruvellir in Kjós. In a couple of places I
found it on damp walls, and on large stones in the ravine »Stóra Gjá«
near Mývatn. Twice I found it on loose lava gravel, on the edge of an
old crater at Skútustaðir, and on the heath between the Isafjörður and
Flateyri in N. W. Iceland. Apart from these latter localities I have only
found it near human habitations or the haunts of animals (bird-cliffs,
caves in which sheep shelter), and Brand is no doubt right when he