The Botany of Iceland - 01.12.1928, Side 46

370
JOHS. BOYE PETERSEN
In the succeeding part, I liave as a rule, under the individual
species, stated in how many samples from fresh water 0strup
found the species in Iceland (östrup 1918). A comparison of these
figures with the number of samples in which I have found the
species will furnish an approximate numerical basis from whicli
we can find out whether the species is chiefly terrestrial or aquatic.
It should be noted, liowever, that since 0strup has examined abt.
500 samples from fresh water, while I have examined abt. 100 from
land, 0strup’s figures should be divided by 5 before comparing.
CENTRICÆ.
Melosira crenulata (Ehrb.) Kutz. van Goor 1924, p. 337.
E. Icel. 4, 7, 24, 77, 113 — N. Icel. L. 169, 256 — W. Icel. 321 -
S. Icel. 295, 372.
I take this species here within the same limits as van Goor 1. c.
The majority of the forms I have found correspond to M. italica, only
in one sample did I fmd frustules of M. crenulata (sensu O. Muller 1904,
p. 264). It is a peculiar fact that only short cells have been observed
in the terrestrial samples. As regards the thickness of the filaments, the
speciraens found corresponded to M. crenulata, M. italica, M. italica tenuis,
and M. italica tenuissima. Only in one sample were living cells ascertained
with certainty, in the rest all cells seemed to be dead. Nevertlieless I
include the species here for the sake of completeness, since the frustules
occur in several samples, and since I believe that a few specimens might
perhaps be able to survive in the earth and reproduce themselves, should
favourable conditions again occur, even though the majority of indivi-
duals die away in periods of desiccation. The species is of common
occurrence and probably lives chiefly in the plankton of lakes and ponds,
whence it passes into the mud of the bottom, while numerous other
specimens are washed up on to the banks. Hence we may expect to
find it on areas near the shores of lakes which are flooded at certain
seasons, and it is presumably from such localities that it is often brought
by human agency on to the land surrounding human dwellings, e. g. in
turf dug up in damp places. The localities in which I have found the
species in Iceland fall into two groups: 1) Very damp places near the
shores of lakes or streams, and 2) farms and their immediate neigh-
bourhood (hlað). 0strup has the species from much more than a
hundred localities. It is difficult to say just how manv, since he states
the nuinher of samples for the individual forms, while it is impossible
to see whether or not several forms occur in the same sample. This
shows that the species is hydrophilous and is only exceplionally found
on land where it can hardly live long.