The Botany of Iceland - 01.12.1928, Side 45

THE AERIAE ALGÆ OF ICELAND
369
opinion of the nature of the smallest systematic units. We must
here consider three possibilities:
1) We can assume a simple fluctualing variability.
2) It can be supposed that the same genotype may give rise to
variants determined by environment and íinallj',
3) There can be a possibility of genotypically diífering forms.
Of these three kinds of forms only the latter two are of any
interest for ecology. Unable for the present to distinguish between
the three kinds of variants, ecology must take them all into account,
and we must hope that we shall subsequently obtain a better insiglit
into the significance of their occurrence in the various associations.
In tlie present work 1 have attempled to unravel the forms of
some of the most commonly occurring species. Owing to the pro-
blematic nature of these forms I have not, in some cases, formally
labelied them as varietates and l'orniæ, but have simply given them
numbers. In other cases I have set up new varieties and forms,
and notably of species in which several such had already been
described and named.
I have previously pointed out (1915, p. 298) that tlie terrestrial
Diatoms nearly all belong to the Pennatæ and among these to the
Raphideæ, the Centricæ and Arraphideæ being only sparingly re-
presented. I connected this fact with tlieir mode of living, thinking
that the presence of a raphe and the consequent molility niight
probably be of great use to them, enabling them to penetrate be-
tween the particles of earth and in similar places in periods of
desiccation, and thus securing protection for them. Furthermore I
showed that most aerial Diatoms belong to small species, and often
to particularly small forms of the species which also occur in fresh
water. This, too, miglit be of importance to these organisms under
their special life conditions, since it seems to be a fact that small
organisms on the whole withstand desiccation better than large ones.
These general observations were fully confirmed by my investiga-
tion of the Icelandic aerial Diatoms, though the facts do not perliaps
appear so clearly in the present paper because I have included
localities of a considerably greater degree of moisture than those
contained in my former investigation. On the whole the line of
demarcation between the aerial and the hydrophilous Diatoms is
not nearly so sharp in Iceland as in Denmark. This is doubtless
due to climatic differences between the two countries.