The Botany of Iceland - 01.12.1928, Side 35

THE AERIAL ALGÆ OF ICELAND
359
undoubtedly of signal importance, being presumably tlie chief factor
in the distribution of the species in associations. In the succeeding
survey of the associations found by me, I shall therefore endeavour
to group tliem chielly in relation to the degree of moisture.
On enquir^r into the pH-value of numerous lieterogeneous loca-
lities, Molholm-Hansen, as previously stated, arrived at tlie result
that pH-values below 5,o are rare, even where there is stagnant water.
He did not examine the water trickling down the rocks, but since
Strom (1925) finds that this kind of water in Norway lias an average
pH-value of 7,3, while the value for marshy ground and similar
tracts with stagnant water averages 4,5, we may no donbt be allowed
to conclude that tlie pH-value for tlie water from dripping rocks in
Iceland must be high, at least as high as in Norway (pH: 7,3).
The amount of light is no doubt another very important factor
for the rock algæ. The greater part of the rocks in Iceland will
be strongly exposed to the sun’s rays, and this may influence their
assimilation, besides bleaching their chlorophyll and other colour
substances active in that process. As a matter of fact, many of
the algæ have their cells surrounded by liighly coloured sheaths,
which has generally been considered as a protective measure against
intense illumination. This applies especially to a number of blue-
green algæ. That the sun may produce strong heat-effects in the
rock is a fact heyond doubt, and likewise that these will be of
great importance to the algæ. I liave, however, had no opportunitv
of making any measurements witli regard to these efTecls in Ice-
land. But there is reason to believe that conditions there will in
several respects resemble those found by Diels (1914, p. 505 f.) in
the southern part of the Tyrol. In particular it will no doubt apply
to all rocky walls that the air immediately above the rocky surface
will be quickly heated, whereas the air within the stone will be
more slowly lieated, and the air outside slowest of all. Furlher, the
rocky surface will lceep warmer than the air as long as the sun is
shining on it. Even in the evening the rock seems to retain the
lieat better than the air, but it is possible that it will cool quicker
than the air in the course of the night; liowever, Diels’s measure-
ments give no information on this point. In shady places such as
narrow íissures, or in places where there is some vegetation, condi-
tions will no doubt be essentially different.
Like Strom (1926, p. 20), I have observed no endolithic algal
vegetation in Iceland.
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