THE AKRIAL ALGÆ OF ICELAND 359 



undoubtedly of signal importance, being presuniably the chief factor 

 in the distribution of the species in associations. In the succeeding 

 surve}' of tiie associations found by me, I shall therefore endeavour 

 to group them chiefly in relation lo the degree of moisture. 



On enquiry into the pH-value of numerous heterogeneous loca- 

 lities, Mølholm-Hansen, as previously stated, arrived at the result 

 that pH-values below 5,o are rare, even where there is stagnant \vater. 

 He did not examine the ^vater trickling down the rocks, bul since 

 Strøm (1925) finds that this kind of water in Norway has an average 

 pn-value of 7,3, while the value for marshy ground and similar 

 tracts wilh stagnant water averages 4,5, \ve may no doubt be allowed 

 to conclude that the pa-value for the waler from dripping rocks in 

 Iceland must be high, at least as high as in Norway (ph: 7,3). 



Tiie amount of light is no doubt anolher very imporlant factor 

 for the rock algæ. The greater part of the rocks in Iceland will 

 be strongly exposed to the sun's raj's, and this maj' influence their 

 assimilation, besides bleaching their chlorophyll and other colour 

 substances active in that process. As a matter of faet, raany of 

 the algæ have their cells surrounded by highly coloured sheaths, 

 which has generally been considered as a proteclive measure against 

 intense illumination. This applies especially to a number of blue- 

 green algæ. That the sun may produce slrong heal-effects in the 

 rock is a faet beyond doubt, and likewise that these will be of 

 great importance to the algæ. I have, however, had no opportunily 

 of making any measuremenls with regard to these effecls in Ice- 

 land. But there is reason to believe that eonditions there will in 

 several respects resemble those found by Diels (1914, p. 505 f.) in 

 the soulhern part of the Tyrol. In parlicular it will no doubt apply 

 to all roeky walls that the air immedialely above the rocky surface 

 will be quickly heated, whereas the air within the stone will be 

 more slowlj' heated, and the air outside slowest of all. Furlher, the 

 rocky surface will keep warmer than the air as long as the sun is 

 shining on it. Even in the evening the rock seems to retain the 

 heat better than the air, but it is possible that it will cool quicker 

 than the air in the course of the night; however, Diels's measure- 

 menls give no information on this point. In shady piaces such as 

 narrow fissures, or in piaces where there is some vegetation, eondi- 

 tions will no doubt be essentially dilTerent. 



Like Strøm (1926, p. 20), I have observed no endolithic algal 

 vegetation in Iceland. 



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