THE AÉRIAL ALGÆ OF ICELAND 349 



freely over Ihe country in most piaces. Even in the woods, which 

 only cover very small areas and consist of ([uite low trees or bushes, 

 there can hardly be much shelter in the ^vinter. Whether it is tlie 

 mechanical effect of the wind or its desiccating po\ver, or both in 

 conjunction, lliat decide the issne, can hardly be determined wilh 

 certainty as long as sure data concerning the force of the wind and 

 the degree of moisture in the atmosphere are not lo band. As 

 known, these Iwo kinds of meleorological data are the most difli- 

 cult to ascertain, just as also the local conditions of quite small 

 areas may play an extremely important part. 



It might be expected that deep mountain ravines would alTord 

 shelter from the wind, and this is undoubtedly the case; but steep 

 ravines are not very common in Iceland where, on the conlrary, 

 the broad U-shaped valleys prevaii. I saw a narrow ravine sheltered 

 from the wind in the so-called »Slora Gja« to the east of Myvatn, 

 but as there were neither trees nor woodwork of any description 

 there, I had no opportunity of observing how algie might have 

 developed on these substrata. It is a characteristic faet that most 

 of the samples of algal vegetation from woodwork come from Isa- 

 fjor(^ur, which lies in a very deep valley surrounded by high moun- 

 tains on all sides, and from Reykjavik where the high and com- 

 paratively closely built houses will be able lo afford some degree 

 of shelter. 



I mav' further point out that the highest mountain regions of 

 Norway present a similar lack of this community of algæ (Strøm 

 1926, p. 17). In a thesis (Nr. 7) in his doctors dissertation (1910) 

 Helg i Jonsson makes the following statement: 



»The cold itself does not seem injurious to hibernating piants 

 in cold climates, whereas desiccalion by frequent, stormy winds is 

 extremely fatal.« True, this statement is made with reference to 

 the phanerogams only, but there can hardly be any doubt that it 

 applies equally to the aerial algæ. 



2. Brickwork. 



The old farms of Iceland were all built of turf and wood, but 

 recently brick-built houses have become more and more common. 

 On the lower part of the walls layers of green algæ are often met 

 with and seem to me lo occur wilh as great frequency as in Den- 

 mark. Evidenlly the algæ find belter life-conditions on the walls than 



