COMMUNITIES OF AERIAL ALGÆ. 



I. ALGAL VEGETATION ON PROMINENT OBJECTS. 

 1. On Woodwork and the Bark of Living Trees. 



THE climate of Iceland is very damp, that is to sav, rain is fre- 

 quent all the year round, and altogether rainy days are very 

 numerous. Thiis it is stated that at Stykkisholm there are on an 

 average 205 rainy days annually. On the other hånd the absolute 

 amount of the rainfall is not very great, and naturally there are 

 considerable iocal variations. 



Consequently, when I went to Iceland in 1914 to study the 

 aerial algæ, I had expected that all woodwork, fence-poles, the 

 wooden gables of houses, and the trunks of the birchtrees would 

 be green with a coating of algæ. My expectalions on this point 

 were, however, entirely disappointed. It turned out that all pro- 

 minent woodwork as well as the trunks of the very sparsely occur- 

 ring trees were practically entirely devoid of any such algal vegeta- 

 tion. The woodwork forming the gables of the turf houses was 

 quite naked, almost white, bleached with the sun, wind, and rain. 

 Only right at the bottom did I occasionally find a scanty algal 

 growth, e. g. at Hrafnkelsta5ir (79) (E. Icel.), where I found a layer 

 of Mijrmecia pyriformis, and at the bottom of a barn-door at M65ru- 

 vellir (Kjos) (292), where I found Desmococcus vulgaris, Pleiirococcus 

 vulgaris, and Trochiscia hirta. 



On two palings in Reykjavik I found layers of green algæ (276, 

 286) consisting in one case of Apatococciis lobatiis, Coccomijxa dispar, 

 Desmococcus vulgaris, in the olher of Apatoccus lobatus, Chlorella 

 ellipsoidea, Pleurococcus vulgaris. 



Fence-poles and telegraph-poles were as a rule quite devoid of 

 vegetation. However, on a fence-pole at MoSruvellir (Kjos) (287) I 

 found Botrydiopsis arhiza, Prasiola sp., Stichococcus bacillaris, while 

 Chlorella rugosa and Stichococcus bacillaris grew on a telegraph pole 

 outside Borgarnes (349). 



