THE AERIAL ALGÆ OV ICELAND 361 



capsa alpina, G. riipestris, some species are met witli here, botli 

 Cj'anophyceæ, Chlorophyceæ, and Dialonis, which will liardly sur- 

 vive complete desiccation. Aniong tlie first-menlioned I shall call 

 attention to Anabæna Cateniila. A. oscillarioides, Plecionema roseohim, 

 Desmonema Wrangelii. Of Chlorophyceæ several species of Desmi- 

 diaceæ occur, Tribonema bombyciniim , Vaucheria borealis, and Hor- 

 midiiim flaccidnm. The Diatoms are often abundantly represented, 

 bolh as regards species and individuals. Often the species usually 

 occurring in streams are dominant. Diatomella Balfoiiriana. Rhopa- 

 lodia gibba, Diatoma hiemalis. Meridian circulare, Synedra Ulna, Epi- 

 themia Zebra are frequently occurring species. 



b. Shady Monntain Clefts. 



It must be assumed that lile condilions in mountain clefts 

 shaded e. g. by low trees differ essentially from conditions on ex- 

 posed rocky walls. Evaporation and, consequently, the risk of 

 strong desiccation must especially be supposed to be far less, as 

 also the amount of light. I have therefore preferred to discuss these 

 localities separatel}" though they have many species in common \vith 

 the rocky walls. The samples which I will include in this category 

 are Nos. 24, 25, 68, 69, 70. Of Chlorophyceæ only Desmidiaceæ 

 occur here, while the Cyanophyceæ are represented by Schizothrix 

 lardacea and Tolypolhrix tennis u. terrestris. Diatoms are numerous, 

 the coninionest are Diatomella Balfonriana, Ennotia prærnpta f. cnrta, 

 Navicnla contenta, N.perpusilla, and Pinnnlaria lata v. Rabenhorstii. 



c. Caues. 



In the lava of Iceland there often occur caves, sometimes of 

 considerable size. Helgi Jonsson writes (1900, p. 93): »Trentepohlia 

 anrea is quite common in the dimly lighted grottos of the lava 

 lields; it grew there in all the small lava holes and was often the 

 sole plant in the grottos«. In addition he found in the lava caves 

 of Snæfellsnes mosses and Lepraria latebrarnm. He also mentions 

 this Trentepohlia vegetation on other occasions, but as I have stated 

 elsewhere, we have no coUected material of this species. I myself 

 have examined a couple of caves in N. Iceland, but found no algal 

 vegetation on the rocky walls, though I found some on the ground 

 on the floor of the caves. In one of these caves (179), located near 

 Skjalvanditljot, the sheep evidently sought shelter in bad weather, as 

 might be seen from their excrements found in considerable amounts 



