THE AKRIAL ALGÆ OF ICELAND 



365 



by its water. Here there will always be a more or less considerable 

 amount of heat ironi Ihe spring and the air will be continually fed 

 witii moisture from the vapours. Is is often very difficult to decide 



Fig. 2. Bird-cliff on Heimaey. 



where to draw the line between the thermal algæ proper and the 

 aérial algæ, and the same species are often fonnd in the water and 

 as half desiccated incrustations on the banks. The algal vegetation 

 growing on and in the often very luxuriant mosses round the springs 

 is of a very hydrophilous character and will hardly ever be exposed 

 to desiccation. Tiie algæ growing on the siliceous sinter or on the 

 ground will no doubt be more exposed to desiccation, but this will 

 probably never be verj' great. Nevertheless I include these algæ be- 

 cause they grow in piaces where the air has free access to them 



