THE AiiRIAL ALGÆ OF ICKLAND 353 



in the island and found Ihat it varies from 4.8 (myri) to 7.6 (running 

 water). Thus very acid soil does not seem to occur, nor very basic 

 soil either. 



Next to the moisture of the soil, the factor of greatest impor- 

 tance to the algæ growing on the surface and having no organs 

 that can penetrale the earth, is no doubt its content of food-stufls, 

 especially manures. I shall be al)le to underlake an approximate 

 dassification based on this factor with the aid of my notes, whereas 

 a dassification according to the degree of moisture would require 

 observations of the individual locahties through protracted periods. 



There is a number of samples which it has been impossible to 

 classify according to more general viewpoints. For these I refer 

 the reader to the preceding list of the samples examined. 



In the following I shall first deal with habitats where manures 

 are present in greater or less amount, taking first the localities where 

 they are found in the greatest quantities. Next I shall mention 

 various natural formations where manures are not found in any 

 noticeable amount. 



The soil in Iceland is in general very moist owing to the fre- 

 quent rains. The localities mentioned in the foUowing under a-e 

 have all abt. the same amount of moisture. The surface itself, where 

 the algæ grow, may apparently become quite dry in dry weather, 

 but lower down there will always be found some moisture which 

 will prevent the desiccation at the surface having any great effect. 

 The homefield as a rule slopes downward from the farm, in its 

 upper part it is therefore as dry as the »hla5«, but in its lower 

 part it is often considerably moister. »Myri« is land where Ihe 

 ground-water stands so high that the surface is always moist, hence 

 it is doublful whether the algæ growing there may be included 

 among the aérial algæ. The algæ on »floi« where the ground-water 

 just reaches the surface or comes a liltle above it, I consider hydro- 

 philous, and so do not deal with them in the present work. 



a. Soil Fertilised by the Droppings of Birds. (Samples 209, 212). 



On the small island of Geitey near the eastern shore af Myvatn, 

 under the dense growths of Archangelica, I found the ground covered 

 with a thick layer of bird's droppings, a regular guano deposit, ori- 

 ginating from the numerous aquatic birds that had their haunts 

 under the piants. The only alga found here was Phormidinm 



