MARINE ALGAL VEGETATION 101 



nalis f. micrococca and Rhizoclonium riparium, which essentially be- 

 long to the lower belts, but can accommodate themselves to life in 

 the air, are also included as members of this association. They are 

 species which, ow r ing to their structure or their manner of growth, 

 are capable of enduring desiccation. Enteromorpha intestinalis f. 

 micrococca has, as is well known, small cells, the walls of which 

 are very thick, the inner walls, in particular, being highly thickened, 

 serving possibly as reservoirs for water. It prefers fissures in the 

 rocks, where, as a rule, it is less exposed than the species charac- 

 teristic of the association. Sometimes, however, I have come across 

 f. micrococca growing, like Prasiola stipitata, on flat rocks manured 

 by birds, but then it had a different aspect and, at first sight, some- 

 what resembled Prasiola. Rhizoclonium also occurs in fissures, where, 

 owing to its manner of growth that is to say its pulvinate form 

 it is protected from desiccation during the long periods of drought. 

 Calothrix scopulorum also occurs as a member of the Prasiola- 

 association and forms Calothriceta of limited dimensions; the indi- 

 viduals are procumbent and are placed so closely together that the 

 rock is completely covered, and thus they protect each other from 

 desiccation. From a biological point of view, the manner of growth 

 of this species, in the dry condition, is similar to that of the cru- 

 staceous algas. Enteromorpha intestinalis f. minima also occurs in 

 this community. 



The species in this community grow, as a rule, in small, pure 

 societies which form a narrow, though not a continuous belt along 

 the coast. This belt is situated higher on exposed coasts than it 

 is on those which are less exposed. 



The Praszo/a-association is undoubtedly commonly distributed 

 in neighbouring countries, but the constituting species may be dif- 

 ferent. In Greenland (Rosenvinge, 63, p. 200) CalothrLv scopulorum, 

 Ectocarpus maritimus, and Rhizoclonium riparium occur only in the 

 uppermost part of the littoral zone. This vegetation, however, scarcely 

 corresponds with the Prasz'o/a- association, but rather with that 

 occurring below. In the Faeroes, however, an exactly corresponding 

 association is found, which Borgesen calls the Chlorophycece- 

 formation (12, p. 712). The Faaroese Chlorophycece-formaiion , how- 

 ever, appears to be more luxuriant and is composed, in part, of 

 other species. A Praszo/a-association (Foslie, 18, p. 127) similar to 

 that of Iceland is evidentlv found in Finmark. 



