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132 H. JONSSON 



10. The Community of Rhodymenia. 



The species are Rhodymenia palmata and Halosaccion ramen- 

 taceum, both of which occur along the coasts in great abundance 

 as pure associations. These two associations most frequently ac- 

 company each other, and seem to demand somewhat similar con- 

 ditions of life. Halosaccion, however, extends the higher up in the 

 littoral zone, but is then almost always submerged in water-filled 

 depressions in that zone, while Rhodymenia extends the further 

 down below the limit of low-tide. In Rhodymenia the form of the 

 thallus is leaf-like and branching; in Halosaccion it is round and 

 excessively or slightly branching. 



The Rhodymenia-association. On regarding a steep rocky 

 coast, where the succession of the associations is always most easily 

 observed, it is seen that a luxuriant Rhodymenia- vegetation com- 

 mences even at the lower boundary of the Fucus-be\t. In many 

 places the species occurs so socially that it predominates as a 

 characterizing plant over relatively large stretches, both in the lower 

 littoral zone and far below the limit of low-tide, to a depth of 

 about 12 metres. On a gently sloping rocky coast, the upper boun- 

 dary of the association is not so sharply defined, but if proper 

 allowance is made for the depressions and elevations in such a 

 littoral zone the boundary is fairly easily ascertained although it 

 may then lie in very many curves. On a gently sloping pebbly 

 coast, the same regular boundary is found as on a steep rocky 

 coast and at about the same height, although the Fzzcacece-community 

 is not present there. 



Various algae may occur on stones under the fronds of the 

 Rhodymenia, as, for instance, Hildenbrandia rosea, Ralfsia clavata, 

 Sphacelaria radicans and others; also a number of epiphytes, espe- 

 cially Myrionemacea.' and Ectocarpacea, often grow on old individuals 

 of Rhodymenia palmetto. 



Rhodymenia occurs also epiphytically on the stems of Laminaria 

 in the upper part of the Laminaria-belt. 



The Rhodymen z'a-vegetation does not seem to be so luxuriant 

 in Greenland (Rosenvinge, 63) as in Iceland. Possibly this is true 

 also of the north of Norway (Foslie, 18). In the Faeroes the 

 Rhodymen z'a-vegetation is luxuriant, but seems to differ somewhat 

 from that of Iceland, as it consists of a form with lower and more 

 dense growth and narrower branches; this form also extends further 

 up (Borgesen, 12, p. 727). 



