102 H. JONSSON 



2. The Community of Filiform Algae. 



It is not easy to find a suitable name for this community. It 

 is composed of several species which are all equally common and 

 are all dominant to almost the same degree. As almost all the 

 species are filiform and non-branching, it seems to me that the 

 community may fitly be named in accordance with the form of 

 the frond. 



The community of filiform algse forms a narrow belt, which 



is often continuous along fairly considerable stretches of the coast, 



at about the average limit of high water. The vertical height of 



the belt is inconsiderable, about one foot, but the breadth conforms 



somewhat to the slope of the coast, and may attain to 3 — 4 feet, 



or even more. This vegetation is very well developed on the face 



of vertical rocks, and the various associations of the community 



can be distinctly seen, one above the other, as parallel bands of 



varying colour. The species which occur most frequently are the 



following: — ■ 



Ulothrix flacca. Bangia fuscopurpurea. 



Urospora mirabilis. Porphyra umbilicalis f. typica. 



Monostroma groenlandicum. 



These are all dominant species, and form extensive associations, 

 of which some are pure and others mixed. Other species may also 

 occur, but only in lesser quantities. 



The Ulothrix-association, as a rule, reaches highest up 

 the cliff. The principal species is Ulothrix flacca, which forms a 

 distinct belt, extending rather far in a horizontal direction. On 

 rock- walls, the filaments are often comparatively long, and are 

 moved to and fro over the entire belt by the beat of the waves 

 or the ripple of the sea at flood-tide ; during the period of exposure 

 they hang down, pressed closely against the face of the rocks. The 

 outer filaments protect the underlying ones from desiccation during 

 low-tide, and thus it happens rather frequently that the outer layer 

 is dry while the protected layer — that nearest to the rocks — is 

 moist. In this way the social growth of the plants protects them 

 against desiccation (cf. Rosenvinge, 63, p. 201), at any rate under 

 normal conditions, and so long as no exceptionally long periods 

 of drought occur. It happens rather frequently, however, that the 

 [//o//m';r-vegetation becomes quite dry during low-tide. This is espe- 

 cially the case when the vegetation occurs on boulders in the lit- 

 toral zone where, when the water subsides, the filaments radiate 



