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152 H. JONSSON 



where there are openings in the Laminaria-xegetation, and beyond 

 the Laminaria-be\t it is very frequently found on a sandy or pebbly 

 substratum, at any rate at inconsiderable depths. In the Laminaria- 

 belt proper it plays the role of a kind of "underwood," but beyond 

 the belt, at greater depths, it protrudes far above the associations 

 of red algse. 



Desmarestia viridis occurs in a similar manner, very often with 

 the other species, both inside and outside the Laminaria-bett. It is 

 of less importance, however, as it is much less common. At depths 

 of between 20—30 metres it may also occur dominantly in patches. 



In E. Iceland, at a depth of 20 — 30 metres, Chorda tomentosa 

 occurs growing very socially with the Desmarestia species. 



Of the epiphytes on Desmarestia acnleata the small Porphyropsis 

 coccinea is of most importance (in S. and SW. Iceland). 



In several respects this association recalls the semi -littoral 

 C/jorda-association. 



Similar Desmarestia - vegetation occurs in the Faeroes (Borge- 

 sen, 14), in Greenland (Rosenvinge, 63) and in northern Norway 

 (Foslie, 18, p. 100). 



16. The Deep-water Community of Fl or ideas. 



A mixed society, which consists mainly of a few species of red 

 algse, generally occurs at a depth of about 15 — 40 metres (over 50 

 metres in E. Iceland). To judge from the dredgings, the plants some- 

 times seem to grow socially — at any rate over small areas — and 

 sometimes to grow scattered , then , as a rule, much intermingled 

 with one another. 



The species which seem to have an abundant local distribution 

 in fairly many places are the following: Delesseria sinuosa at depths 

 of from 14 to nearly 40 metres and somewhat deeper in E. Iceland; 

 Ptilota plumosa from 16 metres to about 40 metres; Odonthalia 

 dentata from 15 — 30 metres, and Polysiphonia arctica from 16 — 40 

 metres (10 to about 60 metres in E. Iceland). Thus each of these 

 species forms associations, but these are often of inferior extent, 

 with the exception, however, of that formed by the last mentioned 

 species. 



The Polysiphonia arctica-association. Polysiphonia arc- 

 tica is of extremely social growth in E. Iceland, in Sey5isfjor5ur 

 and in Rey5arfj6r5ur. It grows most luxuriantly at depths of from 

 16 — 40 metres and forms an enormous, continuous belt along large 



