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158 H. JONSSON 



nities occur intermingled in the Zostera-he\t ; these then grow in 

 small depressions, where the subsoil, usually a solid, clayey and 

 pebbly substratum, appears. Such species are: Chorda Filum, Chor- 

 daria flagelliformis , Castagnea virescens, Dictyosiphon foeniculaceus, 

 Pylaiella littoralis, Ceramium rubrum, Cystoclonium purpurascens and 

 others. They should not be classed in the Zosfera-association, and 

 are mentioned only to explain the appearance of the Zosfera-meadow. 



The Zosfera-association is sublittoral, but hardly extends as far 

 down as do the semi-littoral communities. It cannot endure pro- 

 tracted exposure, and thus a substratum which would adapt itself 

 well to Zostera may be found completely devoid of plants in places 

 which are exposed for a long time during low-tide. From time to 

 time, during extreme ebb-tides, the upper part of the Zosfera-meadow 

 may, however, be seen quite dry. But here two points have to be 

 taken into consideration: the first being that the period of exposure 

 is extremely short, and the second being that the extreme ebb-tides 

 occur so seldom that they ought not to be taken into calculation. 

 As a rule, the Zostera substratum is always submerged during low- 

 tide. The water is so low, however, that the leaves float on the 

 surface of the water, giving it a greenish tinge. When wading in 

 a Zosfe/'a-meadow during low-tide the water reaches to about the 

 knees. 



In SW. Iceland the time of fruiting is during August— October. 



On Zosfera-soil there is in most places a very rich animal-life, 

 but whether this has any influence upon the vegetation or on the 

 nutrient substratum needs further investigation. 



In connection with the Zos/era-assoeiation I will just mention 

 the Brackish-water-vegetation. It is so little known that there 

 is nothing to be said about it, except that I have found Ruppia 

 maritima in one solitary spot, where it grew so luxuriantly that, 

 although scattered, it characterized the bottom. 



