8 
grass-wrack formation is formed chiefly of various plants of the family of 
Potamogetonaceae, Zostera, Ruppia, Zannichellia, Cymodocea, Posidonia etc. 
After this brief summary we may turn to the vegetation in the Danish 
waters, especially the Limfjord, and begin with a consideration of the benthos for- 
mations, which are mainly represented by the grass-wrack formation. 
Zostera marina is everywhere the predominant plant. It extends in a more 
or less broad, broken belt along all our Kattegat and Baltic coasts, oceurring in 
patches over a great part of the Kattegat and in great quantities in all our fjords. 
The occurrence and biological conditions of the Zostera have been deseribed 
in the following papers: 
1. »Det videnskabelige Udbytte af Kanonbaaden Hauchs Togter« by C. 
G. Joh. Petersen, 1883—1886. 
2. C. G. Joh. Petersen: »Fiskenes biologiske Forhold i Holbæk Fjord«. 
Ber. fra den Danske biologiske Stat. I. 1890-(91). 
3. C. G. Joh. Petersen: »Det pelagiske Liv i Fænø Sund«. Ber. f. D. 
biol. Stat: TIL. 71893. 
4. Warming: Dansk Plantevækst I. Strandvegetation. 1906. 
5. Ostenfeld: Aalegræssets (Zostera marinas) Vækstforhold og Udbredelse 
i vore Farvande. Ber. f. D. biol. Stat. XVI. 1908. 
I shall only give a short summary here of the results obtained by these 
investigations, chiefly on the basis of Ostenfeld's work. 
LZostera grows best in a salinity of 1—3'/,, but can also grow in water 
of less salinity. The situation must be somewhat sheltered. In the North Sea, 
therefore, it only occurs in small quantity. The depths to which it descends are 
the following: 
In the Limfjord ca. 6 meters. 
In the Kattegat (Aalborg Bugt) ca. 11 meters. 
In the Baltic (Fakse Bugt) ca. 8 meters. 
In Guldborgsund and Faenø Sund ca. 6 meters. 
The Zostera thus goes furthest out where the water is clearest. The nature 
of the bottom is also of importance for the Zostera. The Zostera which occurs on 
solid ground is shorter and narrower leaved than the fjord Zostera, which grows 
on muddy bottom. 
The distribution and occurrence of the Zostera is illustrated by Chart II in 
»Hauchs Togter« (Kattegat), by the Charts on pp. 17 and 29 in Ostenfeld's 
work (Limfjord and Kattegat). Further, Charts showing the distribution of the 
Zostera in Holbæk Fjord and the Little Belt are given in Ber. f, D. biol. Stat. 
I and III. 
Briefly, the distribution of the Zostera is the following (from Osten- 
Feldnsten: 
In the North Sea the Zostera is known from the waters between Fanø and 
the mainland and further from Hobugt. On the other hand, it is absent or occurs 
very sparsely at least along the whole of the west coast of Jutland. 
In the Limfjord, on the other hand, the Zostera vegetation is extremely 
rich and luxuriant. It follows fairly closely the 6 meter line, occurring therefore 
in greatest quantities in the shallow Brednings, Venø Bugt, in Skibsted Fjord, 
