17 



III. The outer conditions for the growth of the Zostera. 



The grass-wrack is a pronouuced aquatic plant which can only grow when 

 surroimded by water and witliers if exposed to the air for any length of time; thus, 

 its growth ceases at the lowest water mark. The presence of water is not sufficient 

 liowever, tlie water must also be saline. We can best notice how the fresh 

 water hinders the growth of the Zostera at the mouth of larger streams, for example, 

 the outlet of the river Gudenaa in Randers Fjord, where the eel-grass is only 

 to be fonnd a short way up beyond Udbyhøj. The ability of the ecl-grass to 

 grow in salt water is a peculiarity it has in common with only a few other 

 flowering piants and no other northeru flowering plant has it to such a pronounced 

 extent. The Zostera nana Roth, which occurs frequently on our eoasts as low, 

 small growths in quite shallow water, comes close to the common eel-grass in this 

 regard. The few other marine flowering piants which grow in our waters live in 

 more brackish water. 



The eel-grass also grows on those of our eoasts where the salinity of the 

 water is highest (about 3.3 per cent.), namely, along the west coast of Jutland and 

 on the inner side of the uorth point of Jutland; it is not common nor does 

 it grow extensively at these piaces, but this is due to other factors than the salinity. 

 The latter decreases up through the Kattegat and the Belts, and on the east coast 

 of Falster and in Faxe Bay it is as low as l.o per cent., but we still find the 

 eel-grass there. Its distribution extends still further up the Baltic, where its most 

 northern occurrence seems to be in the Åland Sea (according to E. Sernander, 

 Botaniska Notiser, Lund, 1901, p. 276). Here the salinity is not more than about 

 0.6 per cent. 



We see from this that the grass-wrack certainly requires salt water, but 

 we find also that it is not very particular about the degree of salinity. A salinity 

 of 1 to 3 per cent. may be considered the most advantageous for its growth, and 

 as it is just between these limits that the salinity in our Danish Seas varies, we 

 cannot wonder that the eel-grass is at home with us. 



A fairly sheltered locality is another condition for the success of the 

 eel-grass. It cannot grow on an open coast where the waves beat heavily and the 

 motion is felt a good way down in the water; the bottom at such piaces is either 

 too uusteady for the roots to fasten themselves on, or too stony with consequently 



