33 



V. The Zostera vegetation as a habit for other piants and 



for animals. 



At certiiin piaces the grass-wrack is found quite i)ure, tbat is, not mixed 

 witli other piants; at other piaces a great maDy other piants grow amoug its 

 shoots, as has already been meutioued previously. According to these differences 

 one may speak of a »pure« and a »mixed« Zostera vegetation. 



Again, tlie pure Zostera vegetation appears under two totally different 

 forms. On the bare, firm and light sand in the Kattegat and elsewhere, where 

 the waves beat vigorously, there is a pure Zostera vegetation with ratber small, 

 narrow-leaved shoots; it is a poor and sparse growth of no practical importance. 

 The leaves of the shoots are generally clean and without any covering of smaller 

 piants or animals. 



A second kind of Zostera vegetation with no other piants growing amongst 

 it appears on rich, soft muddy bottom and iu protected piaces. Here the Zostera 

 attains its greatest luxuriance, and whereas in the foregoing case it was the uu- 

 sheltered and unfavourable conditions which caused the absence of other piants, 

 here the vigorous and dense growth of the Zostera itself excludes intermixture. 

 The shoots grow as thickly as grass in a meadow, and the leaves attain a length 

 of 1 — 2 meters. The excellent conditions of life are shown also in that the leaves 

 are covered with a multitude of small piants and animals. In springtime the 

 leaves become liairy and brown, with an abundant growth of Diatoms, and later 

 many Algæ are added (Ceramium, Ectocarjms etc). Of animals we always find the 

 honey-combed colonies of Bryozoa (Menihrantpora), single and compound Ascid- 

 ians (Ciona, Bofri/llns etc.^, hydroid polyps (Gonofhyrca etc.j, tubiculous 

 worms (SpirorbisJ, Lamellibranchs (especially the common mussel (Mytilm), 

 see fig. 9) and snails (Hydrohia, Eissoa etc). A more detailed list of the animals 

 living on the leaves of the Zostera will be found in Report 1 from the Biological 

 Station, p. 171 — 176, to which reference may be made. 



The »mixed« Zostera vegetation may also be of different kinds, 

 varying chiefly witli the bottom. The bottom in our sounds with their constantly 

 renewed, streaming water is generally somewhat stou}^ but consists besides of 

 sand-raixed mud, that is, sand mixed to a greater or smaller degree with fine 



