Chap. II] 



MARINE VEGETATION 



795 



region. According to discoveries, to be mentioned subsequently, regarding 

 aphotic plankton, it may be assumed that the aphotic benthos possesses 

 at least bacteria. 



iv. THE BENTHOS OF COLD TEMPERATE SEAS. 



The study of algal floras has already rendered possible the subdivision 

 of the cold temperate seas into a number of well-defined districts, of which 

 however only two, the North Sea and the Baltic Sea, have been studied 

 in detail as regards the character of their formations and their dependence 

 on external factors. These two seas exhibit great differences, and the 

 conclusions reached regarding their benthos would consequently seem to 

 be of wide application. As regards salinity and tidal conditions the North 

 Sea resembles the Atlantic Ocean, while the Baltic, like a true inland sea, 

 has only weak tides and becomes steadily 

 less salt towards the east. The vegetation 

 varies in accordance with these differences. 



As is the case everywhere in cold tempe- 

 rate seas, so in the North Sea and the 

 Baltic, the Phaeophyceae assume the most 

 prominent place both as regards dimen- 

 sions and abundance of individuals. Fucus 

 vesiculosus (Fig. 483) determines the 

 character of vegetation in the uppermost 

 belts of the rocky coasts of Germany ; 

 Fucus serratus (Fig. 484) and species of 

 Laminaria are no less common at a greater 

 depth. Other families of Phaeophyceae 

 also are represented by common species, 

 especially the Ectocarpaceae, small fila- 

 mentous Algae which appear everywhere 

 in great variety of form. The Rhodo- 



phyceae would seem not to occur in such great numbers as at many points on 

 the Mediterranean coast. Nevertheless, they are represented by numerous 

 common species, some of them of fairly large dimensions, for instance 

 Porphyra, Chondrus, Gigartina, Phyllophora, Plocamium, Delesseria, Poly- 

 siphonia, Callithamnion, Ceramium, Corallina, and others. The Chloro- 

 phyceae offer less variety ; they are species of Ulva, Enteromorpha, 

 and Cladophora. Phanerogams are represented by one single but common 

 and social species, Zostera marina (Potamogetonaceae). 



The distribution of species in the horizontal and vertical directions 

 depends on factors similar to those in the Mediterranean Sea, but their 

 influence is less marked. 



The cnicrg-incr belt is much broader in the North Sea with its strong tides, 



Fig. 4S3. Fucus vesiculosus. Piece 

 of thallus. b Vesicles. /Receptacles. 

 One-third natural size. 



