Chap. II] MARINE VEGETATION 797 



also his Porphyra-formation (Porphyra vulgaris), which occupies smooth 

 surfaces of rocks somewhat above the Nemalion-formation. 



The s?ibmcrgcd belt of the photic region generally exhibits a vegetation 

 much richer in forms and more luxuriant than the emerging belt. Here 

 submarine meadows of Zostefa marina cover the sandy or muddy bottom 

 of shallow calm bays; according to Reinke they extend to a depth of 10 

 meters in the Baltic. On looser soil, rooting Algae, such as Caulerpa and 

 many other Siphoneae of warmer seas, are completely absent. All the 

 Algae are lithophytes, epiphytes, or parasites. The vertical and horizontal 

 distribution of species is less regulated by the light, because this is less 

 intense than in the Mediterranean Sea. A differentiation of the photic 

 region into belts of illumination as distinct as that established by Berthold 

 in the Mediterranean, appears to be wanting in the North Sea and the 

 Baltic ; also the distinction between shady and sunny stations of equal 

 depth is but slightly expressed in the vegetation. The horizontal dif- 

 ferentiation is, to a greater extent, caused by the strength of the movements, 

 but also by the salinity of the water. The poverty of the algal flora of the 

 Baltic, when compared with that of the North Sea, is due to its less saline 

 nature, and becomes more accentuated from west to east as the salinity 

 decreases. 



The periodic phenomena in cold temperate seas aie less dependent on 

 differences in the illumination at the different seasons than on those of 

 temperature. While, on the one hand, in seas with a mild moderately 

 bright winter and a very bright summer the algal vegetation is more 

 luxuriant in winter than in summer, on the other hand in cold temperate 

 seas the algal vegetation of 'winter is poorer than that of summer. 



Kuckuck l gives the following description of the vegetation of Algae on 

 the coast of Heligoland (Fig. 487) at the different seasons of the year: — 



' The winter is poorer in species than the summer. Once spring has come, the 

 representatives of the different periods appear consecutively. Thus, the so-called 

 WittklifT, a reef consisting of muschelkalk and situated at the northern extremity of 

 the dune, in March and April becomes covered with fresh green tufts and carpets of 

 various species of Cladophora, and with the yellowish-brown fronds of Laminaria 

 saccharina, which subsequently become divided by the breakers. As spring advances 

 this vegetation is suppressed by Polysiphonia urceolata, which grows with ever- 

 increasing vigour, and eventually, in May, covers all the rocks with dark red plants 

 up to 0-3 meter in length. In June this begins to fall into the background, and in July 

 it has disappeared to give place, merely for a short period, to some very transient 

 species of Enteromorpha. In August and September Cladostephus spongiosus 

 dominates, and tints the rocks with brown until it in turn sheds its fronds and becomes 

 inconspicuous. During the colder winter months, when the emerging rocks are 

 often encrusted with ice, only the maimed stumps of various Algae are visible ; and 



1 Kuckuck, II, p. 446. 



