MARINE ALGAL VEGETATION 99 



10. The Community of Rhodymenia. 



11. The Polysiphonia urceolata-association. 



12. The Community of Corallina. 



13. The Crustaceous Alga-association, 

 c. The Sublittoral Zone. 



14. The Community of Laminariacese. 



15. The Desmarestia-association. 



16. The Deep-water Community of Florideae. 



17. The Lithothamnion-association. 



18. The Community of Crustaceous Algae. 

 B. The Sea-grass Vegetation. 



1. The Zostera-association. 



A. The Marine Algal Vegetation. 



a. The Littoral Zone. 



The Littoral Communities. The littoral vegetation is com- 

 posed of several communities. These are left dry during low-tide, 

 with the exception of the pool-vegetation; but the period during 

 which the different communities are left dry varies greatly. I am 

 not prepared to state accurately the period of exposure, but I pre- 

 sume that the lowest littoral communities will be laid bare for 

 about 1—2 hours under normal conditions, while the uppermost 

 communities will be submerged for about one hour, and will be 

 laid bare for about 11 hours. On exposed coasts the period of ex- 

 posure is shortened by the beat of the waves. The ecological factors 

 differ not a little in the uppermost and lowermost part of the littoral 

 zone, and because of this difference the vegetation is divided into 

 longitudinal belts along the coasts. The littoral vegetation falls na- 

 turally into three divisions: the light-loving or strictly littoral com- 

 munities, the shade-loving communities, and the pool-vegetation. 

 In the following description of the vegetation, the communities in 

 each of the three zones are arranged as far as possible according to 

 depth, and in such a way, that the uppermost come first and the 

 lowest come last. This rule, however, cannot always be adhered to. 



aa. The Photophilous or strictly Littoral Communities. 



These communities are found in places in the littoral zone 

 which are directly exposed to light during low-tide. 

 They are composed of blue-green, green, brown and red algae; the 



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