3i8 MARINE ECOLOGY 



which is a summary of the various schemes that have been proposed 

 for classifying the vegetation from this region. 



In concluding this section a word may be said about the perio- 

 dicity of the vegetation in the Mediterranean as compared with that 

 of the English Channel. First of all there is the same pronounced 

 difference in the floral aspects of the summer and winter months 

 that has been observed on other coasts. Boreal Atlantic species 

 such as Ulothrix flacca, U. siihflaccida, U. pseudoflacca, Bangia 

 fusco-purpurea and Porphyra spp. dominate the flora in winter, 

 whilst in summer it is the tropical and subtropical species such as 

 Siphonocladus pusillus, Acetahularia mediterranea, Pseudobryopsis 

 myura, Liagora visctda, etc., which form the dominant species. In 

 comparing the behaviour of the Mediterranean vegetation with that 

 of the Boreal Atlantic one may distinguish several types of algal 

 periodicity: 



(i) Algae with a summer vegetation period in both the English 

 Channel and the Mediterranean. These algae usually occur at a 

 considerable depth where there is little or no temperature variation, 

 e.g. Sporochnus pedunculatuSy Arthrocladia villosa. 



(2) Algae with a winter and spring vegetational period in both 

 the Mediterranean and the English Channel, e.g. species of cold 

 waters such as Ulothrix flacca. 



(3) Algae appearing in the winter and spring in the Mediter- 

 ranean but during the summer in the English Channel. For these 

 algae it might be supposed that the temperatures of the winter and 

 spring in the Mediterranean correspond more or less to the summer 

 temperatures of the Channel, e.g. Nemalion helminthoides. 



(4) Algae found during the summer months in the English 

 Channel but persisting throughout the year in the Mediterranean, 

 e.g. Padina pavonia. Their absence in the Channel at other times of 

 the year may be associated with the low temperatures, and it 

 ought to prove possible to ascertain the minimum temperature at 

 which such algae will survive. 



(5) Algae of spring and winter in the Mediterranean but per- 

 sisting throughout the year in the Channel, e.g. Porphyra umbili- 

 calis, Callithamnion corymbosum. This again is probably related to a 

 temperature correlation, but in this case the algae concerned will 

 not tolerate the high temperatures that are reached during the 

 summer months in the Mediterranean. Comparisons of this 



