340 THE ALGAE 



such algae (e.g. Nemalion helminthoides) it may be considered 

 that the winter and spring temperatures of the Mediter- 

 ranean approach most closely to the summer temperatures 

 of the Channel. 



(4) Algae foimd during the simimer months in the English 

 Channel but persisting throughout the year in the Mediter- 

 ranean, e.g. Padinapavonia. It is hkely that low temperatures 

 inhibit their growth in the Channel and it ought to be pos- 

 sible to determine the minimum adult survival temperature. 



(5) Spring and winter algae of the Mediterranean but persisting 

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

 calis, Callithamnion corymbosum. Here again there is probably 

 a temperature correlation, but in this case the algae will not 

 tolerate the high summer temperatures of the Mediterranean. 



Comparisons of this nature are most valuable in helping us to 

 imderstand something of the biological requirements of different 

 species. They also indicate profitable lines of enquiry concerning 

 the temperature relations of the algae. 



The zonations to be observed in South Africa are especially in- 

 teresting (Stephenson, 1939, 1944). On the West coast the waters 

 are relatively cold and a cold water flora and fauna predominates. 

 The east coast is bathed by the warm waters of the Indian ocean 

 and a very different flora and fauna exists. In the intermediate zone 

 of the Cape itself one can find all transitions from the cold water 

 zonation to the warm water zonation (Fig. 193). Similar changes 

 could no doubt be observed north and south of Cape Lookout in 

 North CaroUna and in other parts of the world. Very few of these 

 changes have, as yet, been worked out. 



Seasonal differences in the floras of different regions have been 

 observed in various parts of the world. There are two general 

 phenomena: one is the actual replacement of certain species by 

 others, e.g. Mediterranean, North Carohna and elsewhere; the 

 other is changes in the vertical levels of certain algae on the shore. 

 Thus, in the Isle of Man successive generations of Cladophora 

 rupestris move vertically up and down several feet annually, the 

 movement being rendered possible because there is a monthly re- 

 production when the sporehngs only survive at the most favourable 

 level for the particular time of the year, and this is not necessarily 

 that in which the parent plants are growing. Some algae migrate up 

 in winter and down in summer, whilst others do the very reverse. 



