ECOLOGICAL FACTORS 4II 



different localities depending upon the nature of the tide (p. 384), 

 and there will also be seasonal variations in quantity and quality of 

 Ught available at different depths. At the lower levels work has still 

 to be carried out to determine the extent to which competition 

 with other organisms may be important. At high latitudes where 

 the alga is reaching its southern limit, competition from other algal 

 species is probably more severe and as a factor it may be more im- 

 portant here than at its northern limit. It is not suggested that the 

 final picture has yet been reached even in this case and there are 

 further aspects that may or may not be of significance. Thus it has 

 been found that Hormosira, like certain other marine algae, pos- 

 sesses an Na+ efflux ptmip, removing excess sodium, and a K+ in- 

 flux pump which enables it to accumulate potassium (Bergquist, 

 1959). Whether either or both of these pumps may have a critical 

 operating value, depending either on length of period of sub- 

 mergence or depth of submergence has yet to be studied. 



In conclusion, it is evident that work during the last few years 

 brought us to a clearer understanding of the factors controUing algal 

 zonation to the sea-shore, but there is stiU scope for much more, 

 especially of a physiological and biochemical nature. 



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