458 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



grows best in the temperate zone. I have no doubt that 

 the distribution of the species of marine alga? depends on 

 the same factor as that of terrestrial phanerogams. The 

 limits of the distribution of phanerogamic species as a 

 rule coincide roughly with isothermal lines. 



The distribution of the plant form represented by 

 Chondrus crispus in any given small district is dependent 

 not on the temperature, but on quite different factors. It 

 is impossible to say as yet fully what these factors are. 

 The following account is therefore only short. 



To begin with, it may be stated that a firm sea bottom 

 is generally necessary for the growth of algae in general. 

 Stones which roll about with every tide never bear red or 

 brown seaweeds, at the most only a few green ones. Sand 

 is always quite barren. 



Certain alga? occur very regularly at certain heights 

 above or below certain fixed levels. I have lately been 

 fixing these heights for a few alga? in Port Erin Bay as a 

 preliminary to some more detailed investigations into the 

 vertical distribution of marine alga?. 



If we call the level of dead low-water mark of an 

 ordinary spring tide 0, then we can divide the shore into 

 a series of regions. We will begin from the highest point. 

 Pelvetia canaliculata extends from 12 ' to 17 ' above O. These 

 plants are often left exposed by the sea water for days. 

 The highest individuals are often moistened only by the 

 spray of dashing waves. 



Fucus vesiculosus extends from 3' to 13', but not in the 

 same condition. In an upper region, 9' to 13' above 0, 

 the plants are small, rarely fertile, and possess no vesicles. 

 In the lower region the plants are normal. 



Ascophyllum nodosum extends from 6' to 11' above 0. 



Fucus serratus forms a very distinct region, 3' to G' 

 above O. 



