30 



grows best in tlie temperate zone. I have no dou'bt that 

 the distribution of the species of marine algte 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 

 Chondriis 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 algse 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 algae occur very regularly at certain heights 

 above or below certain hxed levels. I have lately been 

 fixing these heights for a few algae in Port Erin Bay as a 

 jjreliminary to some more detailed investig-ations into the 

 vertical distribution of marine algse. 



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. AVe will begin from the highest point. 

 Pelvetia canalictdata 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 onlv by the 

 spray of dashing waves. 



Fucus vesicidosus 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. 



Ascojdii/IIum ?wdosum extends from 6' to 11' above 0. 



Fucv.t scrratus forms a very distinct region, 3' to 6' 

 above 0. 



