1915] 



SETCHELL DISTRIBUTION OF MARINE ALGAE 



301 



three 



Spot distributions 



less easy to 



enough 



are 



detect than other anomalous distributions but 

 sufficiently known to make apparent their influence and im 

 portance in any scheme of representation of geographical 

 distribution. 



While the distribution of any particular 



of plant 



depends upon a complex of conditions controlling continued 

 existence, both vegetative and reproductive, certain more 

 general factors may be distinguished as prevailing over larger 

 areas, while others, less general, may account for local and 

 usually discontinuous distribution within particular provinces 

 and districts, and as components of various formations, bathy 



metric belts, and associations. 



Temperature has come to be considered as one of the most 

 important of the conditions controlling, or governing, the dis- 

 tribution of plants and animals (cf., e.g., Merriam, '94, '98, 

 etc.; Livingston and Johnson, '13; and others). Any bio- 

 logic factor has, of necessity, two variables (cf. Livingston 

 and Johnson, '13, p. 351), intensity and duration, and 



ables present considerable 



m 



the case of land plants. For marine plants, particularly for 



those species 



tantly submerged 



amplitude of 



variables is less than for the land plants. The surface waters 

 of the ocean, while influenced by the temperature of the air, 

 change slowly and only within cert 



limits 



Mo 



con- 



siderable is the variation through the influence of 



pwell 



Yet on the whole 



the temperature variables are seemingly, at least, much less 

 in amplitude than are those of the land. For those plants 

 exposed during tidal changes the temperature variables may 



be considerable in amplitude. Yet such 



are 



only 



occasional and of short duration, except, perhaps, for the 

 plants of the uppermost tide limits. One matter of import- 

 ance as to all factors in plants submerged entirely or for the 

 greater portion of the time, is the uniformity of exposure to 

 the same conditions. While the land plant may have its roots 

 buried in the soil of one temperature and its aerial organs 



exposed 



considerably different temperature, the entire 



