i 3 2 THE PLANTS 



the north of Cape Cod, under the influence of the 

 Labrador current, Arctic and Northern European forms 

 occur ; to the south, with its Gulf Stream, species char- 

 acteristic of warmer seas. Professor Setchell was able 

 to show that a change in the kelp flora of Western North 

 America takes place with the increase of every 5° C. 

 of surface temperature. 



We touch here upon the question of the geographical 

 distribution of algae. This is a question wanting careful 

 investigation, for so many agencies exercise an influence 

 on the occurrence of algae in a given locality. The 

 researches of Berthold in the Gulf of Naples, of Kj ell- 

 man in the Arctic Sea, of Kolderup Rosenvinge on 

 the coast of Greenland, of Borgesen at the Faeroe 

 Islands, of Svidelius, and many others, have opened a 

 rich field of study with regard to the conditions of 

 algal life and the physiognomy which algae give to the 

 coast. When searching for algae, we observe that 

 large stretches of the coast are characterized by one 

 or two — sometimes more — algae living, in large num- 

 bers, under the same outward conditions. Other algae 

 may be mixed with them, but they are always in 

 smaller numbers, or less conspicuous ; they do not put 

 a stamp on the vegetation, as the others do. Thus, 

 for instance, Ecklonia buccinalis is a most character- 

 istic alga of the Cape, covering large stretches of the 

 coast in the sublittoral region. Little red algae live 

 parasitically on its stem, but they are entirely hidden 

 under the mass of brown blades floating on the sea. 

 We may call this an Ecklonia " association," and when 

 several associations live under the same outward condi- 

 tions we speak of a " formation." Borgesen distin- 

 guishes, for instance, a formation of encrusting algae on 

 the coast of the Faeroe Islands, and all encrusting algae 

 belonging to it live under the same external conditions. 



