Distribution of the Laminar iacem. 369 



In discussing this subject of the limiting of the distribution of 

 kelps by variations in temperature we must keep two things in 

 mind. In the first place we shall be dealing entirely with the sur- 

 face waters; and in the second place, the temperature which is to be 

 considered is the summer temperature principally. 



Kelps do not descend very deep into the ocean, so that the surface 

 waters, and those just below are the only ones influencing their 

 growth. So that the temperature of the surface waters is the one 

 principally concerned. 



In regard to the matter of season, it has been shown in the case 

 of organisms extending from the tropics toward the poles, that it is 

 the winter temperature which determines the limits of distribution^ 

 and consequently in the case of organisms extending from the poles 

 towards the tropics, it is the summer temperature that has the most 

 to do with limiting the range of the species. Of course we find 

 that there are several sets of species, the inhabitants of the coldest 

 waters, then of the less cold, and then even of the warm waters 

 but their distribution towards the equator is limited by the summer 

 temperature while their distribution toward the poles, if limited by 

 temperature at all, is limited in this case only, by the winter tem- 

 perature. 



In studying charts on which the temperatures of the surface 

 waters are given in both isotheral lines and isocrymal lines, it is 

 interesting to note the limitations of the different subtribes.^ 



The Laminariese, Alariese, and Agarefe do not descend below the 

 20° C. isothere except in a fe^v species which may extend over 

 toward the 25** line, while the most luxuriant growth of this portion 

 of the kelp-flora is found to be bounded towards the equator by the 

 10° isothere. In the northern hemisphere the waters about northern 

 Norway and of Behring Sea possess what is probably the greatest 

 numbers of individuals as well as species of these tribes and these 

 are within the isothere of 10°. In the southern hemisphere, Fuegia 

 which is wholly below the 10° line and Kerguelen Land and the 

 Crozets which are said by Hooker^ to be the " great nurseries " for 



' Cf. J. D. Dana, Crustacea of the U. S. Exploring Expedition, Pt. 2, p. 1452, 1852. 



2 The isotheral and isocrymal lines for surface waters are given in the " charts show- 

 ing the surface temperatures of the Atlantic, Indian, and Pacific Oceans," published 

 by The British Meteorological Council in ) 884, in degrees of Fahrenheit, and in Berg- 

 haus's " Physikahscher Atlas " (1892) in degrees of the centigrade scale. The temper, 

 atures used in this paper are of the latter. 



•^ Flora Antarctica, Pt. 2, p. 465. 



