8o2 



ZONES AND REGIONS 



[Pt. Ill, Sect. V 



requires less than a year. In general, here also, vegetative activity prevails 

 in summer, and reproductive activity in winter, the latter at a temperature 

 from —i° to —2° C. 



In spite of their connexion the various portions of the polar seas possess 

 distinct algal floras, the differences in which can only partially be accounted 

 for by existing causes, such as milder temperature (west coasts of Greenland 

 and Norway), unequal salinity (the Siberian Sea is poor in salt). Kjellman 

 distinguishes three chief districts, namely, those of Spitzbergen, of Siberia, 

 and of America. The dominant Laminariaceae in these districts are in part 

 different species. 



Fig. 491. Lithothamnion glaciale. Two-thirds natural size. After Kjellman. 



2. THE PELAGIC PLANKTON 1 . 



The vegetable kingdom is represented in the pelagic plankton chiefly by 

 Diatomaceae, Peridinieae, and Cyanophyceae. Diatomaceae are present 

 everywhere, and are equally prominent by reason of the number of forms 

 and of individuals. The Peridinieae in cold seas are rich in individuals, in 

 warm seas rich in forms. The Cyanophyceae, chiefly Oscillariaceae, are 

 very abundant in warm seas. 



A few other classes of Algae occur in plankton, but are usually less frequent 

 or more local. Two species of the genus Halosphaera (Protococcaceae) are not 

 devoid of significance in the plankton of warm seas. True Flagellatae are represented 

 in cold seas by species of Dictyocha among others ; the Pyrocysteae, a group of 



1 Schiitt, I and II. 



