CHAP, xxxvui PLANKTON-FORMATION 161 



concerning the matter ; in temperate regions the associations during 

 the cold season are identical with those occurring during summer in 

 the arctic region, but in the warm season they are different. Diatomaceae 

 dominate in the temperate region, except in autumn (the warmest season, 

 with a temperature of about 20 C.) when Peridineae or, in brackish 

 water, Cyanophyceae (Nodularia) may be the characteristic plants. 

 As regards the arctic neritic plankton it is known that during spring- 

 time Diatomaceae congregate in numbers on the under-surface of the 

 ice, 1 but that when the ice disappears pelagic diatoms and yellow Flagel- 

 lata dominate. 



Oceanic or pelagic plankton, i.e. plankton of the open sea, mainly 

 consists of Peridineae and Coccolithophoridae, but also of Cyanophyceae, 

 Diatomaceae (relatively few species), and Halosphaera. It includes a 

 large number of species of Peridineae and Trichodesmium in the tropics, 

 and of Peridineae and Diatomaceae in temperate and arctic regions. 



These terms * neritic ' and ' pelagic ' or ' oceanic ' plankton approxi- 

 mately correspond to Haeckel's ' neroplankton ' and ' holoplankton ' 

 respectively ; as most species belonging to neritic plankton spend only a 

 part of their life as pelagic organisms, and during the other part are 

 associated with the soil as spores and the like ; whereas pelagic forms 

 always occur as organisms floating free in the water. 



Among oceanic species resting spores are unknown, but some 

 species, including Rhizosolenia styliformis, produce microspores that are 

 apparently allied to auxospores. 2 



The depth to which phytoplankton descends in the sea differs 

 according to various conditions, such as transparence of the water and the 

 like ; in general it may be asserted that phytoplankton occurs to a depth 

 of 200 metres, but that even at 100 metres its amount is small 3 : diurnal 

 migrations are unknown among phytoplankton-organisms. 



In recent years investigators 4 have established a number of associa- 

 tions in phytoplankton, which are brought into existence by difference 

 in temperature, and salinity of the water ; but very little is known 

 of their oecology. Cleve has given names to them in accordance with 

 their dominant genera or species, thus : tricho-plankton (after Thalassi- 

 othrix), styli-plankton (after Rhizosolenia styliformis), chaeto-plankton 

 (after Chaetoceras), sira-plankton (after Thalassiosira), tripos-plankton 

 (after Ceratium tripos). 



B. Limnoplankton. Fresh- water Plankton. 



This form of plankton in fresh water is constituted of autophytic 

 species. In large lakes it may be differentiated into two zonal sub- 

 divisions : the pelagic in the open water, and the neritic near the shore. 



Other subdivisions may be recognized, such as potamoplankton, 

 heloplankton, and probably several more. 



Limnoplankton appears to be one of the most cosmopolitan of for- 

 mations. Concerning that in arctic and tropical countries we are almost 

 entirely ignorant, and it is only in regard to temperate Europe and 



1 Vanhoeffen, 1897. * Gran, 1902; G. Karsten, 1905-7; P. Bergen, 1907. 



1 Compare previous remarks concerning shade-flora. See G. Karsten 1905. 

 * P. Cleve, 1897, I 9 I 5 Gran, 1900, 1902 ; Ostenfeld, 1898-1900. 



WARMING Vf 



