438 



Ecology 



The double maximum of diatoms in general does not appear to be so 

 marked in the freshwater plankton as in the marine plankton, and the 

 double maximum of the same species, as instanced by A sterionella gracillima, 

 Gyclotella compta, Rhizosolenia morsa (W. & G. S. W., '12) and Fragilaria 

 crotonensis (Wesenberg-Lund, '08), is of great interest. 



A number of the freshwater diatoms are perennial constituents of the 

 plankton of certain lakes and can be collected in a living condition all the 

 year round. 



cc 



rr 



rrr 



SEPT. 



1.^0 



OCT. 

 9 



NOV. 

 7-2 



DEC. 

 3-2. 



JAN. 

 1-1 



FEB. 

 0-2. 



MAR 



0-4 



APR. 



1-7 



CC 



XANTHIDIUM SUBHASTIFCRUM v 



STAURASTRUM PARADOXUM 

 ASTERIONELLA GRACILLIMA 



---- MELOSlRA GRANULATA --- 

 ANAB/5-NA LEMMERMANNH oooooooo. 

 CERATIUM HIRUNDI NELL-A -- -- + -* + + 



Fig. 270. Chart showing the periodicity of six of the most abundant constituents of the plankton 

 of Windermere, in the English Lake District, from September 1907 to August 1908. The 

 temperatures are in degrees Centigrade. 



In the European lakes the most notable forms are Asterionella, with a 

 range of form and size which embraces both A. formosa (fig. 73 A) and 

 A. gracillima, and the two species of Tabellaria. On the whole, T.fenestrata 

 is much more abundant than T. flocculosa, except in the English Lake 

 District. The chain-forms of T. fenestrata are the most frequently observed, 

 but the star-dispositions of the frustules are common except in the lakes of 

 North Wales. T. fenestrata var. asterionelloides (fig. 73 B) is one of the 

 dominating features of the late spring, the summer and the early autumn 



