1908.] 



The British Freshwater Phytoplankton. 



193 



forming only an average of 11 per cent, of the entire phytoplankton. This 

 low percentage is probably due to the small numbers of adventitious species 

 washed into the lakes from the mountain sides, and is possibly accentuated 

 by the stony character of the lake margins and lake bottoms. They occur in 

 the greatest variety in the Scottish and Irish lakes, probably owing to the 

 large number of adventitious species washed into the lakes by the rains. 

 The Pennate Diatoms are much the most numerous and conspicuous. 



Among the commonest forms are Asterionella (with a range of form and 

 size which embraces both A. formosa and A. gracillima) and the two species 



Fig. 3. — Peculiar Form of Ceratium Mrundinella, 0. F. M., in which the first antapical 

 horn («£]) is greatly deflected to one side, x 200. 



of Tabellaria. T. fenestrata is much more abundant than T. flocculosa, except 

 in the English lakes, where the reverse obtains. The chain-forms of 

 T. fenestrata are the most frequently observed, but the star-dispositions (var. 

 asterionelloides) are common except in the Welsh lakes. T. fenestrata,, var. 

 asterionelloides, is one of the dominating features of the late spring, the 

 summer, and the early autumn plankton of many British lakes, and it 

 exhibits great variability in the relative strength and breadth of the girdle 

 view of the cells.* That these differences are of no varietal importance is 

 proved by the occurrence of all intermediate stages. 



We have not observed any star-dispositions of T. flocculosa in any of the 

 British lakes, although such have been observed by Holmboe in Norway and 

 by Wesenberg-Lund in Denmark (T. flocculosa, var. pelagica, Holmboe). 



The numerous plankton-forms of Asterionella almost convince one that 

 A. formosa and A. gracillima are merely states of the same species. No 

 chains of Asterionella, were observed in any of the lakes. 



The genus Fragilaria, is somewhat rare, and of the species which occur 

 F. capucina is the commonest. F. crotonensis is very general in Scotland 



* Cf. W. and G. S. West, in 'Roy. Soc. Edin. Trans.,' vol. 41, 1905, Plate 2, figs. 1—3. 



