1908.] The British Freshwater Phytoplankton. 



195 



especially by the Desmidiaceae. Apart from the latter, Botryococcus Braunii 

 and Sphcerocystis Schroeteri are the most general and abundant. Species of 

 Oocystis are frequent, but never occur in quantity. Dictyosphwrium 

 pulchellum often occurs abundantly, but it also at times occurs in equal 

 abundance in bogs. 



Eudorina elegans is fairly general, even in large lakes such as Lough Corrib, 

 although it reaches its maximum abundance in small lakes. 



Pediastrum Boryanum and P. duplex are frequent in the plankton of the 

 shallower lakes, but P. simplex is very rare. Several species of Ccelastrum, 

 Scencdesmus, and Crucigenia occur in many of the lakes, but never in 

 quantity. 



Species of Zygnema, Spirogyra, and Mougeotia occur in the plankton of 

 most of the lakes, principally in the late spring and summer. They are 

 usually the slender species of these genera, and are almost invariably sterile. 

 In the smaller alpine lakes Mougeotia is often abundant, and forms no small 

 part of the phytoplankton.* The curious coiled Mougeotia-fH&ments of some 

 of the Scottish lochs have already been referred to. It would appear that 

 the coiling is a limnetic character.^ developed to augment the floating- 

 capacity of the filament, and the fact of its presence is direct evidence that 

 some of these solitary filaments of Mougeotia are adapting themselves to a 

 life in the plankton. 



The most interesting feature of the British freshwater phyto- 

 plankton is the dominance of Desmids. In 1903, and again in 1905, we 

 showed that in contrast to any previously known plankton that of the 

 Scottish lakes was unique in the abundance of its Desmids. Since then we 

 have found that this dominance of Desmids is not confined to the lochs of the 

 Scottish Highlands, but is a feature of the plankton of the four lake-areas of 

 the British Islands, and that the plankton of the western British lake-areas 

 differs markedly from all other European plankton in the abundance of its 

 Desmids.% 



* In the alpine lakes of the Pike's Peak Eegion, Colorado, Shantz states that species of 

 Spirogyra and (Edogonium form a large part of the summer plankton {vide 'Anier. 

 Microscop. Soc. Trans.,' March, 1907). Fragmentary filaments of various species of 

 QUdogoniurn are also very frequent in the summer plankton of the British lakes. 



t Consult G. S. West in 'Linn. Soc. Bot. Journ.,' vol. 38, 1907, pp. 85 and 86. 



% Among European lakes, only those of Norway and certain parts of Sweden approach 

 the British lakes in the possession of a conspicuous Desmid-flora in the plankton (consult 

 Huitfeldt-Kaas, loc. cit., 1906 ; and Lemmerniann, in 'Archiv for Bot. utgiv. af K. Sv. 

 Vet.-Akad.,' Bd. 2, 1904), and it should be mentioned that many of these plankton 

 Desmids are identical with the British ones. Tanner- Fullemann has recorded the 

 occurrence of a number of Desmids in the plankton of the Schoenenbodensee (vide ' Bull, 

 de l'Herb. Boissier, 2me ser., t. 7, 1907), but the species which he records, when it is 



