Limnoplankton 437 



deflected first antapical horn which so far is only known from a few lakes in 

 the Outer Hebrides and the west of Ireland ( W. & G. S. W., '06 ; '09 B). In 

 some lakes seasonal form -variations have been observed, such as those described 

 by Wesenberg-Lund ('08) from the Danish Lakes, but these variations have 

 not been observed in the British lakes, although several forms frequently 

 occur simultaneously in the plankton of one lake. 



Ceratium cornutum is another summer type, but of less frequent occurrence. 

 It may occur in very small numbers in lakes in which G. hirundinella is 

 abundant, but more often it is found in those lakes, generally with very pure 

 water, from which G. hirundinella is absent. It is common in the Welsh 

 lakes and in the lake-area of Carnarvonshire it occurs in the well-aerated 

 Sphagnum-bogs. 



Many species of Peridinium are characteristic of lake-plankton. In 

 European areas P. cinctum and P. Willei (fig. 44) are the most conspicuous, 

 the latter being a summer form with a maximum at some period during the 

 warmer months ( W. & G. S. W., '09 A). As in the case of Ceratium hirundinella, 

 Peridinium Willei is a perennial constituent of the lake-plankton of more 

 southern latitudes. Species of Peridinium, like many diatoms, do not attain 

 a universal maximum at one definite period of the year, but the various 

 species reach their greatest vegetative development at different times of the 

 year (W. & G. S. W., '12). In some of the pools of the English Midlands 

 there is a summer species (a var. of P. cinctum), a spring species (P. anglicum; 

 fig. 51), and a very early spring almost a winter species (P. aciculiferum ; 

 fig. 50), each of which has been shown to form resting-cysts at the close 

 of the vegetative period, even though the vegetative periods are all at 

 different seasons. The observations on the Peridinieae of Bracebridge Pool 

 in Sutton Park, Warwickshire (consult G. S. W., '09 A) show that temperature 

 is really an important factor in the occurrence of each species. 



BACILLARIE^E. Only in the more contaminated lakes (i.e. those with 

 a relatively high percentage of dissolved salts) do diatoms attain great 

 maxima. Vast maxima occur periodically in the central European Lakes, in 

 the Danish lakes and in a few of the British lakes ; also in Tanganyika. The 

 Pennate Diatoms are much more numerous and for the most part much more 

 conspicuous than the Centric Diatoms. In lakes with very pure water, such 

 as the Carnarvonshire lakes, the plankton contains very few diatoms. The 

 evidence at present available shows that, although many of the plankton- 

 species of diatoms occur in greatest quantity in the spring, some of them 

 attain their maxima in the summer and autumn, and several of them have 

 a double maximum, one in the spring and the other in the autumn. An 

 instance of the latter is afforded by Asterionella gracillima in Windermere 

 (W. & G. S. W., '09 A), in which the spring maximum is of greater bulk but 

 not so prolonged as the autumn maximum (consult fig. 270). 



