West: Phytoplankton of English Lake District. 193 



occurrence of this species in the lakes of western Europe. In 

 the plankton of an Australian lake, on the other hand, M. 

 granulata was found to reach its maximum abundance in the 

 middle of the warm period with a water-temperature of 2i°C.* 

 There is no doubt, however, that the Melosira phase of the 

 Windermere plankton is from January to April or May (con- 

 sult text-fig. 3), and this agrees with a similar M^/os^Va-plank- 

 ton described by Lemmermann as occurring in the ' Zwis- 

 chenabner Meeres' from January to April. f In the Ploner 

 See the same author also J records a first Diatom period from 

 January to July, and a second one, consisting principally of 

 Melosira distans (a closely related form to M. granulata) from 

 December to January. 



Asterionella gracillima has a great maximum in May and 

 June (temp. 4.4 — 8.3°C.) at which period it completely domi- 

 nates the plankton (consult PI. VII. and text-fig. 3), and a 

 second maximum, more prolonged but not so great, from 

 November to January (temp. 7.2 — i.iX.). This double 

 maximum of Asterionella, first in spring and then in autumn, 

 appears to be fairly general in deep lakes, and Wesenberg- 

 Limd § also remarks upon a similar double maximum of 

 Fragilaria crotonensis in the Danish lakes. 



In investigating the periodic appearance of Asterionella 

 in the reservoirs which supply the city of Boston with water, 

 Whipple II suggested that the maxima were due to disturbing 

 influences having raised up quantities of individuals from the 

 bottom to the limnetic region (either in the vegetative con- 

 dition or in the form of spores), where they found suitable 

 conditions for rapid multiplication. He regarded summer and 

 winter as two periods of stagnation, whereas the spring and 

 autumn were largely periods of storm, with the necessary 

 conditions for disturbing the water and setting up convection 

 currents such as would raise this bottom-flora to the surface. 



* G. S. West in ' Journ. Linn. Soc. Bot.', 1909, XXXIX., p. 21. It 

 seems quite likely that under the general name ' Melosira granulata,' 

 more than one form is included, these forms attaining their maxima under 

 different conditions of temperature. 



f Lemmermann in ' Ber. Deutsch. Botan. Ges.', XVIII. , 1900, p. 140 



X Lemmermann in ' Forschungsb. Biol. Stat. Plon.', X., 1903, p. 126. 



§ Wesenberg-Lund, ' Plankton Investigations of the Danish Lakes, 

 Copenhagen,' 1908, p. 50. 



II G. C. Whipple in ' Technol. Quarterly,' VII., 1894 ; Whipple and 

 Jackson in ' Journ. of New England Waterworks Association,' XIV,, 1899, 



[To be continued). 



1909 May I. 



N 



