559 



our own, raises a doubt as to the value of any conclusions de- 

 rivable from such data. Steuer ('01) has suggested that the 

 geographical position of the bodies of water in a large measure 

 determines the character of the planktograph. I should pre- 

 fer rather to put the emphasis upon temperature, w^hich is not 

 everywhere merely a matter of latitude, and also to insist upon 

 the dominance of purelj/ local eondifions over those more gener- 

 ally operative, such, for example, as temperature, in determin- 

 ing the amplitude of the movements of the planktograph and 

 the general position of seasonal maxima. Illustrations in sup- 

 port of this view can be found in our own records — for example, 

 in Phelps and Quiver lakes. These are bodies of water within 

 three miles of each other, and with quite similar temperatures, 

 yet their planktographs are in some years quite as different as 

 any of those whose difference Steuer seeks to explain by lati- 

 tudinal positions. Furthermore, the " Sommerschlaf " which 

 he predicates as probable in tropical waters is least of all evi- 

 dent in Phelps Lake, the warmest of all our localities, where 

 temperatures during midsummer approximate those of the 

 tropics. More chronological series of collections at brief in- 

 tervals, not exceeding a fortnight, from many localities are 

 needed before general conclusions of permanent value concern- 

 ing the seasonal course of plankton production will be possible. 

 The )}i((x'nnuiu pjvd/frfioH has been used by some writers as 

 a t>asis of comparison of plankton production in different 

 bodies of water. It doubtless has a slight value in suggesting 

 the relative productivity of waters, though it would seem that 

 an annual average of weekly or fortnightly collections would 

 be very much more accurate. Difficulty attends these com- 

 parisons when deep and shallow waters are brought into con- 

 trast. If the volume of plankton under one square meter is 

 made the basis the shallow waters are at a disadvantage, while 

 if the amount per m.'* is made the basis the more barren deeper 

 strata reduce the average plankton content of deep w^aters to 

 a relatively small figure. Since, however, all strata, at least of 

 most bodies of fresh water, are productive of plankton, it 



