44 



temperatures below 40. The data .are insufficient to trace the 

 pulses satisfactorily. This species is distinguished with difficulty 

 from A. gracillima, and may include only old, and in our planktons 

 often heavily incrusted, individuals; or it may be only a low-tem- 

 perature variety of the species above named, which in the grand 

 total of all our collections outnumbers it ten thousand to one. 



Asterionella gracillima Heib. Average . number of individual 

 cells, 28,860,160. In 1897 the species was only one third as abun- 

 dant, a contrast which finds its explanation in the fact that the 

 June rise of that year (Pt. I., PI. XI.) did not reach the stage of 

 overflow, and a June pulse is absent in the collections of that year. 

 The seasonal distribution of this organism is one of the best-defined 

 and most striking of all the components of the river plankton. It is 

 peculiar in the fact that it appears in numbers only during spring and 

 the beginning of summer, and in the absence of any autumnal pulse 

 upon the return of the temperatures in which the spring pulse ap- 

 peared. This species was recorded in every month of the year but 

 October, but always in small numbers after July 1. In 1894, collec- 

 tions were not commenced until after the time of the spring pulse. 

 In 1895 the spring collections were few, and at intervals so great as to 

 preclude the detection of the full course of the spring pulse. The 

 maximum number in the collections 'of that year appears April 9 at 

 1,203,100 and falls to 445,995 on April 29 which is approximately 

 the time of the maximum of subsequent years. This was a year of 

 unusually low water during the spring, and overflow stage was at no 

 time reached (Pt. I., PL IX.), which may account for the apparent 

 suppression of the spring pulse. The species does not reappear in the 

 collections of that year until December, but it continues in small 

 numbers (less than 5,000 per m. 3 ) until the end of March, 1896, when 

 there is a rapid increase which culminates April 24 at 26,281,400. It 

 disappears entirely from the records at the end of a fortnight, and save 

 for a single entry in June and two in September it does not again 

 appearin 1896. In 1897 the culmination of thespring pulse occurs April 

 27 at 324, 633, 600 three hundred-fold larger than in the previous 

 year. There is a normal March flood (Pt. I., PI. XL), on the declining 

 stages of which this pulse appears. With the close of June the 

 species disappears from the records. The June rise does not reach 

 the stage of overflow, and the scanty records show but this single 

 pulse throughout the year. Beyond a single entry in August and in 



