27 



which have been described in this genus. This genus includes the 

 most abundant of the larger algae in the plankton of fresh waters, 

 and it affords an attractive field for the study of variation by statis- 

 tical methods and for the determination by the experimental method 

 of the effect of environmental changes upon structure. The two 

 groups of individuals included here under P. boryanum and P. 

 pertusum give typical curves of seasonal distribution which are so 

 similar that their combination in a single series would not greatly 

 modify the resultant seasonal curve. In the sum total of all 

 collections P. boryanum (1,034,000) includes about one tenth of the 

 number referred to P. pertusum (10,830,117). 



A few scattering individuals, generally less than 1,000 per m 3 ., 

 appear at irregular intervals, during the colder months, from the 

 first of December until the end of March. The number increases as 

 the temperature rises, and the species appears in all collections un- 

 til November, when it again becomes irregular in its occurrence in 

 the plankton. The fluctuations in numbers during this period are 

 very marked, the pulses of frequency being set off by intervals in 

 which the numbers are small. A slight pulse of 2,120 appears on 

 November 17, 1894. In 1895 the vernal pulse attains the very 

 unusual number of 572,824 in the unusually low water of that year, 

 and the autumnal pulse of September 5 is but 10,600, and is followed 

 by a secondary one on November 27 of 4,081, perhaps as a result 

 of the stable conditions and the abnormally high temperatures 

 (above 45) which then prevailed (Pt. I., PI. IX). In 1896 the 

 vernal pulse culminates May 18 at 31,164, while the autumnal pulse 

 is scarcely visible and the numbers throughout the summer are 

 small, as a result, it may be, of the repeated floods of that year 

 (Pt. L, PI. X). In 1897, with few vernal data, the vernal pulse does 

 not appear, though a rise to 8,000 occurs on July 21. The major 

 autumnal pulse culminates on September 14 at 14,400, and another 

 one on October 12 at 6,000, attending the late autumn of that 

 year. In 1898 there are vernal pulses on May 10 of 6,400 and on 

 June 14 of 32,000. The autumnal pulse on September 27 reaches 

 the considerable number of 65,600. In the winter of 1898-99 

 Pediastrum was seemingly absent from the plankton. The pulses 

 are thus somewhat irregular, though there is in this species a 

 suggestion of vernal and autumnal pulses at corresponding 



