27 
which have been described 1n this genus. This genus includes the 
most abundant of the larger alge 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%., 
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 Novernber, 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 vear, 
and the autumnal pulse of September 5 is but 10,600, and 1s followed 
by a secondary one on November 27 of 4,081, perhaps as a result 
of the stable conditions and the abnormally high temperatures 
lalgome. +5. wince ethen prevailed (Pt. I; Pl. 1X). 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. I., Pl. X). In1897, 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 
