28 
temperatures. The optimum conditions seem to lie above 60° 
and the maximum numbers to occur at or near 70°. 
Pediastrum pertusum Kutz.—Average number of ccenobia, 
44,372. This species appears in the plankton in all months of the 
year and in almost all of our collections. It is the most abund- 
ant representative of the Chlorophycee which is retained by the 
silk of the plankton net, and is quantitatively an important 
factor in the ecology of the plankton. The numbers during the 
colder months, from November to April, when the water is from 
32° to 40°, are few, and the sequence of their appearance is fre- 
‘quently interrupted. As the temperature rises in April the num- 
bers increase, and the vernal pulse culminates in a maximum in May 
or June. There is no indication of the vernal pulse in the scattered 
collections of 1894. In 1895 the pulse is extreme, reaching 5,264,860 
on June 19, in a period of exceptionally low water. In 1896 a pre- 
liminary vernal pulse culminates May 8 at 23,580 and 1s followed on 
June 17 by one of 107,200. In 1897 the few spring collections do 
not reveal any vernal pulse, while in 1898 a minor one on May 17 
reaches 5,600, declines to 600 at the end of the month, and rises 
again to 56,000 by June 21. These vernal maxima all occur—or at 
least pass through their period of development—before the water 
reaches its midsummer temperature of approximately 80°. They 
develop during the transition from 60°to 80° (Pt. I., Pl. [X. to XI.). 
Autumnal pulses during the decline from 80° to 60° appear on Sep- 
tember 5, 1895, (105,996), on September 30, 1896 (9,200), on Octo- 
ber 12, 1897 (231,200), and on September 27, 1898 (259,200). In 
addition to these pulses there are others at irregular intervals during 
the summer: on July 30, 1894 (154,548), on July 2, 1896 (68,400), 
on August 15, 1896 (22,000), on July 14 (289,600) and on August 
31, 1897 (442,000), and on August 2 (295,200) and 30 (326,400), 
1898. 
The optimum conditions of development thus lie above 60°, 
and pulses are more frequent in spring and late summer or early 
autumn near 70°, though they appear somewhat less frequently 
during the summer in our maximum temperatures near 80°. The 
cause of these pulses is not conclusively demonstrable from the data 
at hand, owing in part to the interval between examinations. 
Daily examinations of the plankton and chemical analyses seem to 
be desirable for such demonstration. There are indications, how- 
