28 



temperatures. The optimum conditions seem to lie above 60 

 and the maximum numbers to occur at or near 70. 



Pediastrum pertusum Kiitz. Average number of coenobia, 

 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 Cklorophyceaz 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 is 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 60to 80 (Pt. I., PI. IX. to XL). 

 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- 



