5 
conditions and food requisite for such development. This species 
appears in our collections in every month of the year, though in 
much smaller numbers and less frequently from November to 
April—that is, below 50°. Pulses of noticeable magnitude appear 
only above this temperature, and usually above 60°. 
Slight traces of vernal and autumnal pulses appear in the col- 
lections of the silk net in 1894-1896. In the filter-paper collections 
of 1897-1898 they are well defined. The vernal pulse appears in 
1897 on May 25 at 46,800,000, and in 1898 on May 10 at 70,200,000. 
The autumnal maximum in 1897 is remarkable both for its large 
numbers and its prolongation, culminating twice—first on Septem- 
ber 21 at 151,200,000, and again on October 19 at 154,800,000. 
This remarkable development, combined with the stable conditions 
and higher temperatures (Pt. I., Pl. XI.) of that low-water autumn, 
is responsible for the continuance of the species in our collections 
throughout the winter. In 1898 the species declined earlier, in 
November, and was but sparingly represented in collections of the 
winter of 1898-1899. As in other species of the genus and other 
Chlorophycee, midsummer pulses appear at intervals, often of four 
weeks, but ranging from three to six. In 1897 these occurred on 
July 14 at 55,800,000 and on August 31 at 21,600,000. In 1898 
they appear on June 28 at 10,800,000, on July 19 at 79,200,000, on 
August 9 at 39,600,000, and on August 30 at 54,000,000. At inter- 
vals between the pulses the numbers decrease, and in the regular 
collections of 1898 the minima between the pulses do not in any case 
exceed 30 percent. of the adjacent maxima, and are usually very 
much less. The distribution of the pulses of this species coincides 
very closely with that of the other species of the genus, and also with 
that of other Chlorophycee. For example, Pediastrum pertusum, 
the most abundant of the larger alge, has seven of its thirteen pulses 
on the same dates with those of Scenedesmus quadricauda and three 
others on adjacent dates, leaving but three which are not practically 
coincident. The operation of some common and general factor in 
the environment is suggested by such phenomena. 
The wide seasonal range of this organism gives it a claim to rank 
as a perennial planktont, though its quantitative distribution 
shows clearly that the optimum temperatures for its growth lie 
above 60°. The largest number recorded in 1897 appears October 
19 at a temperature of 65°, and in 1898 on July 19 at 84°. It is 
