205 
PULSES OF POLYARTHRA PLATYPTERA—continued. 
Year Date Temp. No. Date Temp. No. Date Temp. No. 
| 
1894 Oct. 17 58° ie eNO) |) —= aa SSS See —== 
1895 Oct. 23 5c 408 | Nov. 27 33° 74,942 Dec. 18 39° 21,147 
1896 soos === = Dec. 29 35 37,560 
1897 Oct.s5 file 816,000 | Nov. 15 47° 22,400 | Dec. 14 40° 7,300 
1898 Octo 11 65° 47,500 | Nov. 22 40° 6,000 | Dec. 20 33° 63,400 
sot’) 49° 37,500 
An examination of this table and the graphic presentation (PI. 
V.) of the seasonal distribution will show at once the uniformly 
small numbers attained at low temperatures. Between October 
15 and April 15, that is below 60°, no pulse exceeding 100,000 is 
reached save one of 122,400, February 21, 1899, at 33°. Of all the 
records in this period only seven exceed 50,000. On the other hand, 
during the warmer months, above 60°, the pulses have a much 
_ greater amplitude. Four of them exceed 400,000, and there are 
~ twenty-two records above 100,000. The summer pulses are often 
separated by minima which approach midwinter levels, but in spite 
of this the general level of summer occurrences is much higher than 
that of the colder season. In 1898 the average from April 15 to 
October 15 was 30,861 per m.’, and for the other months of the year, 
15,813, or about half the number in the warmer season. From 
these facts of distribution it is apparent that though perennial the 
species finds its optimum conditions at temperatures above 60°. The 
statement of Hempel (’99) that it thrives best in cold water is not 
borne out by the statistical examination in any of the years. 
The recurrent pulses of this species vary greatly in amplitude. 
The largest pulse recorded was that of 816,000, October 5, 1897, at 
71°. It appeared in a period of prolonged low water and at the 
close of one of high temperatures continued beyond the usual 
september limit (Pt. I., Pl. XI.),in a very unusual development of 
Carterta and the smaller algee of the water-bloom (Pl. II.). Similar 
autumnal pulses do not appear in other years, the autumnal develop- 
ment as a rule not exceeding to any noticeable degree that of mid- 
summer. There has been in every fully tested spring a large vernal 
pulse, usually at the time of the spring volumetric maximum, or 
thereabouts. In 1896 and 1898 it was the largest pulse of the year. 
