205 



PULSES OF POLYARTHRA PLATYPTERA continued. 



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. 3 , 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 XL), in a very unusual development of 

 Carteria and the smaller algae 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. 



