1920] Allen: Plankton of the San Joaquin River 49 



Discussion of Species 

 Asterionella gracillinia Heib. 



Station I Station II Station II] Daily Hourly 



Average 344,378 1,743,406 1,381,583 3,516 12,206 



Identification certain. Records show numbers of colonies. Average 

 size of colonies at Station I was three, at Station II four, at Station 

 III three. Two forms were noted, a typical and a large form. They 

 were recorded separately in tables 1 to 5 though the above averages 

 are for the two combined. As in the case of Actinasirum the two were 

 alike except for size but the larger was usually only about 50 per cent 

 larger, rarely twice as large. There were sometimes all gradations in 

 size but in most cases the distinction was plain. It will be noticed by 

 reference to the tables that the pulses ran somewhat the same with 

 both forms at all stations but that the large form appeared rather 

 late at all stations and that it was only prominent in May and June 

 at Stations I and II, June at Station III. Perhaps this condition 

 warrants the inference that the large form is favored by a tempera- 

 ture above 20° C. especially since the June maximum comes at the 

 highest temperature (26?5 C. and 29° C. respectively at Stations II 

 and III) and very nearly so at Station I (22° C). 



Both forms were practically absent at all stations through July, 

 August, September and October. Both reappear in November. The 

 typical form is abundant at all stations except during the four months 

 just mentioned. At Station I it showed a strong pulse in January and 

 a maximum pulse in March. Another strong pulse came in December. 

 At both of the other stations the maximum pulse came in February and 

 a very strong pulse in December. If both forms be counted together, 

 Station II is seen to have almost as large a pulse in June as in Febru- 

 ary. The combination also shows a very large pulse in June at Station 

 III. The abrupt disappearance of both forms in July at all stations 

 must be due to some other factor than temperature since the tempera- 

 ture change is neither abrupt nor very marked. Stagnation may have 

 a strong deterrent influence. There are fairly well marked, recurrent 

 pulses at four to six weeks intervals at all stations. 



Very few single cells of Asterionella were recorded. This can be 

 accounted for in two ways: first, escape through the net; second, con- 

 fusion with other single cells. 



