89 



optimum temperatures lie below 50, and its greatest development 

 has taken place in minimum temperatures under the ice. Rising 

 floods and disturbed hydrographies conditions tend to reduce its 

 mimbers or to suppress its development, while declining floods 

 initiate increase in numbers and favor the appearance of pulses. 

 A "late" autumn delays the appearance of Synura. 



Not only are colonies of Synura found in the collections, but at 

 times large numbers of free cells make their appearance. These 

 are released by the breaking up of colonies, and occur in all degrees 

 of isolation. It seems to be a natural phenomenon, and occurs 

 most abundantly with or immediately after the crest of the pulse. 

 Thus the pulse of December 29 (1,999,500 colonies) was attended 

 by 21,600,000* free cells on that date. A week later there were 

 1,693,500 colonies and 57,600,000 free cells. There are in the rec- 

 ords several instances of meteoric increases of free cells at other 

 times than at those of apparent pulses. It does not seem possible 

 from the data at hand to determine whether this is due to environ- 

 mental influences or to the accidents of collection and subsequent 

 handling. In the discussions of Mallomonas and Syncrypta, sugges- 

 tions have been made that these organisms may be stages in the 

 life cycle of Synura. Synura is the largest and by far the most 

 important synthetic organism of the winter plankton. It shares 

 appreciably in the winter volumetric pulses as, for example, those 

 of December, 1898 (Pt. I., PI. XII.). 



Its fluctuations do not seem to produce any marked effect upon 

 the nitrates, possibly because the latter are present in excess of the 

 needs of Synura. In the winter of 1898 nitrates are high, 1.25 

 parts per million with the pulse of 1,999,500 colonies on November 

 29, but decline rapidly to .1 on December 13 with a fall of Synura 

 to 78,000. On December 20, Synura rises to 2,764,800, but the ni- 

 trates rise only to .35. It is evident that the nitrates are not the 

 only factor regulating the fluctuations of Synura. 



Marsson ('00) reports Synura as abundant in the winter plankton 

 of lakes about Berlin, and Brunnthaler ('00) finds it in the winter 

 plankton of the Danube. There is, however, no recorded instance 

 in which Synura forms so prominent a part of the plankton of a body 

 of water as it does of that of the Illinois River. It may be that a 

 closer analysis than has yet been given the potamoplankton of other 

 streams will reveal its prominence there also. It is present (Kofoid 



