89 
optimum temperatures lie below 50°, and its greatest development 
has taken place in minimum temperatures under the ice. Rising 
floods and disturbed hydrographic conditions tend to reduce its 
numbers 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 Syucrypta, sugges- 
tions have been made that these organisms may be stages in the 
lite 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. -1., Pl. XIT.). 
Its fluctuations do not seem to produce any marked effect upon 
the nitrates, possibly because the latter are present 1n 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,Syunura 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 
