301 



The extreme limits are 12 and 49 days, and the average duration 

 is 29.9 days. 



From the data here presented it is evident that the pulses are in 

 the main from 3 to 5 weeks in duration, averaging approximately 

 29 + days a little less than one calendar month. 



The amplitude of the pulses is affected profoundly by seasonal 

 and local influences, such as the factors of temperature and chemical 

 constituents of the water, and the hydrographic conditions. These 

 have been discussed in connection with the volumetric-data in Part I. 

 and in the discussion of species in the first part of the present paper. 

 Rising, or even uniform, temperatures, hydrographic stability, 

 decaying vegetation or access of sewage or other fertilizing constitu- 

 ents, all serve to increase the amplitude of the pulses. Declining 

 temperatures, dilution or suspension of access of fertilizers, competi- 

 tion of gross vegetation, access of flood waters and increase in 

 current, all tend, in the main, to depress the amplitude of the pulses. 

 The duration of the pulses is not, however, thereby essentially 

 modified, though a tendency to override subsequent pulses and 

 partially, rarely wholly, to submerge them is at times of major pulses 

 often apparent in the data. 



The cause and significance of the phenomenon of recurrent pulses 

 is not clearly and unmistakably evident, owing, on the one hand, to 

 the irregularity of the data, and, on the other, to the great complex- 

 ity of the problem, especially in the fluctuations and varying 

 combinations of environmental factors. 



The plankton method itself is subject to great errors, but these 

 are largely distributed, and careful examination, especially of the 

 matter of dilution and computation, has failed to reveal any probable 

 or even possible source in the method to which these recurrent pulses 

 can be traced. 



It is not impossible that the rhythm here noted is merely a 

 chance outcome of the statistical method and without biological 

 significance; that it is wholly accidental, the resultant of the con- 

 flicting and varying factors of the environment and not predomi- 

 nantly or continuously initiated by any one factor. On the other 

 hand, its nature, as we have described it, is such that we are led to 

 look for some factor in the environment with which this rhythm of 

 repetition in growth of the plankton organism might be correlated, or 

 to some internal or inherent factor within the organisms constituting 



