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 7 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 
