a 
THE PERIODICITY IN THE MULTIPLICATION OF THE ORGANISMS OF 
THE PLANKTON. 
One of the most obvious conclusions brought to light by the 
detailed study of the volumetric fluctuations of the plankton pub- 
lished in Part I. of this report, and most strongly reinforced by the 
statistical data showing the fluctuations in the numbers of the indi- 
viduals of the various species and in the sums total of the various 
biological groups represented in the limnetic fauna and flora, is that 
plankton production is fundamentally rhythmic or periodic in 
character, viewed either in its constituent elements or as a whole. 
This total result is simply the sum of a like phenomenon pervading 
more or less completely and coincidently the reproductive cycles, 
the rise and decline in the numbers of the typical constituents of 
the plankton. The exceptions to this rhythm are usually found in 
those organisms which are adventitious in the plankton and have 
their centers of growth and distribution in other regions than the 
open water. 
Many illustrations of this periodic movement in the multiplica- 
tion of organisms of the plankton have been cited in the preceding 
pages and may be seen in the accompanying plates. As an illustra- 
tion for discussion in detail we may take the pulse of July, 1898, 
shown in the volumetric data of Table III. and Plate XII. of Part I. 
The fluctuations in the biological population during this period are 
also tabulated in Table I. of this paper, and graphically presented 
in Plates II. and IV., which exhibit the movement in the totals of 
the Chlorophycee, Bacillariacee, and chlorophyll-bearing Masti- 
gophora, and of the Rottfera and Crustacea. 
In the volumetric data the pulse rises from a minimum of .14 cm.* 
per m.%on July 5 to a maximum of .88 cm.’ on the 19th, declining 
again on the 26th to the second minimum, of .67 cm.? Its duration 
is thus four weeks and its amplitude, in comparison with many 
other pulses in the records, relatively slight. It occurs in the more 
stable conditions of declining river levels and midsummer tempera- 
tures. The following list gives the names of the more or less typical 
planktonts considered in the discussion of this pulse. Others, 
largely adventitious or insignificant in numbers, might be added 
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