503 



environmental factors might be treated under this head. But 

 this has not been my method nor is it now my purpose to 

 adopt it. 



The phenomena of growth and reproduction of the con- 

 stituent organisms of the plankton, on the other hand, owing 

 to our ignorance of their controlling factors, can at present be 

 treated only under this head. The volumetric data in them- 

 selves contain little evidence bearing directly upon the prob- 

 lem, but in the light of the statistical results the fluctuations 

 in the plankton become dependent upon fluctuations in the 

 rate of growth, and especially in that of the reproduction of its 

 constituent organisms. These fluctuations are often concur- 

 rent, or, at most, shortly consequent, in many species at the 

 same time and in several different localities, and give rise to 

 the coincident volumetric pulses to which attention has so 

 often been called in the preceding pages. Somewhat regular 

 alternations of growth and rest, of fission and spore formation, 

 or of parthenogenesis and sexual reproduction, are funda- 

 mentally the basis of the cyclic movement in production. The 

 amplitudes, and to some extent the location and duration of the 

 pulses, are plainly affected by the various factors of the envi- 

 ronment discussed in preceding pages — by light, temperature, 

 vegetation, tributary water, various hydrographic factors, and 

 by food supply, and, possibly, also, by chemical conditions not 

 directly concerned in nutrition, but the available data fail com- 

 pletely to afford any satisfactory environmental factor or group 

 of factors which stands in correlation, even remotely obvious, 

 with this cyclic movement in production. T therefore class 

 this periodic growth, these sexual cycles which cause volumetric 

 pulses, under the head of internal factors. The element of 

 periodicity in itself does not seem to be consequent upon any 

 known external factor. 



NORMAL REGIMEN OF PLANKTON PRODUCTION. 



The records of plankton production in the Illinois River, 

 its tributaries, and backwaters, contained in this paper raise 



