511 



The normal regimen is accordingly not delineated by a 

 planktograph of marked definiteness in its course, but is one 

 formed by a sequence of recurring pulses of approximately a 

 month's duration and varying amplitudes, low in winter floods, 

 rising with the temperature to a vernal maximum of consider- 

 able magnitude, often, but not always, declining during the 

 summer months, frequently rising again in late summer or au- 

 tumn, and in some localities and years to an extent exceeding 

 that of the vernal season, and falling with autumn tempera- 

 tures but increasing in stable winter conditions after the min- 

 imum winter temperature is reached, — such is the general reg- 

 imen of plankton production in the Illinois River and its back- 

 waters. 



If this cyclic movement in production be characteristic of 

 the plankton generally, fresh water and even marine, it must 

 follow that scattered and irregular collections, or those at in- 

 tervals exceeding a week or at most a fortnight, may fail en- 

 tirely to give an adequate representation of the course of pro- 

 duction or relative fertility of a body of water. Chronological 

 series throughout the whole seasonal range of climatic condi- 

 tions and at close intervals — of one week or less — are neces- 

 sary for any accurate delineation of production and fertility 

 of water by the plankton method. 



SOURCE AND MAINTENANCE OF THE P0TAM0PLANKT0N. 



The existence of a very characteristic and abundant plank- 

 ton in the Illinois River at once raised the question as to its 

 source and maintenance. We find at Havana a stream which 

 year in and year out carries by a burden of life, microscopic as 

 to its individuals, to be sure, but in the aggregate a volume of 

 surprising extent. This stream of life exhibits a routine of 

 seasonal changes in constitution and quantity which neither 

 flood, the ice of winter, nor the drouth of summer wholly inter- 

 rupts. It recurs year after year with a regularity which stands 

 in strong contrast with the fluctuations of the environment. 

 Although the water in the stream is subject to continuous re- 



