121 
lies primarily in the length of time afforded for breeding. The 
rate of the current is but one of the elements determining this 
time. If this be true, the expression of this relation should 
take another form, as, for example, the volume of plankton 
present in any body of fresh water varies with the length of 
time afforded for breeding. 
THE DISCHARGE. 
The total annual production of plankton in a given body of 
water can be estimated only when the total discharge for that 
time is known. The seasonal fluctuations of the discharge also 
profoundly affect the local and seasonal distribution of the 
plankton, and modify alike its quantity and its constituent or- 
ganisms. For these reasons the consideration of this element 
in the environment of the plankton is of prime importance. 
The discussion of the subject naturally falls under two heads, 
namely, the rainfall and the run-off. 
The Rainfall.—The political boundaries of [linois do not 
coincide with the watersheds of the Illinois River basin, though 
almost half the area of the state hes within this basin, which, 
moreover, extends through more than two thirds the length of 
the state, and is fairly typical of four fifths of its area. The 
small portion of the basin (4,287 square miles) which lies out- 
side of the state does not present conditions of rainfall which 
materially differ from those of parts of the basin within the 
state. The northern part of Indiana, in which the Kankakee 
basin lies, has a mean annual rainfall, according to Leverett 
(797), of 35.49 inches, which is somewhat less than the average 
(87.858 in.) for the whole State of Illinois; but since in I]li- 
nois as in Indiana the rainfall in the northern part is less than 
that in the southern part it is not improbable that the precipi- 
tation in the Kankakee basin in Indiana is about the same as 
that in corresponding latitudes of Illinois. Under these cir- 
cumstances, the statistics of rainfall for the whole State of LI- 
linois may be taken to represent with considerable accuracy 
the conditions within the basin of the Illinois River. 
