539 
and more rapid in the tributary than in the main stream, and 
their relative effect upon the plankton is greater in the former 
than in the latter. 
How far the conclusions here drawn regarding the rela- 
tions of tributary waters to the main stream will hold true for 
other localities can be determined only by examination. The 
local conditions in this instance are in some respects peculiar. 
The large amount of sewage in the main stream and the rela- 
tively small size of the tributary have here enhanced the con- 
trast between their plankton content. The distinction between 
tributary and main stream in this instance is only an illustra- 
tion of a wider generalization, broached in the discussion of the 
relation of the current to the plankton. In all types and con- 
ditions of the environment, time for breeding is a fundamental 
factor, and inthe relation of tributary and main stream a con- 
trast in the length of time afforded for this development is evi- 
dent. Wherever this contrast occurs we may reasonably ex- 
pect, other things being equal, that the o/der stream water will 
support the more abundant plankton, and that it will also ex- 
hibit the greater diversification in its constituent organisms. 
The facts derived from the examination of the tributary 
waters of Quiver Creek and Spoon River indicate clearly that 
streams of this kind add but little to the total plankton of the 
main stream, and diversify it only by increasing the proportion 
of littoral species already present. Owing to the slight devel- 
opment of plankton in their waters, the immediate effect of 
their access isa dilution of the plankton per cubic meter of 
channel water. The initial steps in the sequence of plankton 
development in these tributary waters have been taken in the 
growth of the smaller algee and diatoms, but the degree attained 
is still below that in the channel. 
Channel plankton is therefore wot in any considerable de- 
gree derived from or maintained by the contributions of plank- 
ton from tributary streams of the kind here examined, and 
these are typical of most of the tributaries of the [linois not 
modified by industrial agencies. It is not merely the mingled 
