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 in the 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 older 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 is a 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 alga? and diatoms, but the degree attained 

 is still below that in the channel. 



Channel plankton is therefore not 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 Illinois not 

 modified by industrial agencies. It is not merely the mingled 



