513 



siderable in comparison with that from the other sources. 

 Wherever the river encroaches upon the bluffs, as, for instance, 

 below Havana, seepage areas of some extent and springs of 

 some size are to be found. Such banks, however, in immediate 

 contact with the main stream itself, are of very limited extent. 

 Furthermore, their contributions to the plankton are relatively 

 still smaller. These springy banks abound in life — planarians, 

 amphipods, isopods, oligochaetes, and rhizopods, mainly limic- 

 olous species, which rarely leave their habitat to enter the 

 river with the spring water. Such springy banks, exposed to 

 midsummer's heat during low water, do at times teem with 

 species common in the plankton. For example, a bank of this 

 sort on the levee at Havana was covered with a brownish scum 

 composed principally of Synedra acus, a diatom abundant in 

 the plankton at cooler seasons of the year. The temperature 

 of spring waters along the bluffs in midsummer is about 60°. 

 Elsewhere in the warmer waters of this springy shore are to 

 be found patches of green and red scum, where Euglena viridis 

 and E. sanguinea were abundant, both species being common in 

 the plankton at that time. Tiny rills of cool water traverse 

 the oozy bank and carry stray individuals of these various 

 species into the river, but their clear waters are poor in com- 

 parison with the brown water of the stream which they join, 

 turbid with plankton. Their contributions are thus insignifi- 

 cant in amount and, while adding a trifie to its diversity, their 

 main action is that of diluents of the potamoplankton. 



RELATION OF TRIBUTARY STREAMS TO CHANNEL PLANKTON. 



The relation of tributary streams to the potamoplankton in 

 the channel of the Illinois is a much simpler problem than that 

 presented by the backwaters. Their contributions enter the 

 river in well-defined channels, and the areas of their respective 

 basins are an index to the quantity of water they bring to the 

 river. Their share in the formation of the potamoplankton can 

 thus be more readily tested and estimated. Under conditions 

 prior to the opening of the Chicago drainage canal the river 



