289 
The hydrographic conditions and the location of the test 
in the stream are responsible for a large percentage of this va- 
riation. ‘Though the low-water levels cut off and reduce the 
diversifying action of impounding backwaters, the slight cur- 
rent minimizes the equalization due to mingling by the flow 
of the water in the channel, and, most of all, the location of 
the test just below the outlets of Quiver Lake and Spoon Riv- 
er (Pl. II.) involves the full effect of the diluent action of 
their relatively poorer waters. In Spoon River, on the day of 
the test, 3.12 cm.’ of plankton per m.* of water was found 
(Table IV.), while in Quiver Lake on October 1 there was only 
07 cm.* per m.* (Table V.). The discharge from Quiver Lake 
is reinforced by the seepage from springs along the eastern 
shore,and these diluents are probably the cause, to some extent, 
of the low plankton content in the two collections nearest 
the eastern shore—2.4 and 3.88 em.’ to an average of 6. for the 
ten collections. The effect of Spoon River isseen in the much 
smaller decline in the inshore collection on that side of the 
river. Combined with the diluent action of these plankton- 
poor tributaries may also be the effect of shoal water and the 
horizontal stratification of the plankton. 
If we eliminate from the test the two collections made in 
the marginal belt of spring-fed waters, 24 meters wide along 
the eastern shore, the +departures from the mean fall from 
+ 22.3 and +33.8 to +12.1 and +20.2. These latter figures 
more truthfully represent the variation in distribution of 
plankton in channel waters including four fifths of the width of 
the stream—a lateral extension far beyond the range in that 
direction of the mid-channel collections of our chronological 
series which form the basis of the conclusions of the present 
paper. 
The data concerning the local distribution of the plankton 
in the Illinois River in longitudinal and transverse directions 
presented in the preceding pages may be summarized as fol- 
lows: The average + departure from the mean longitudinal 
distribution in consecutive catches at the same point in the 
