532 
bly a fairer statement. The number of species noted is 20 and 
49 respectively. The main stream has a greater variety of the 
smaller Chlorophycee and of the blue-green alge, while in both 
individuals and species the desmids are well represented in the 
tributary. Littoral species predominate in the latter, though 
limnetic forms also occur. It is significant that the alge, pri- 
mary links in the chain of food relations, have already attained 
a considerable development in the tributary waters. 
The diatoms present the most striking contrast in the table, 
the ratio being 1 to 78. This great disproportion is caused by 
the larger numbers of certain limnetic diatoms, notably Melo- 
sira, Asterionella, Fragilaria, and Synedra, in the plankton of 
the main stream. These are present in the tributary, but only 
in smaller numbers, while species of littoral habitat present in 
both streams are relatively more abundant in Spoon River. 
The contrast between Quiver Creek and Spoon River in the pro- 
portions of their diatom flora is significant. In the creek the 
shores and bottom are more immediate and effective features 
of the environment than they are in the larger tributary, hence 
we find there that the diatoms are largely littoral forms, and 
they and the Rhizopoda which feed upon them constitute 
almost the total plankton. In Spoon River the same species 
occur, though data are lacking for quantitative comparisons 
with Quiver Creek. These littoral species are, however, over- 
shadowed by the development of otherand more typical plank- 
ton organisms, so that they constitute here a smaller relative 
proportion of the planktonts than they do in Quiver Creek. 
The littoral species find a place for development along the shores 
and bottom of the tributary streams, and by reason of the pro- 
tection there afforded, or of their sessile habit, they have time 
for increase. The limnetic forms, on the other hand, are more 
at the mercy of the current, and though at low water they ap- 
’ pear in numbers in Spoon River, they rarely find sufficient time 
at other seasons for their characteristic increase until they reach 
the main stream. The greater fertility of the water in the 
river is doubtless also a factor in causing the marked difference 
in the diatom flora of the two streams. 
