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 Chlorophycece and of the blue-green algse, 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 algse, 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 other and 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. 



