The Population of Stream Beds 357 



ous organisms as time permits, or the special conditions 

 of nutrition and sewage contamination facilitate. 

 Though continually discharging, the stream maintains 

 the continuous supply of plancton, largely by virtue of 

 the reservoir backwaters — ^the great seedbeds from 

 which the plancton-poor but well fertilized contribu- 

 tions of tributary streams are continuously sown with 

 organisms whose further development produces in the 

 Illinois River a plancton unsurpassed in abundance." 



Doubtless, in every stream the plancton supply is 

 constantly renewed from sheltered and well populated 

 basins, which serve as propagating beds. And, indeed, 

 on every solid support diatoms are growing, and the 

 excess of their increase is constantly being released 

 into the passing current. In the swiftly flowing, 

 plancton-poor streams about Ithaca there is not time 

 for much increase of free planctons by breeding. The 

 waters run so swift a course they can only carry into 

 the lake such forms as they have swept from their 

 channels in their rapid descent. 



While there has been much study of the life of the 

 open waters of rivers there has hitherto been little 

 study of their beds. Where the beds are sandy with 

 flow of water over them we know the life differs from 

 that of muddy basins. The heavier-shelled mussels and 

 snails are on the sand ; and the commoner insects there 

 are the burrowing nymphs of mayflies and Gomphine 

 dragon-flies, and the caddis- worms that live in portable 

 tubes of sand. 



The beds of the smallest streams are easy of access, 

 and a few observations are available to indicate that 

 their study will bring to light some interesting ecological 

 relations. A few very restricted situations will be cited 

 in illustration. 



