256 



The factors determining the decrease are the proportions 

 of silt and plankton and the character of each. When floccu- 

 lent debris is abundant, or when filamentous diatoms or algae, 

 Copepoda, or the Cladocera with long antennae are present in 

 numbers, the decrease upon centrifuging is greater. When the 

 silt is earthy or contains considerable quartz, and when the 

 plankton consists of Protozoa such as Synura, or Rotifera such 

 as Syrichceta, or Cladocera such as Chydorus or Bosmina the de- 

 crease is less. The amount of plankton placed in the tubes of 

 the centrifuge also slightly affects the ratio of decrease in vol- 

 ume. For example, one of our largest planktons, measuring 

 11.15 cm. 3 by the gravity method, fell but 8 per cent, when cen- 

 trifuged in a single tube. When divided among three tubes 

 the decrease became 14 per cent. This was a plankton largely 

 composed of Chydorus and Bosmina. Another large plankton, 

 measuring 11.85 cm. 3 by the gravity method, fell to 7.6 cm. upon 

 centrifuging in a single tube — a loss of 36 per cent. This con- 

 sisted very largely of Synura. As other large catches decreased 

 as much as 50, or even 60, per cent., it is clear that large vol- 

 umes do not necessarily yield only slight decreases. 



The instances in which the decrease exceeded 70 per cent. 

 are 5 in number. Of these, 3 contained Melosira or Fragillaria, 

 1 was rich in Oscillaria, and 2 contained considerable floccu- 

 lent debris from aquatic vegetation. All of the 12 whose de- 

 crease was less than 30 per cent, occur in April and May, when 

 Chydorus and Bosmina are at their maximum and constitute 

 a large, if not the greater, part of the plankton. In a few in- 

 stances these catches which showed slight reduction in volume 

 contained Hydra, insect larvae, and other adventitious forms 

 from surrounding vegetation. 



Both extremes contain numerous instances in which the 

 plankton catch is made up of typical plankton organisms, and 

 consequently the range in the decreases here recorded is normal 

 for the range in planktons occurring in our waters throughout the 

 year. It is therefore reasonable to assume that the centrifuged vol- 

 umes here reported for plankton in the Illinois must, on the average, 



