41 
The relation is here more apparent, but the resulting pulse is no 
larger than those following upon floods during summer, and but 
little larger than the ones which precede it in the autumn. The 
effect of this overturning upon the plankton of the Ilinois River 
may thus be detected, though it is here of less importance than in 
lakes and reservoirs since it 1s overshadowed or replaced by other 
and more potent factors. 
The relation of the seasonal distribution of the diatoms to that 
of the total plankton is not readily unraveled. The latter is the 
resultant of a most complex series of factors, whose number and 
‘relative potency are subject to constant change and readjustment 
in the unstable environment of the stream. It is the biological 
expression of the state of tension among these various sactors which 
for the moment exists. Of these factors the diatoms are but one, 
though an important one, in the food cycle and ecology of the 
plankton. The volumetric determinations in the diagram (p. 
37) do not give the true seasonal distribution of the total plankton 
owing to the escape of an unknown quantity through the meshes of 
the silk net. They represent more truly that of the animal plank- 
ton than that of the phytoplankton. A comparison of the seasonal 
distribution of the diatoms and total plankton may serve, in spite 
of the errors involved in the volumetric determinations and the 
disparity of individuals among the diatoms, to throw some light on 
the effect of the fluctuations of the latter upon the movement in the 
volume of plankton. A close comparison of the two seasonal curves 
reveals the fact that the diatom curve is not identical with the vol- 
umetric curve. It is true that the double vernal (April-May) pulse 
of diatoms coincides in location with the vernal volumetric pulse. 
This is also true of the pulses of June 14 and July 19. The crest of 
the volumetric vernal pulse is, however, lodged between the double 
apices of the diatom curve, and all the subsequent volumetric 
pulses from July on lie in depressions of the diatom curve, and vice 
versa. It is apparent at once on examination of our planktons that 
the catches of the silk net are from the volumetric standpoint 
largely, indeed overwhelmingly, of animal origin. These volu- 
metric pulses are as a rule largely pulses of the zodplankton. It is 
therefore to be expected that the diatoms would decrease at such 
times, since they form the food of many Eutomostraca and not a few 
Rotifera. The appearance of the diatom pulses before or after the 
