42 



volumetric (animal) pulse may therefore in a measure present the 

 wavering tendency to establish an equilibrium between these two 

 elements of the plankton. The presence of an abundant animal 

 plankton may therefore be a cause of some of the minimum periods 

 between diatom pulses. Other causes, such as decline of food ele- 

 ments, may also arise, but in our waters the nitrates at least rarely 

 ever reach a level where an unutilized margin capable of support- 

 ing a large diatom population is not still present. Data concerning 

 other food elements are not at hand, but their paucity in water 

 derived from such varied sources and so liberally fertilized by 

 organic wastes seems improbable. There is also the further possi- 

 bility and, indeed, from the data in hand the probability of the 

 existence among diatoms of reproductive cycles, interrupted by 

 resting periods. The available data do not, however, throw any 

 light upon the nature of this internal factor or the cause for the 

 running down of the energy of reproduction, and but little upon the 

 operation of environmental factors which stimulate anew the 

 process of reproduction. 



The seasonal distribution of the diatoms as a whole, and that of 

 individual species also, offer repeated instances of recurrent pulses 

 at intervals approximating four weeks the lunar month. In 1898 

 thirteen such pulses can be detected. These often correspond 

 roughly to minor flood intervals, but not always so, for occasionally 

 two pulses occur on the decline of a single flood. Similar appear- 

 ances may be traced in other years, when collections were frequent 

 enough to exhibit minor pulses. They are, however, in all cases 

 quite irregular, and exceptions are frequent. 



That cosmic factors may indirectly, through immediately environ- 

 ing factors, affect the reproductive phenomena of pelagic organisms 

 has been suggested by the work of Kramer ('97), Mayer ('00), and 

 Friedlander ('01) in the case of the "Palolo" worm, a coral-reef 

 annelid whose seasonal swarming for reproductive purposes occurs 

 at somewhat definite lunar intervals. 



While the data concerning the seasonal distribution of diatoms 

 in the Illinois River may serve to suggest the operation of an enig- 

 matic cosmic factor, I wish distinctly to state that in my opinion 

 they are wholly inadequate to establish either its presence or its 

 potency. It is much more probable that we have to deal merely 

 with some matter of food relations between the plants and animals 



