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the plankton, or to the interaction of environmental and internal 

 factors. 



That there is a periodicity in the reproductive processes of 

 organisms, of both plants and animals, is generally apparent. We 

 see it in the flowering and fruiting seasons of the phanerogams, and 

 in the breeding seasons of many invertebrates, of mollusks and 

 insects, and of the vertebrates generally, of fishes, amphibians, 

 reptiles, birds, and most mammals. Fluctuations in environmental 

 conditions, notably in food and temperature, influence these re- 

 productive processes. The phenomenon of rise and decline of the 

 microscopic population in laboratory aquaria is likewise an illustra- 

 tion of the periodicity of organisms, but usually within a briefer 

 interval than that of the organisms above mentioned. The studies 

 of Maupas ('88) and Calkins ('02) have shown that even in the 

 seemingly uniform conditions of the laboratory, the reproduction 

 of the ciliate Protozoa is essentially periodic. 



On a priori grounds it seems highly improbable that in the case 

 of the organisms of the plankton, internal factors should determine 

 the coincidence of the periods of growth and reproduction in several 

 hundred species. While it is not impossible, or indeed improbable, 

 that these species of the plankton if bred in pure cultures or uniform 

 environment would still exhibit a periodic reproduction, it seems 

 highly improbable that so diverse an assemblage of algae, diatoms, 

 flagellates, protozoans, rotifers, and entomostracans as is found in 

 the Illinois River, would exhibit in laboratory cultures under 

 uniform conditions any such coincidence in the location and duration 

 of their pulses as is found in the waters of the stream. Whatever 

 the internal factors involved in the growth and reproduction of 

 plankton organisms may be, it is patent that we must look for some 

 environmental factor or factors lying at the foundation of the 

 coincidence of seasons of growth and reproduction of plankton 

 organisms, which results in the phenomenon of recurrent pulses in 

 species, groups, and volumetric plankton. 



We may simplify the problem somewhat by recognizing at the 

 outset the importance of nutrition in supplying the basis for the 

 periodic growth of any organism. The rotifers and entomostracans, 

 at least the limnetic types, depend in large measure, either directly 

 or indirectly, upon the synthetic planktonts, such as the algae, dia- 

 toms, and flagellates, for their food. Since the pulses of these animal 



