THE PERIODICITY OF FRESHWATER ALGAE* 
Edgar Nelson Transeau 
The normal life of a freshwater alga such as a Spirogyra consists 
of a period of germination of spores, a period of growth and develop- 
ment, and a period of sexual or asexual reproduction, commonly 
followed by a period of dormancy. An Oedogonium has in addition 
to these periods one or more phases during the vegetative period when 
zoospores are produced. Other green algae have part or all of these 
several periods. Consequently if we follow the changes in the algae 
occurring in a given pond or stream throughout the year, we find a 
rather regular succession of Hfe phases for each of the species present. 
Since the life cycles of the different species vary in their duration, we 
also find an orderly sequence of appearing and disappearing species. 
To discover the causes underlying these periodic changes, efforts 
have been directed in two rather distinct lines of investigation: (i) 
The observation of algae under laboratory conditions, and (2) The 
observation of algae under natural conditions. 
To the first class belong the investigations of Klebs, Artari, Benecke 
and Danforth. These experiments have given us a considerable body 
of information concerning the effects of light, temperature, concentra- 
tion and the chemical nature of the medium. The results of the 
experiments with variations in light and temperature as factors in 
accelerating or retarding vegetative and reproductive activities are 
for the most part qualitative. They still await a quantitative state- 
ment of their relations. The experiments with concentration and 
chemical composition of the medium not only show very inharmonious 
results, but the conclusions to which they have led are scarcely 
applicable to the explanation of the periodicity of algae in nature, since 
the concentrations used are many times the concentrations of our 
natural waters, and many of the substances used do not occur in our 
pond and stream solutions. These results may be of great interest 
in cell physiology, but they do not appear to be applicable to the condi- 
tions out of doors. 
* Invitation paper read before the Botanical Society of America and affiliated 
societies at Columbus, December 29, 1915. 
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