THE PERIODICITY OF FRESHWATER ALGAE

The normal life of a freshwater alga such as a Spirogyra consists of a period of germination of spores, a period of growth and development, 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 life 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, concentration 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 statement 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 ccarcely 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 conditions out of doors. * Invitation paper read before the Botanical Society of America and affiliated societies at Columbus, December 29, I9I5.