to 30 minutes in darkness. I n the case of dinoflagellates, however, reports by several observers (Zacharias, '05 ; Moore, '08; Kofoid and Swezy, '21; Harvey, '52) suggest that luminescence is under the control of a type of diurnal rhythmic mechanis8m. This conclusion is based mainly on visual observations that dinoflagellates kept in total darkness fail to luminesce during the day, but regain their ability to do so during the nighttime. The studies described in this paper confirm this general conclusion with quantitative measurements, and give additional information concerning other effects of light.
Methods
A unialgal culture of G. polyedru was isolated by one of the authors (B. M. S.) from a net sample collected off the pier at Scripps Institution of Oceanography, on September 22, 1952. Since that time, the cultures have been maintained in Fernbach flasks placed in a window (north exposure, 400-800 foot candles) at room temperature (23"-26"C.). The culture medium was composed of 75% filtered sea water, 2% soil extract (Pringsheim, '46), 2 X M KNO,, 2 X lo-* hf K,HPO,, 6 X M FeCl,, and 2.7 x 10-5 M ethylendiaminetetraacetic acid (EDTA) at pH 8.0-8.2. All distilled water used in these experiments was distilled a second time in a Pyrex glass still.
For experimental purposes, some cultures were grown and maintained in a temperature-controlled cabinet (25 "C.) under constant illumination at an intensity of about 700 foot candles.
Illumination was provided by "cool white " fluorescent lamps.
Cultures grown in the window were used for experiments in which it was desired that organisms be exposed to normal day-night changes in light intensity. Cultures were inoculated at a cell density of about 800 cells per milliliter, and were used for experiments 10-15 days later at a population density of 6000-13000 cells per milliliter.