Effects of Daylength and Temperature on the Reproduction of Vaucheria sessilis

Abstract

1. Gametangium formation in Vaucheria sessilis was both much earlier and much more abundant under long (18-hour) days than under short (8-hour) days. The plants were maintained entirely under artificial light of 50-65 ft-c. from fluorescent tubes. 2. Interruption of the long dark periods had no effect on gametangium formation, but addition of glucose and meat peptone to the culture media promoted earlier and more abundant gametangium formation under short-day conditions. 3. In all four environments (warm long day, cool long day, warm short day, cool short day) gametangia were formed only after abundant fat globules had accumulated in the filaments, there being a particularly high concentration of fat globules at the site of subsequent gametangium initiation. Fat formation was both much earlier and more abundant under long-day conditions. 4. The results suggest that gametan gium formation in V. sessilis is not truly photoperiodic but that low total photosynthesis and consequently limited food availability were the limiting factors in short-day conditions. 5. Light duration had no effect on zoospore formation or on zoospore or oospore germination, but low temperatures were much more favorable to all these than high temperatures. 6. Variations in composition and concentration of mineral culture media as used in these experiments had little effect on reproduction of V. sessilis.