Effect of external factors on phototaxis of Chlamydomonas reinhardtii. I. Light.

1. A fully automated phototaxis monitoring device is described for measuring photo-topotactic responses of flagellated organisms. 2. Photokinesis can be demonstrated in Chlamydomonas cells only after a dark period of about 72 hrs. 3. Pre-darkening of a few hours duration raises the phototactic disposition, whereas pre-illumination has no significant effect. 4. Circadian rhythms can be initiated by only one period of darkness or lower light intensity, whereas a period of higher intensity does not induce rhythms. The period length of the circadian rhythms is about 24 hrs.