Germination tests were done with five varieties of celery on thermo-gradient bars set to cover the range 0 to 40°C. Optimum germination at constant temperatures occurred between 10 and 15°C in all five varieties, with maxima between 20 and 30°C. A method is described for obtaining detailed descriptions of responses to diurnally fluctuating temperatures using combinations of thermo-gradient bars and incubators. In three varieties tests of the effects of daily cycles including temperature fluctuations demonstrated closely similar qualitative responses. Germination was consistently improved by fluctuating temperatures, but the range of temperature combinations resulting in high germination was closely circumscribed. Quantitative differences between varieties at particular temperatures were sometimes considerable. Optimal conditions for germination were found in combinations involving temperatures from 22 to 25°C in one phase of the daily cycle, and 12 to 15°C in the other.