Growth and differentiation in hydra. I. The effect of temperature on sexual differentiation in hydra littoralis

Hydras cultured for long periods at constant temperature (23°, 21°, 18°, 15°, 10°, or 5°C) were exposed to a new, higher or lower temperature for four weeks. It was found that at constant temperature, the percentage of hydras entering sexuality and the duration of the sexual periods decreased with decreasing temperature. There was no sexual differentiation at 5°. No hydras became sexual within four weeks after a decrease in temperature. The results of the temperature rise experiments show that, in general, the percentage of hydras entering sexuality depended on the absolute temperature change. The results indicate that this percentage also depended on the initial and final temperatures. An hypothesis is presented, accounting for sexual differentiation in terms of a temperature effect on an initiator and an inhibitor.