An investigation of photoperiod and temperature in relation to the life cycles of Mnium hornum Hedw. and M. undulatum Sw. (Musci) with reference to their histology

Antheridia and archegonia of Mnium hornum are produced according to an endogenous rhythm that approximates to an annual cycle. Short days appear to have a delaying effect on gametangial production, but continuous exposure to 7.25-hour days at both 10° and 20°C is incapable of causing complete suppression. The cycle shows some tendency to precede spring conditions and it is suggested that the effect of short days in winter is to preserve its co-ordination with the seasons. Archegonial induction in M. undulatum takes place in response to long day treatment, although an alternative stimulus can be provided by a rise in temperature, the critical day length being between 7.25 and 12 hours at 10° C. Male M. undulatum is also a long day plant, but a diurnal fluctuation in temperature is essential to inflorescence production. Temperature ranges of 5.5°-11° C and 10°-20°C are suitable for antheridial induction in 15-hour days.