1. Dormancy: Seeds of some plants do not germinate just after their development on the plant, even if all conditions are favourable. This state of seed in which germination is temporarily suspended is called dormant stage and this condition is called dormancy. Seed dormancy may be due to internal factors or external factors.
2. After Ripening Period: Seeds of some plants do not germinate immediately after their formation and enter in the resting phase. During the resting phase, some internal changes occur in the seeds and after this only they acquire the capacity to germinate. This resting period is called after the ripening period. Such seeds germinate after completing this period.
3. Photoperiodism: The phenomenon of developmental responses of plants, in terms of transfer of plant from the vegetative phase to reproductive phase, to the relative light and dark period is called photoperiodism. Some plants enter flowering when exposed to uninterrupted light for more than a critical period. These are called long-day plants. On the other hand, some plants enter flowering when the day length is shorter than a critical time period. These plants are called short-day plants.
