Survival of annual plants, and desert annuals in particular, seems to be dependent on three mechanisms: (1) seed dispersal or topochoric behavior for determining suitable microhabitats; (2) germination at the right time; (3) germination of only part of the seeds at one time, even under optimal conditions (heteroblasty). Environmental factors that determine seed germinability, such as day length and temperature, seed position in the capsule, in the dispersal unit, in the inflorescences and in different inflorescences of the same plant are discussed. These factors affect heteroblasty even during the last days of seed maturation. In plants of different taxa the photoperiodic induction that influences germination is absorbed by the leaves or even by the post-harvested fruit and is transferred to the seeds. Flowers along the same branch produce seeds that differ in their germinability according to the day length during the last few days of their maturation.