Fruits (drupes) of Symphoricarpos orbiculatus Moench ripen in autumn and are dispersed from autumn to spring. Seeds (true seeds + fibrous endocarp) are dormant at maturity, and they have a small, linear-shaped embryo that is underdeveloped. In contrast to previous reports, the endocarp and seed coat of S. orbiculatus are permeable to water; thus, seeds do not have physical dormancy. No fresh seeds germinated during 2 wk of incubation over a 15/6-35/20oC range of thermoperiods in either light (14 h photoperiod) or constant darkness; neither GA3 nor cold stratification alone overcame dormancy. One hundred percent of the seeds incubated in a simulated summer --> autumn --> winter --> spring sequence of temperature regimes germinated, whereas none of those subjected to a winter --> spring sequence did so. That is, cold stratification is effective in breaking dormancy only after seeds first are exposed to a period of warm temperatures. Likewise, embryos grew at cold temperatures only after seeds were exposed to warm temperatures. Thus, the seeds of S. orbiculatus have nondeep complex morphophysiological dormancy. As a result of dispersal phenology and dormancy-breaking requirements, in nature most seeds that germinate do so the second spring following maturity; a few may germinate the third spring. Seeds can germinate to high percentages under Quercus leaf litter and while buried in soil; they have little or no potential to form a long-lived seed bank.

Key words: embryo growth, germination phenology, imbibition, morphophysiological seed dormancy, Symphoricarpos orbiculatus, underdeveloped linear embryo.