XVI International Botanical Congess

Physical dormancy (PY) is caused by a water-impermeable seed (or fruit) coat, in a few species, the embryo also is physiologically dormant. Before seeds can germinate, a structurally-specialized part of the seed (or fruit) coat must be disrupted. Natural factors that break PY include high temperatures, high-fluctuating temperatures, fire, drying, freezing/thawing, and passage through animals. Nondormant seeds can germinate over a wide range of environmental conditions. PY occurs in all major world vegetation types, but is most prevalent in tropical deciduous forests, savannas, hot deserts, steppes, and matorral. PY occurs in at least 15 angiosperm families, most of which are phylogenetically advanced. The dense macrosclereid layer in fossil fruits of Rhus rooseae (Middle Eocene) suggests that PY probably had evolved by the Early Tertiary.