Physiological dormancy

Physiological dormancy is initiated by the embryo or surrounding tissues. The mechanism may be the presence of hormones or enzymes inside the seed, which prevent germination. Here are a few examples:

In forests of Africa, shade-intolerant plants stand a better chance of germination when the dense, tall vegetation of the upper storey is cleared by fire, creating a temporary gap in the canopy for light to flood in. Physiological dormancy allows some seeds to wait on the forest floor until the dormancy is broken by one of the chemicals in smoke.

In woodlands of Northern Europe, the pattern of cold winters and warm summers means that spring is the best time for some seeds to germinate: those seeds may go into dormancy during the cold months of winter. This type of physiological dormancy is broken by warmer temperatures of spring.

In the sand dunes of Western Australia, seeds stand the best chance of germination when buried deep underground, where the sand is more stable, and oxygen is present: for instance the dune-stabilizing rhizomatous grass Spinifex hirsutus will not germinate unless it is dark and there is oxygen available.