<P> Often, seed dormancy is divided into four major categories: exogenous; endogenous; combinational; and secondary . A more recent system distinguishes five classes: morphological, physiological, morphophysiological, physical, and combinational dormancy . </P> <P> Exogenous dormancy is caused by conditions outside the embryo, including: </P> <Ul> <Li> Physical dormancy or hard seed coats occurs when seeds are impermeable to water . At dormancy break, a specialized structure, the' water gap', is disrupted in response to environmental cues, especially temperature, so water can enter the seed and germination can occur . Plant families where physical dormancy occurs include Anacardiaceae, Cannaceae, Convulvulaceae, Fabaceae and Malvaceae . </Li> <Li> Chemical dormancy considers species that lack physiological dormancy, but where a chemical prevents germination . This chemical can be leached out of the seed by rainwater or snow melt or be deactivated somehow . Leaching of chemical inhibitors from the seed by rain water is often cited as an important cause of dormancy release in seeds of desert plants, but little evidence exists to support this claim . </Li> </Ul> <Li> Physical dormancy or hard seed coats occurs when seeds are impermeable to water . At dormancy break, a specialized structure, the' water gap', is disrupted in response to environmental cues, especially temperature, so water can enter the seed and germination can occur . Plant families where physical dormancy occurs include Anacardiaceae, Cannaceae, Convulvulaceae, Fabaceae and Malvaceae . </Li>

Where does a seed develop in a flower