Few things are more important for plants than ensuring that germination takes place in the right place and at the right time. Sometimes this requirement is satisfied by germination as soon as seeds are shed, but in most plants there is a delay of anything from days to decades. One important mechanism for achieving this delay, although not the only one, is seed dormancy.

Types of seed dormancy

There are three fundamentally different types of seed dormancy, at least two of which have evolved on several separate occasions (Baskin & Baskin, 1998). These dormancy types are morphological, physical and physiological. In morphological dormancy, the seed is immature when shed and a period of growth and/or differentiation is required before germination can take place. Seeds with physical dormancy have impermeable testas or pericarps; the embryo is therefore dry until the seed coat is broken and water enters. Physiological dormancy prevents germination until a chemical change takes place in the seed. Dormancy types may be combined in the same seed – a combination of morphological and physiological (morphophysiological) dormancy is very common, but physical and physiological dormancy are rarely combined. The combination of physical and morphological dormancy is, of course, impossible. Note the crucial distinction between physiological dormancy, which is reversible, and the other two types, which are not. Thus, generally speaking, physiological dormancy permits a more flexible response to the environment than the other two types of dormancy. Morphological dormancy seems to be the most primitive type (Baskin & Baskin, 1998).