Plant seeds and fruits have evolved as the typical propagation
and dispersal units of angiosperms (Linkies et al.
2010). Seeds are very complex and diverse plant organs;
they store nutrients and allow far-distance dispersal as well
as persistence of a species in the local habitat. A typical
mature angiosperm seed consists of the embryo covered by
the maternal seed coat (testa) and in most cases by a more
or less abundant layer of endosperm tissue between the
embryo and testa (Fig. 1). The endosperm is a product of
double fertilization and serves as a nutrient source for the
embryo during seed development. It may also serve as a
nutrient source for the emerging embryo during germination
and seedling establishment, but another important role
of the endosperm in mature seeds is its involvement in the
regulation of germination timing (see below). Due to their
structure, low water content and mechanisms such as dormancy
and desiccation tolerance, orthodox seeds are able
to survive for long periods and harsh environmental conditions.
These features enable the timing of seed germination
to weather conditions that are favorable for
germination and further seedling development. It has
therefore been concluded that the evolution of seeds was
one of the major reasons for the rapid distribution