The word seed is normally used for the part of the plant that is used for sowing,
while the word grain refers to the same part of the plant, but when it is used for
consumption by humans or animals.
One physical feature of a seed which can be useful for identification purposes, of
legume varieties for example, is the part known as the hilum, the point of attachment to
the funiculus, which leaves a scar, normally oval in shape, on the seed where it has
broken away.
Germination is the process whereby a seed is transformed into an independent,
established seedling. It is a complex and intensively researched topic, but in essence a
seed will only germinate when it is has sufficient air,moisture and warmth.
- Seed Dormancy
Sometimes a healthy, living seed is unable to germinate even though all three of the
conditions are favourable—the seed is then said to be dormant.
Seed dormancy can be a very beneficial survival strategy of plants in the wild state
because it helps to ensure that the seed only germinates when the conditions are
favourable for plant growth. But in cultivated crops dormancy can sometimes be a
problem for food producers. For example some species of Beta vulgaris and some of the
legumes such as clovers sometimes will not germinate for weeks or even months, even
when all the conditions appear to be favourable.
Sometimes if seed is planted which has only recently been harvested it germinates
very slowly or not at all. Sorghum seed for example is dormant for about a month after
harvest, and most indica type rices need a one to three-month “rest period” after harvest
in order to reach maximum germination and vigour (japonica type rices normally do not
have a dormancy period).
Fortunately this characteristic has been more or less eradicated from the commonly
grown food crops due to selection pressure from plant breeders. The dormancy period
for these crops is normally passed either just before or just after the seed falls from the
seed head.
Mechanisms of Seed Dormancy
Dormancy is imposed on seeds by a number of mechanisms, which fall into two
categories:
- Embryo (Internal) Dormancy—when the seed embryo holds the mechanism, and
- Hard Seed (Coat Imposed or External) Dormancy—when the seed testa or outer
membrane is hard and does not readily imbibe (take in) water.
In some species germination is either stimulated or inhibited by the seed being exposed
to light, either in continuous light or even short bursts. Examples of food crops that will
not germinate in the light include chives, garlic and amaranths.