Structure of seeds

A seed is the result of sexual reproduction and contains an embryo which will grow into a new plant. In angiosperms they are contained within a fruit, in gymnosperms they are fruitless. The structure of seeds varies from plant to plant, but most have the same main features which are outlined below, using a runner bean seed as an example.

Embryo

This is the immature plant which will grow within the seed and eventually emerge from it in a process called germination. The key parts of the embryo are:

Radicle

This will develop into the plant’s primary root. It is usually the first part of the embryo to push its way out of the seed during germination.

Plumule

The first bud, which will develop into the shoot and first true leaves of the plant, is called the plumule.

Epicotyl/hypocotyl

These sections attach the radicle to the plumule. The epicotyl is the part above the point of attachment to the cotyledon, the hypocotyl the part below it. Depending on the type of germination (epigeous or hypogeous) one of these sections will be the first to push above ground.

Cotyledon

This is the seed leaf. In some plants this contains high quantities of starch and will provide a source of food for the developing embryo prior to germination, in other plants this role is performed by an endosperm (see below). In monocotyledons there is just one seed leaf, in dicotyledons there are two. Depending on the type of germination (epigeous or hypogeous) the seed leaf may remain below ground or be pulled above ground. When the latter happens seedlings may appear to have the ‘wrong’ shape of leaf at first, until the true leaves appear.

Testa

This is the hardened seed coat which protects the seed. It is formed from the outer layers of the ovule.

Micropyle

Not shown in the above illustration, the micropyle is a weak point in the testa resulting from the ‘hole’ through which the pollen entered the ovule in order to fertilise it.

Hilum

This is the point at which the seed attaches itself to the fruit (this isn’t present in gymnosperms where there is no fruit). When the seed is dispersed it is released from this point.

Endosperm

In some seeds (including monocotyledons such as wheat and maize and dicotyledons such as caster oil plants) a separate store of starchy food develops, called the endosperm. It is this starchy content that gives us many of our cereal crops such as wheat, oats and barley.