Seed development

The  process by which seeds are formed is almost unbelievable. The flowers in this  picture are grown to make a garden look attractive. However, their main purpose  is to contain the sex organs for the plant.

This is a fuchsia plant in flower.

Let’s look more closely at flowers, as this is where all the action happens for sexual reproduction in plants.

Here is a cross-section diagram of a fuchsia flower.

Pollination

• For  seeds to be produced, the male sex cells in the pollen and female sex cells in  the ovary need to get together. The first part in this process is pollination.  Pollination is when the pollen lands on the female part of a flower. In most  plants the stigma needs to receive pollen from another flower.
• Cross pollination occurs when  pollen from a flower lands on the stigma of a different flower of the  same species.
• Self-pollination occurs when  the pollen from a flower lands on the stigma of the same flower.

Cross pollination of a flower.

Self-pollination of a flower.

Pollen travel

There  are two main ways that pollen gets from one flower to another:

Insect pollinated

When insects like bees or flies visit flowers and pollen sticks onto their hairy legs. When they visit another flower the pollen rubs off on its stigma.

Insect pollinated flowers may have large brightly coloured petals, large pollen grains, and be scented.

Wind pollinated

The wind carries pollen from one plant to another. Wind pollinated flowers may have small petals, no scent, a lot of small light pollen grains on long, droopy stamens.

Grass pollen is carried by wind. Grass has feathery stigmas to catch the pollen.

A  few plants, for example, cucumbers, have separate male and female flowers on  one plant.

Female and male cucumber flowers.

Kiwifruit  have male and female flowers on separate plants. These are called dioecious plants.  This means a male plant must be grown alongside female plants for pollination  to occur.

Fertilisation

Once pollination has taken place fertilisation can occur. Fertilisation is  the joining of a male and female sex cell. This is what happens:

• a pollen grain grows a tube down the middle of the pistil (via the style) to the egg inside the ovary
• when the tube reaches the female sex cell (egg), the tip dissolves. The male sex cell (sperm) is released
• the sperm and the egg join in the ovule
• once these cells are joined, the ovules turn into seeds inside the ovary
• if there is more than one egg in the ovule then more than one seed forms as in a pea pod if each egg is fertilised by a separate sperm
• the ovary develops into a fruit and surrounds the seed.

The seeds will have characteristics from the two different parent plants. Just like you, they will be individuals and in some way different from their parents.

The development of seeds

Seeds contain a food store and a tiny plant inside a  protective coat.

The food store is called the cotyledon.

Monocotyledon seeds,  such as corn, have one cotyledon to store food for the developing seed. Some  seeds such as bean seeds have two cotyledons. These are called dicotyledons.

The tiny plant is called the embryo, and the protective coat covering the  seed is called the testa.

Understanding the structure of seeds will help you propagate plants more successfully from seed.

This is a diagram of the inside of a split lupin seed.

Here is a cross-section diagram of a corn seed.

It has one cotyledon. It is called a monocotyledon seed. The cotyledon’s role in a monocotyledon is slightly different from that of a dicotyledon. The cotyledon transfers food to the embryo.

Fruit formation

When fertilisation is complete, the ovary surrounding  the seeds develops into a fruit. The fruit helps to protect the seed. Some  fruit is dry like gorse or lupin pods and some are juicy like an apple.

Seed dispersal

This  is the scattering of seeds away from the parent plant. Seeds need to be  dispersed to reduce the competition from the parent plant and germinating  seedlings.

When  the pod of the lupin dries it splits open explosively to scatter the seeds away  from the plant.

Seeds  can be dispersed by the wind, on animal hair or bird feathers, in bird and  animal faeces or they can float in water.

What's next?

Go to: 3 Seed germination.