16.4 - Fertilisation & Germination

Fertilisation in flowering plants

1. After a pollen grain lands on the stigma, it does not fertilise the female gamete straight away - the male gamete still has to reach the ovule.
2. A pollen tube grows out of the pollen grain and down through the style towards the ovary, entering the ovule.
3. A nucleus from the male gamete then travels down this tube to join the female gamete inside the ovule.
4. Fertilisation is the moment the male nucleus and female nucleus fuse to form a zygote.
5. The zygote divides by mitosis to form an embryo.

What the flower becomes after fertilisation

• Each ovule containing a fertilised female gamete becomes a seed.
• The ovary that surrounded the ovules develops into a fruit around the seed or seeds.

The structure of a seed and its food reserves

A seed has three main parts:

1. Embryo - The young plant, made up of an embryo root and an embryo shoot that grow into the adult plant
2. Food store - A supply of lipids and starch that the young plant lives on at first
3. Seed coat - A tough protective outer covering that keeps the inside safe until the seed germinates

Why the seed relies on its food store

1. At first the seed cannot photosynthesise - it has no green leaves and may be underground in the dark - so it lives entirely on its food store.
2. When the seed germinates, it draws glucose from its food store and respires it. This provides the energy and materials the seedling needs to grow its first root and shoot.
3. The seed keeps relying on its food store until the young plant has grown above the soil and produced green leaves.
4. Once it has green leaves, it can make its own glucose by photosynthesis and no longer depends on the food stored in the seed.

The conditions needed for germination

• Germination is the process by which a seed develops into a young plant.
• It's controlled by enzymes inside the seed.
• A seed often lies dormant (inactive) until conditions are right.
• Germination only begins when three conditions are all present together: water, oxygen, and warmth.
• If any one of these is missing, the seed stays dormant and does not germinate.

Why each condition is needed

• Water - Makes the seed swell and activates the enzymes inside it, which control the growth of the seedling.
• Oxygen - Needed for aerobic respiration, the process that provides the energy the seed needs to grow.
• Warmth (a suitable temperature) - Raising the temperature raises enzyme activity and so the rate of growth.
  • If it's too cold, the enzymes work too slowly.
  • Very high temperatures denature the enzymes and stop germination altogether.
  • The temperature that suits germination depends on the type of seed.