Fertilisation

Growth of a pollen tube

1. A pollen grain lands on the stigma.
2. It is stimulated to grow by sugars produced by the stigma.
3. A pollen tube grows through the style and into the ovule at the micropyle.
4. The pollen tube grows towards chemicals released from the ovule – chemotropism.
5. The tube nucleus then dies.
6. As the haploid generative nucleus moves down the pollen tube, it divides by mitosis to form 2 haploid sperm nuclei. These are the male gametes.
7. The 2 haploid sperm nuclei move down through the pollen tube.

Double Fertilisation

1. One of the male gamete nuclei joins with the egg nucleus to form a diploid zygote.
2. The second male gamete joins with the polar nuclei to form a triploid endosperm.
3. Because two fertilisations take place, this is known as double fertilisation.

Seed Formation

1. After fertilisation, the ovule becomes the seed.
2. The triploid endosperm grows rapidly by absorbing the inner layer of the ovule, called the nucellus.
3. The zygote grows by mitosis to form the embryo (which is the new plant).
4. The embryo is made up of the;

• Plumule (which will become the shoot) and
• The radicle (which will become the roots).

5. As the embryo grows inside the seed, it produces one or two seed leaves called cotyledons.

• The cotyledons are diploid and are a food store in the seed.

6. The testa is a protective cover that forms around the seed.

Types of Seeds

Monocots

• Plants that form a seed with a single cotyledon are called monocots.
• In monocots, the cotyledon absorbs a small amount of the endosperm to form an endospermic seed (e.g., grass, cereals).

Dicots

• Plants that form a seed with two cotyledons are called dicots.
• In dicots, the cotyledons absorb all the endosperm to form a non-endospermic seed (e.g., carrot, oak tree).

Seed Features	Monocots	Dicots
Cotyledons	1 cotyledon	2 cotyledons
Endosperm	Present (endospermic seed)	None (non-endospermic seed)
Examples	Grass, cereals	Carrot, oak tree

• A pollen grain lands on the stigma.
• The tube nucleus controls growth of a pollen tube, which grows down through the style towards the micropyle (guided by chemotropism).
• The generative nucleus moves down the pollen tube and divides by mitosis, forming two haploid male gametes (sperm nuclei).
• Double fertilisation occurs:
• 1st sperm nucleus (n) + egg nucleus (n) —> zygote (2n)
• 2nd sperm nucleus (n) + 2 polar nuclei (n) —> triploid endosperm (3n)

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