Pollination
Pollination is the transfer of pollen grain from the anther to the stigma. Following pollination the process of fertilisation can now begin.
Double Fertilisation
Fertilisation is the process in which a male gamete fuses with a female gamete to produce a zygote. In flowering plants, the female gamete is in the ovule, protected within the ovary. The male gamete is the nucleus contained in the pollen grain. The male gamete is delivered to the female gamete by a pollen tube.
Steps of double fertilisation.
- When a compatible pollen grain lands on the stigma, pollen grains lands on the stigma, pollen grains absorb water this must occur to initiate germination, it germinates in the sucrose solution secreted by the stigma and produces a pollen tube.
- The pollen tube grows down the style controlled by the tube nucleus, followed by the generative nucleus which divides via mitosis to give two male nuclei the pollen tube tip secretes enzymes which digest the tissues of the style.
- The pollen tube grows out pf the pollen grain through a gap in the cell wall called a pit, and down the style, up a gradient of chemoattractants.
- The pollen tube nucleus codes for the production of hydrolases including cellulases and proteases, and it digests its way down the style’s tissue. The products of digestion are used by the growing pollen tube.
- The pollen tube enters the embryo sac through a gap in the integumants known as the micropyle.
- As the pollen tube penetrates the embryo sac, the tip opens and the two male gamete nuclei enter, the pollen tube nucleus having disintegrated earlier.
- The male and female gametes are haploid.
- The first male nucleus fuses with the female nucleus or egg nucleus to form a diploid zygote.
- The second male nucleus fuses with the two polar nuclei to form a triploid endosperm nucleus.
- Following fertilization the developmen of the seed and fruit take place.
Development of a seed
• Pollination occurs (pollen lands on stigma).
• Pollen tube grows down the style to the ovule.
• Double fertilisation occurs:
• One male nucleus fuses with the egg cell → diploid zygote.
• One male nucleus fuses with two polar nuclei → triploid endosperm.
• Zygote divides by mitosis → embryo develops.
• Endosperm develops as a food store.
• Ovule develops into a seed.
• Integuments form the seed coat (testa).
• Seed dehydrates and becomes dormant.
Development of fruit
• After fertilisation, the ovary wall develops.
• The ovary wall forms the pericarp (fruit wall).
• The ovary becomes the fruit.
• Seeds are enclosed within the fruit.
• Fruit develops to aid seed protection and dispersal.
Structureof seed
• Testa (seed coat) – tough outer layer formed from the integuments; protects the embryo.
• Hilum – scar showing where the seed was attached to the ovary wall.
• Micropyle – small pore that allows water uptake during germination.
• Embryo, consisting of:
• Radicle – develops into the root.
• Plumule – develops into the shoot.
• Cotyledon(s) – seed leaves; may store food.
• Endosperm (in many seeds) – food store containing starch, lipids or proteins.
Stucutureof fruit
• Develops from the ovary after fertilisation.
• Pericarp (fruit wall), made of three layers:
• Exocarp – outer layer (skin).
• Mesocarp – middle layer (often fleshy).
• Endocarp – inner layer surrounding the seed(s).
• Contains one or more seeds.
• May be fleshy (e.g. apple) or dry (e.g. pod).
