Sexual Reproduction in Flowering Plants

Words to know

The flower: who's who

Parts of a flower:

• Androecium (male) = all the stamens. Each stamen = filament (stalk) + anther (makes pollen).
• Gynoecium (female) = all the pistils. Each pistil = stigma (receives pollen) + style (tube) + ovary (contains ovules).

Male side: anther and pollen

• A typical anther has 2 lobes, and each lobe has 2 chambers (theca), so it is bilobed and dithecous.
• So an anther has 4 microsporangia (pollen sacs).
• Wall of the microsporangium, outside to inside: epidermis → endothecium → middle layers → tapetum.
• The tapetum is the innermost layer. It feeds the developing pollen.

How pollen is made (microsporogenesis)

Microspore mother cell (2n) → meiosis → microspore tetrad (4 cells, n) → each microspore becomes a pollen grain.

Pollen grain: a tough little packet

• Exine is the hard outer wall, made of sporopollenin. It is one of the toughest natural materials: it survives high heat, strong acids and alkalis, and no enzyme can break it. That is why pollen is found preserved as fossils.
• Germ pores are gaps in the exine where there is no sporopollenin. The pollen tube comes out through them.
• Intine is the thin inner wall, made of cellulose and pectin.
• Inside a mature pollen grain there are 2 cells:
  • Vegetative cell: big, has food, and forms the pollen tube.
  • Generative cell: small, divides to make 2 male gametes.
• About 60% of flowering plants shed pollen at this 2-celled stage.

Female side: ovule and embryo sac

• The ovary contains ovules, attached to a cushion called the placenta.
• Parts of an ovule: funicle (stalk), hilum, integuments (coverings), micropyle (small opening), chalaza (base) and nucellus (food tissue).

How the embryo sac is made

Megaspore mother cell (2n) → meiosis → 4 megaspores (n) → 3 degenerate, 1 stays functional → 3 rounds of free-nuclear mitosis → embryo sac with 8 nuclei in 7 cells.

• Only one megaspore survives, so this is called monosporic development.

• Synergids have a filiform apparatus, which works like a "landing guide" that attracts and guides the pollen tube.

Pollination: getting pollen to the stigma

• Chasmogamous flowers open normally.
• Cleistogamous flowers never open, so they always self-pollinate (e.g. Viola, Oxalis, Commelina). They give an assured seed set, even without pollinators.

Who carries the pollen?

• Abiotic (non-living) agents:
  • Wind: light, non-sticky pollen and feathery stigmas (grasses, maize).
  • Water: rare; Vallisneria, Hydrilla and Zostera.
• Biotic (living) agents: mostly insects (especially bees), also birds and bats. Their flowers are large, colourful, fragrant and rich in nectar.

Outbreeding devices: stopping self-pollination

Plants want variety, so many have tricks to avoid selfing:

1. Pollen release and stigma readiness happen at different times.
2. Anther and stigma are at different positions.
3. Self-incompatibility: the pistil rejects its own pollen (a genetic lock).
4. Unisexual flowers:
   • Monoecious plants (castor, maize) have male and female flowers on the same plant. This stops autogamy but not geitonogamy.
   • Dioecious plants (papaya) have male and female flowers on different plants. This stops both.

Artificial hybridisation (plant breeding)

• Emasculation means removing the anthers from a flower before they release pollen.
• Bagging means covering the flower with a butter-paper bag so no unwanted pollen gets in.
• The breeder then dusts the chosen pollen on the stigma and bags the flower again.

Double fertilisation (a favourite exam topic)

The pollen tube brings 2 male gametes. Both of them fuse with something, so it's called double fertilisation. This happens only in flowering plants (angiosperms).

• Syngamy: male gamete 1 + egg (n) → zygote (2n) → becomes the embryo.
• Triple fusion: male gamete 2 + 2 polar nuclei (n + n) → primary endosperm nucleus, PEN (3n) → becomes the endosperm (food).

After fertilisation: what becomes what

Endosperm: the baby's lunch box

• Endosperm forms before the embryo, because the baby needs food first!
• Most common type: free-nuclear (many nuclei, no walls yet).
• Coconut: the water is free-nuclear endosperm and the white kernel is cellular endosperm.
• Non-albuminous seeds use up all the endosperm (pea, groundnut).
• Albuminous seeds keep some endosperm (wheat, maize, castor).
• Perisperm is leftover nucellus (black pepper, beet).

The embryo

• The zygote grows: proembryo → globular → heart-shaped → mature embryo.
• A monocot embryo (like grass) has 1 cotyledon called the scutellum. The shoot tip is covered by the coleoptile and the root tip by the coleorhiza.

Fruits and seeds: fun facts

• False fruits grow from the thalamus as well as the ovary (apple, strawberry, cashew).
• Parthenocarpic fruits grow without fertilisation, so they have no seeds (banana).
• Oldest seeds: a Lupinus arcticus seed germinated after about 10,000 years, and a date palm (Phoenix dactylifera) seed after about 2,000 years.

Apomixis and polyembryony

• Apomixis means forming seeds without fertilisation (some grasses, Asteraceae). It is useful for farmers because hybrid seeds made this way stay the same every year, so farmers don't need to buy new hybrid seed every season.
• Polyembryony means more than one embryo in one seed (citrus, mango).

30-second revision

• Anther = 4 microsporangia; tapetum feeds the pollen.
• Exine = sporopollenin (the toughest material). Germ pores have no sporopollenin.
• Embryo sac = 7 cells, 8 nuclei (monosporic).
• Xenogamy = the only pollination type that brings new genes.
• Double fertilisation = syngamy (2n zygote) + triple fusion (3n PEN).
• Ovary → fruit, ovule → seed, integuments → seed coat.