Sexual Reproduction in Plants

A typical flower has 4 parts (from outside in):

1. Sepals. Green; protect the bud before flowering. Often fall off as the flower opens.

2. Petals. The colourful display.

• Insect-pollinated: bright, large, often scented.
• Wind-pollinated: small, dull, often missing.

3. Stamen (male part).

• Filament: thin stalk.
• Anther: at the top; produces POLLEN GRAINS (which contain male gametes).

4. Carpel (female part, also called pistil).

• Stigma: top, sticky in insect-pollinated, feathery in wind-pollinated.
• Style: stalk linking stigma to ovary.
• Ovary: contains OVULES (each contains a female gamete).

Most flowers have BOTH male and female parts — they're 'hermaphrodite' or 'bisexual'. Some species have separate male/female flowers (e.g. squash, holly).

Worked qualitative. Why are bee-pollinated flowers often blue/purple, while bird-pollinated flowers are often red?

• Bees see UV light well, can't see red.
• Birds (especially hummingbirds) see red well.
• Plant evolution matched flower colour to the pollinator's vision.
• A nice example of co-evolution.

Cambridge tip. Memorise the four parts and which is male/female. Cambridge often gives a labelled diagram and asks to identify or describe roles.

Insect-pollinated flowers:

Feature	Reason
Brightly coloured petals	Attract insects
Scented	Attract insects
Nectar (sweet liquid)	Reward for insect visit
Sticky pollen	Sticks to insect body
Sticky stigma INSIDE the flower	Catches pollen brought in
Small amounts of pollen	Few but precise transfers

Wind-pollinated flowers:

Feature	Reason
Small, dull petals (or absent)	No need to attract insects
No scent or nectar	Same
Light, dry, smooth pollen	Carried easily by wind
Lots of pollen	Most blows away; need extras
Anthers OUTSIDE the flower (dangling)	Pollen released into the air
Feathery stigma OUTSIDE the flower	Catches drifting pollen

Examples.

• Insect-pollinated: rose, snapdragon, lavender, sunflower.
• Wind-pollinated: grasses, oak, wheat, hazel, pine (cones, not flowers).

Hay fever — wind-pollinated plants release LOTS of pollen, which causes allergic reactions in some people.

Worked qualitative. Why do farmers introduce BEEHIVES into orchards during flowering?

• Apples, almonds, etc. are insect-pollinated.
• More bees = more pollination = more fruit.
• Without pollinators, no fruit forms.
• Bee decline is a major agricultural concern.

Cambridge tip. Pair each adaptation with its REASON. Cambridge marks both halves.

Self-pollination.

• Pollen transferred to stigma of the SAME flower (or another flower on the SAME plant).
• Produces less variation.
• Reliable — works even alone.

Cross-pollination.

• Pollen transferred to stigma of a flower on a DIFFERENT plant of the SAME species.
• Produces more genetic variation.
• Needs another plant nearby + a vector (insect/wind).

How plants encourage cross-pollination.

• Stigmas mature at different times from anthers (so own pollen isn't viable when stigma is ready).
• Stigma above anther so pollen falls past it.
• Self-incompatibility: own pollen rejected by stigma.
• Separate male and female flowers.

After pollination — fertilisation.

1. Pollen grain lands on stigma.
2. Pollen TUBE grows down through the style to the ovary.
3. Pollen tube delivers MALE GAMETE to the ovule.
4. Male gamete nucleus FUSES with egg nucleus → ZYGOTE.
5. Zygote → embryo → SEED. Ovary wall thickens → FRUIT.

Worked qualitative. Why do many flower species REFUSE their own pollen?

• Cross-pollination = more variation = better adaptation.
• Plants that always self-pollinate have less variation → vulnerable to disease.
• Self-incompatibility evolved to FORCE outcrossing.
• Common in apples, plums, many wild species.

Cambridge tip. Memorise the chain after pollination: pollen tube → ovule → fertilisation → zygote → seed; ovary → fruit.