Plant Reproduction

The flower is the reproductive structure of an angiosperm (flowering plant). Most flowers are bisexual (hermaphrodite), carrying both male and female parts.

Outer (non-reproductive) parts:

• Sepals — usually green; enclose and protect the developing flower bud.
• Petals — in insect-pollinated flowers they are large, brightly coloured and may produce scent to attract pollinators.

Male part — the stamen:

• Anther — produces and releases pollen grains (which contain the male gametes).
• Filament — stalk that holds the anther in position.

Female part — the carpel (pistil):

• Stigma — sticky/feathery surface that receives pollen grains.
• Style — supports the stigma and is the route the pollen tube grows down.
• Ovary — contains one or more ovules; each ovule holds an egg cell and will become a seed after fertilisation.
• Nectary — produces nectar, a sugary reward, in insect-pollinated flowers.

Pollination is the transfer of pollen from an anther to a stigma.

• Self-pollination — pollen lands on the stigma of the same plant (or even the same flower). Reduces genetic variation but guarantees a mate.
• Cross-pollination — pollen is carried to the stigma of a different plant of the same species. Increases genetic variation, which is favourable for evolution and disease resistance.

Adaptations for pollination — a comparison:

Feature	Insect-pollinated	Wind-pollinated
Petals	Large, brightly coloured	Small, dull/green or absent
Scent & nectar	Present (attract insects)	Absent
Pollen	Sticky/spiky, large grains, small amount	Smooth, light, large amount
Anthers	Inside flower, firm	Dangle outside, loose
Stigma	Sticky, inside flower	Large, feathery, hangs outside to catch pollen

Mutualism in pollination. Insect (and bird/bat) pollination is a mutualism: the pollinator gains food (nectar/pollen) while the plant gains pollen transfer. This relationship underpins the reproduction of most flowering plants, many food crops (e.g. apples, almonds, oilseed) and overall biodiversity — so pollinator decline threatens both ecosystems and agriculture.

Fertilisation in flowering plants.

1. A compatible pollen grain lands on the stigma and germinates.
2. A pollen tube grows down through the style toward the ovary, carrying the male gametes.
3. The pollen tube enters an ovule through a small pore (the micropyle).
4. Double fertilisation (SL concept): one male gamete fuses with the egg cell → diploid zygote; the other fuses with the central cell → endosperm, a nutritive tissue.

After fertilisation:

• Zygote → embryo (the young plant, with embryonic root and shoot).
• Endosperm / cotyledons → food store for the embryo.
• Ovule → seed (a dormant embryo + food store inside a protective seed coat/testa).
• Ovary → fruit (protects seeds and often aids their dispersal).

Seed structure. A seed contains the embryo (radicle = future root, plumule = future shoot), a food store (endosperm or cotyledons), and a protective testa (seed coat). The micropyle is a small pore through which water enters.

Seed dispersal spreads seeds away from the parent, reducing competition and colonising new areas:

• Wind — light/winged seeds (e.g. dandelion, sycamore).
• Animal — fleshy fruits eaten then excreted, or hooked seeds that cling to fur.
• Water — buoyant fruits (e.g. coconut).
• Explosive/mechanical — pods that split and fling seeds (e.g. pea).

Germination is the start of growth of the seed into a seedling. Three conditions are required:

• Water — rehydrates tissues and activates enzymes.
• Oxygen — for aerobic respiration to release energy for growth.
• A suitable temperature — for enzyme activity.

Mobilisation of stored food. Water uptake triggers release of the hormone gibberellin, which switches on genes for the enzyme amylase. Amylase digests stored starch into maltose/glucose, which is transported to the growing embryo and respired for energy until the seedling can photosynthesise.