Sexual Reproduction in Flowering Plants

The flower: the plant's reproductive shoot

A flower is a short shoot changed for reproduction. Its outer whorls (sepals and petals) protect and attract. Its inner whorls make gametes (sex cells).

• Stamen (male part) = a thin stalk called the filament + a head called the anther. All stamens together are the androecium.
• Pistil (female part) = stigma (landing pad for pollen) + style (long neck) + ovary (swollen base). All pistils together are the gynoecium.
• The ovary holds ovules. Each ovule later becomes a seed.

Inside an anther

A typical anther has two lobes and four pollen sacs (microsporangia). Each sac has four wall layers. The innermost layer, the tapetum, feeds the young pollen. In the centre, pollen mother cells divide by meiosis. One mother cell gives a group of four cells (a tetrad). This is microsporogenesis.

Inside an ovule

An ovule sits on a stalk (funicle). It has one or two coats (integuments) with a small opening, the micropyle. The body inside is the nucellus. The base end, opposite the micropyle, is the chalaza.

Male and female gametophytes

A gametophyte is the small plant body that makes gametes. In flowering plants both are tiny and live inside the flower.

Male gametophyte: the pollen grain

• The wall has a tough outer layer, exine, made of sporopollenin. It resists heat, acid and time, so pollen fossils survive. Gaps in the exine are germ pores.
• The inner soft layer is the intine.
• A mature pollen grain has 2 cells: a big vegetative cell (food, later makes the tube) and a small generative cell. The generative cell divides once by mitosis to make 2 male gametes. In many plants pollen is shed at the 2-cell stage; in others at the 3-cell stage.

Female gametophyte: the embryo sac

• In the nucellus, one megaspore mother cell does meiosis and gives 4 megaspores. Usually 3 die and 1 survives (monosporic development). This is megasporogenesis.
• The surviving megaspore nucleus divides 3 times by mitosis without walls: 1 → 2 → 4 → 8 nuclei.
• Then walls form. Final shape: 7 cells, 8 nuclei. At the micropyle end: the egg apparatus (1 egg + 2 synergids). At the chalaza end: 3 antipodal cells. In the middle: one big central cell with 2 polar nuclei.

Synergids have finger-like ingrowths (the filiform apparatus) that guide the pollen tube in.

Pollination: types and agents

Pollination means pollen moving from an anther to a stigma.

Three types

• Autogamy: same flower. Easy in flowers that never open (cleistogamous, e.g. some peas and Commelina), which set seed even with no pollinator.
• Geitonogamy: another flower on the same plant. It needs an agent, but the genes are the same, so genetically it is like selfing.
• Xenogamy: a flower of a different plant. This brings new gene mixes.

Agents

• Wind: light, dry, non-sticky pollen made in huge amounts; long feathery stigmas; open stamens (grasses, maize).
• Water: rare; a few water plants such as Vallisneria and sea grasses. Pollen has a mucus coat against wetting.
• Animals (mostly insects, also birds and bats): bright colours, scent, nectar and sticky pollen. Some flowers give a safe egg-laying place in return (the yucca plant and its moth).

Outbreeding devices: how plants avoid selfing

Selfing again and again makes weak offspring (inbreeding depression). So many plants have ways to favour cross-pollination:

• Timing gap: pollen is released when the stigma is not ready, or the stigma is ready before pollen.
• Distance: anther and stigma are placed apart, so pollen cannot fall on its own stigma.
• Self-incompatibility: the pistil chemically blocks pollen from the same plant.
• Separate sexes: unisexual flowers. If male and female flowers are on the same plant (monoecious, e.g. castor, maize) only autogamy is stopped. If they are on different plants (dioecious, e.g. papaya) both autogamy and geitonogamy are stopped.

Pollen-pistil interaction

The pistil can tell right pollen from wrong pollen. Chemicals on the pollen talk to chemicals on the stigma.

• Right (compatible) pollen: it germinates. A pollen tube grows out of a germ pore, down through the style, into the ovary, and enters the ovule, mostly through the micropyle. It enters one synergid and releases the 2 male gametes.
• Wrong (incompatible) pollen: the pistil stops germination or stops the tube.

Plant breeders use this knowledge in artificial hybridisation: remove the anthers of the female parent early (emasculation), cover the flower with a bag (bagging), dust chosen pollen on the stigma, and bag it again.

Double fertilisation

Flowering plants do two fusions in one embryo sac:

1. Syngamy: male gamete (n) + egg (n) → zygote (2n). The zygote grows into the embryo.
2. Triple fusion: male gamete (n) + 2 polar nuclei (n + n) → primary endosperm nucleus, PEN (3n). It grows into the endosperm (food store).

Two fusions together = double fertilisation. It happens only in flowering plants.

After fertilisation: endosperm, embryo, seed and fruit

Endosperm comes first

The PEN divides again and again to make endosperm before the embryo grows much, because the embryo needs food. In coconut, the water is free-nuclear endosperm and the white kernel is cellular endosperm. In pea and groundnut, the endosperm is eaten up before the seed is ripe (non-albuminous seed). In wheat, maize and castor some stays (albuminous seed).

Embryo

The zygote sits at the micropyle end and divides. A dicot embryo has an axis with two cotyledons (seed leaves); the tip above is the plumule (future shoot) and below is the radicle (future root). A monocot embryo such as grass has one cotyledon called the scutellum.

Seed

The ovule becomes the seed: integuments turn into the seed coat, the micropyle stays as a tiny pore, water is lost and the seed can rest (dormancy). A seed can stay alive for many years.

Fruit

The ovary becomes the fruit; its wall becomes the pericarp. If only the ovary makes the fruit it is a true fruit (mango). If other parts such as the thalamus join, it is a false fruit (apple, strawberry).

Special cases: apomixis, parthenocarpy and polyembryony

• Apomixis: seeds form without fertilisation. The embryo grows from a diploid cell of the ovule. The offspring are copies of the mother. Farmers like it because hybrid seeds made this way keep their good traits year after year, so no new hybrid seed has to be bought each season.
• Parthenocarpy: a fruit forms without fertilisation, so it has no seeds (banana). It can be started by spraying plant hormones.
• Polyembryony: one seed has more than one embryo. Extra embryos grow from nucellus cells (citrus, mango). Plant one orange seed and several seedlings may come up.

Try it: open a flower and a seed

1. Take a hibiscus (gudhal) flower. Pull off the petals. Count the stamens around the central column. Touch the stigma tips: they are sticky, like the pollen trap in step 1 of the 3D.
2. Soak 5 chickpeas or groundnuts overnight. Peel the coat. Split the seed: you see two big cotyledons and a tiny embryo axis. This is a non-albuminous dicot seed.
3. Cut a maize grain lengthwise and put a drop of iodine: the endosperm turns blue-black (starch).

Predict first, then check: which part of the seed turns darkest with iodine?

Key formulas and definitions

• Pollen grain = male gametophyte: vegetative cell + generative cell → 2 male gametes
• Embryo sac = female gametophyte: 7 cells, 8 nuclei (egg + 2 synergids, central cell with 2 polar nuclei, 3 antipodals)
• Syngamy: male gamete (n) + egg (n) → zygote (2n) → embryo
• Triple fusion: male gamete (n) + 2 polar nuclei (2n) → PEN (3n) → endosperm
• Ovule → seed; integuments → seed coat; ovary → fruit; ovary wall → pericarp
• 1 microspore mother cell (meiosis) → 4 pollen grains; 1 megaspore mother cell → 4 megaspores, 1 survives

Worked examples

Common mistakes

• Saying the embryo sac has 8 cells. It has 8 nuclei but only 7 cells, because the 2 polar nuclei share one central cell.
• Thinking geitonogamy is cross-pollination. An agent is needed, but the pollen comes from the same plant, so the genes are the same as selfing.
• Mixing up apomixis and parthenocarpy. Apomixis = seed without fertilisation; parthenocarpy = fruit (usually seedless) without fertilisation.
• Calling endosperm diploid. It comes from triple fusion, so it is triploid (3n).