Why Some Flowers Refuse Their Own Pollen

A pollen grain is useless until it reaches a suitable stigma, and even then it must grow a tube all the way to an ovule. Flowers have evolved remarkable ways to move pollen, to avoid their own pollen, and finally to fuse gametes twice over.

This part covers types of pollination, pollinating agents, outbreeding devices and artificial hybridisation, and pollen-pistil interaction with double fertilisation.

Autogamy is pollination within the same flower, geitonogamy is between different flowers of the same plant, and xenogamy is between flowers of different plants; only xenogamy brings genetically different pollen to the stigma.

The three types:

- Autogamy — pollen reaches the stigma of the same flower; the anther and stigma must mature together and lie close to each other
- Geitonogamy — pollen moves to another flower on the same plant; it needs a pollinating agent, so it works like cross-pollination but is genetically the same as autogamy
- Xenogamy — pollen comes from a different plant, bringing genetically different gametes

Cleistogamous flowers never open, so they are invariably autogamous and set seed even without pollinators. Plants such as Viola, Oxalis and Commelina produce both open, chasmogamous flowers and closed, cleistogamous ones.

An everyday example. Pea and wheat are largely self-pollinated, so farmers can save their seed and grow the same variety again, while maize, with separate male and female flowers, depends on cross-pollination.

The substance. Geitonogamy is the trap — although an agent carries the pollen, the offspring are no more varied than from self-pollination.

Wind- and water-pollinated flowers release light pollen in large amounts and lack showy petals, while insect- and bird-pollinated flowers attract animals with colour, scent and nectar and have sticky pollen.

Wind pollination:

- Light, non-sticky pollen produced in large numbers
- Well-exposed stamens and large, often feathery stigmas
- Common in grasses; the silky threads of a maize cob are its styles and stigmas, waiting for windblown pollen

Water pollination is rare and found mostly in monocots, such as Vallisneria and Hydrilla in fresh water and seagrasses such as Zostera. In Vallisneria, the female flower reaches the surface on a long stalk, and male flowers released onto the water drift to it.

Insects and birds:

- Large, colourful, fragrant flowers rich in nectar, or small flowers grouped into showy clusters
- Sticky pollen that clings to the animal's body
- Bees are the dominant animal pollinators; sunbirds also pollinate many flowers
- Some flowers offer shelter or egg-laying sites, as the moth of Yucca lays eggs in its ovary while pollinating it

An everyday example. Honeybees buzzing over flowering mustard fields carry pollen from flower to flower while collecting nectar.

The substance. Floating on water does not make a flower water-pollinated — water lily and water hyacinth are pollinated by insects or wind.

Because continued self-pollination causes inbreeding depression, flowers avoid it by releasing pollen and making the stigma receptive at different times, by keeping anther and stigma apart, through self-incompatibility, and by bearing unisexual flowers; plant breeders cross chosen parents by emasculation and bagging.

Outbreeding devices:

- Asynchrony — pollen is released before the stigma becomes receptive, or after it stops being receptive
- Spatial separation — anther and stigma sit in different positions
- Self-incompatibility — a genetic mechanism that stops pollen from the same flower or plant germinating or growing its tube
- Unisexual flowers — male and female flowers on one plant (monoecious, as in castor and maize) prevent autogamy but not geitonogamy; on separate plants (dioecious, as in papaya) they prevent both

Artificial hybridisation:

- Emasculation — removing the anthers from a bisexual flower bud with forceps before they shed pollen
- Bagging — covering the emasculated flower with a butter-paper bag to keep out unwanted pollen
- Controlled pollination — dusting the receptive stigma with pollen from the chosen male parent, then re-bagging
- In unisexual flowers, the female buds are simply bagged before they open

An everyday example. Papaya growers keep both male and female trees in an orchard, because a dioecious female tree cannot pollinate itself.

The substance. Monoecy stops autogamy but not geitonogamy; only dioecy rules out both.

The pistil recognises compatible pollen and lets it grow a pollen tube through the style into the ovule, where one male gamete fuses with the egg in syngamy to form the diploid zygote and the other fuses with the two polar nuclei in triple fusion to form the triploid primary endosperm nucleus — together called double fertilisation.

Pollen-pistil interaction:

- Chemical signals between pollen and pistil decide whether the pollen is compatible
- Compatible pollen germinates, and a pollen tube grows out through a germ pore and down the style
- In 2-celled pollen, the generative cell divides into two male gametes as the tube grows
- The tube enters the ovule through the micropyle and then one synergid, guided by its filiform apparatus

Double fertilisation:

- Syngamy — a male gamete (n) fuses with the egg (n), giving the zygote (2n), which becomes the embryo
- Triple fusion — the other male gamete (n) fuses with the two polar nuclei (n + n), giving the primary endosperm nucleus (3n), which forms the endosperm

Worked example. In a plant with $2n=20$, the zygote has $20$ chromosomes and the primary endosperm nucleus has $3\times 10=30$.

An everyday example. The white, starchy bulk of a rice grain is endosperm built from triple fusion, while the tiny germ is the embryo formed from syngamy.

The substance. Double fertilisation is an event unique to flowering plants — two separate fusions take place in one embryo sac.