Pollination - Kinds

Pollination is the transfer of pollen grains from the anther to the stigma. Based on the source of the pollen there are three kinds:

  • Autogamy (self-pollination) - pollen reaches the stigma of the same flower. It requires synchrony in the release of pollen and stigma receptivity, and the anther and stigma must lie close together.
  • In chasmogamous flowers (which open and expose the anther and stigma), autogamy is possible but not guaranteed.
  • In cleistogamous flowers (which never open, e.g. Viola, Oxalis, Commelina), the anthers and stigma lie close and autogamy is assured - they are invariably autogamous. Their advantage is that they set seed even without pollinators; the disadvantage is that there is no chance of cross-pollination and hence no new genetic recombination.
  • Geitonogamy - transfer of pollen from the anther of one flower to the stigma of another flower of the same plant. It is functionally cross-pollination (needs a pollinating agent) but is genetically the same as autogamy, because the pollen is from the same plant.
  • Xenogamy - transfer of pollen to the stigma of a different plant. This is the only kind that brings genetically different pollen to the stigma.

Agents of Pollination

Plants use abiotic agents (wind, water) and biotic agents (animals). Since pollen transfer by abiotic agents is by chance, such plants compensate by producing enormous quantities of pollen.

Wind pollination (anemophily)

The commonest abiotic mode. Adaptations: pollen is light and non-sticky; stamens are well-exposed; stigmas are large and often feathery; flowers often have a single ovule per ovary and are packed in an inflorescence. Examples: grasses, and maize (Zea mays) with its tassels.

Water pollination (hydrophily)

Quite rare, limited to about 30 genera of mostly monocotyledons. Two forms:

  • Epihydrophily - pollination at the water surface, as in Vallisneria: the female flower reaches the surface on a long stalk and the male flowers or pollen grains are released onto the surface and carried passively by water to the stigma.
  • Hypohydrophily - pollination below the water surface, as in sea-grasses (Zostera). Here the pollen grains are often long, ribbon-like and protected by mucilage.

In many water-pollinated species the pollen is protected from wetting by a mucilaginous covering. Note: most aquatic plants (e.g. water hyacinth, water lily) are not water-pollinated - they emerge above water and are pollinated by insects or wind.

One-liners: autogamy = same flower, geitonogamy = another flower same plant (genetically self), xenogamy = different plant (genetically different); cleistogamous = always autogamous; wind = commonest abiotic (light pollen, feathery stigma; grasses, maize); water pollination is rare - Vallisneria (epihydrophily), Zostera (hypohydrophily).

Pollination by Animals (Zoophily)

Most flowering plants use animals - chiefly insects (bees, butterflies, moths, beetles, wasps), and also birds (e.g. sunbirds) and bats. Bees are the most dominant pollinators.

  • Animal-pollinated flowers are usually large, colourful, fragrant and rich in nectar. Small flowers may be clustered into an inflorescence to be conspicuous.
  • Flowers pollinated by flies and beetles may emit foul odours to attract them.
  • The usual floral rewards are nectar and pollen grains. Some flowers offer a safe place to lay eggs - e.g. the tallest flower Amorphophallus, and the Yucca-moth relationship, in which the plant and the moth cannot complete their life cycles without each other (an obligate mutualism).
  • In some orchids (Ophrys), a petal mimics the female of the pollinator insect so the male pseudocopulates with the flower and pollinates it - pollination by sexual deceit.

Pollen-Pistil Interaction

After a pollen grain lands on the stigma, the pistil is able to recognise whether the pollen is of the right (compatible) type. This pollen-pistil interaction is a continuous dialogue mediated by chemical components:

  • If compatible, the pistil accepts the pollen, which germinates on the stigma to give a pollen tube; the tube grows through the tissues of the style, is guided to the ovule and enters the embryo sac.
  • If incompatible, the pistil rejects the pollen by preventing its germination or the growth of the pollen tube.

Artificial Hybridisation

In crop improvement, plant breeders make desired crosses by ensuring that only the wanted pollen fertilises a chosen female parent:

  • Emasculation - if the female parent bears bisexual flowers, the anthers are removed from the flower bud before the anther dehisces, using forceps.
  • Bagging - the emasculated flower is immediately covered with a butter-paper bag to prevent contamination by unwanted pollen. When the stigma becomes receptive, mature pollen of the desired male parent is dusted on it and the flower is re-bagged.
  • If the female parent produces unisexual flowers, emasculation is unnecessary - only bagging is needed.

One-liners: bees = dominant pollinators; animal flowers = colourful, fragrant, nectar; Yucca-moth = obligate mutualism; Ophrys = sexual deceit; compatible pollen germinates, incompatible is rejected; emasculation + bagging for hybridisation (bisexual parent); only bagging if unisexual.

Outbreeding Devices

Continued self-pollination leads to inbreeding depression. Many flowering plants have evolved outbreeding devices to discourage self-pollination and promote cross-pollination:

  1. Dichogamy - the pollen release and stigma receptivity are not synchronised: either the pollen is shed before the stigma of the same flower is receptive (protandry), or the stigma matures before the pollen (protogyny).
  2. Herkogamy - the anther and stigma are placed at different positions so that self-pollen cannot easily reach the stigma.
  3. Self-incompatibility - a genetic mechanism that prevents self-pollen (from the same flower or plant) from germinating or fertilising the ovules, by inhibiting pollen-tube growth.
  4. Production of unisexual flowers -
  • If male and female flowers are on the same plant (monoecious), e.g. castor and maize, it prevents autogamy (self within a flower) but not geitonogamy.
  • If they are on different plants (dioecious), e.g. papaya, it prevents both autogamy and geitonogamy.

One-liners: outbreeding devices prevent selfing -> dichogamy (protandry/protogyny), herkogamy (different positions), self-incompatibility (genetic), unisexuality; monoecious (castor, maize) stops only autogamy; dioecious (papaya) stops autogamy AND geitonogamy.