Pollination

The Core Idea: Why Pollination Exists

Flowering plants produce male and female gametes — the pollen grain and the embryo sac, respectively. Neither of these gametes can move on its own. For fertilisation to happen, they must be physically brought together. Pollination is the mechanism that achieves this: the transfer of pollen grains from the anther (where they are shed) to the stigma of a pistil.

Plants cannot move to find mates, so they have evolved an extraordinary range of adaptations to get pollen from one flower to another. They rely on external agents to carry the pollen. (See the in-text question on naming these agents, below.)

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Kinds of Pollination

Pollination is classified into three types based on the source of the pollen.

1. Autogamy (Self-pollination within the same flower)

• Pollen is transferred from the anther to the stigma of the same flower.
• In a normal, open flower, complete autogamy is rare. It requires two conditions to be met simultaneously:
  • Synchrony: Pollen release and stigma receptivity must occur at the same time.
  • Proximity: The anthers and stigma must lie close to each other.
• Some plants get around this by producing two kinds of flowers:
  • Chasmogamous flowers: These are typical, open flowers with exposed anthers and stigma. They can be cross-pollinated.
  • Cleistogamous flowers: These flowers never open. The anthers and stigma are already close together inside the bud. When the anthers dehisce (split open), pollen lands directly on the stigma. Because the flower never opens, there is zero chance of pollen from another flower landing on its stigma. Cleistogamous flowers are invariably autogamous. (See the in-text question on whether this is advantageous or disadvantageous, below.)
• Advantage: They produce assured seed-set even when no pollinators are available.
• Disadvantage: They produce no genetic variation — all offspring are clones of the parent.
• Examples: Viola (pansy), Oxalis, Commelina.

2. Geitonogamy (Pollination between flowers of the same plant)

• Pollen is transferred from the anther of one flower to the stigma of another flower on the same plant.
• Functionally, this is cross-pollination because a pollinating agent (wind, insect, etc.) is involved.
• Genetically, however, it is identical to autogamy — the pollen and ovules come from the same parent plant, so there is no new genetic material introduced.

3. Xenogamy (Cross-pollination between different plants)

• Pollen is transferred from the anther of one flower to the stigma of a flower on a different plant of the same species.
• This is the only type of pollination that brings genetically different pollen grains to the stigma. It is the source of genetic variation in the offspring.

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Agents of Pollination

Plants use two abiotic (non-living) agents — wind and water — and one biotic (living) agent — animals. The vast majority of flowering plants rely on biotic agents. Only a small proportion use wind or water.

1. Wind Pollination (Anemophily)

Wind pollination is the most common form of abiotic pollination.

Adaptations of wind-pollinated flowers:

• Pollen grains: Light, dry, and non-sticky so they can be carried easily by air currents.
• Stamens: Well-exposed, often hanging out of the flower, so pollen is easily shaken loose into the wind.
• Stigma: Large, often feathery or branched, to efficiently trap airborne pollen grains.
• Ovary: Often contains only a single ovule.
• Inflorescence: Flowers are often numerous and packed into a compact inflorescence (e.g., the tassel of a corn cob — what you see are the stigmas and styles waving in the wind).
• Common examples: Grasses, corn (maize).

2. Water Pollination (Hydrophily)

Water pollination is quite rare in flowering plants, limited to about 30 genera, mostly monocotyledons. (In contrast, lower plants like algae, bryophytes, and pteridophytes commonly use water to transport male gametes.)

Not all aquatic plants use water for pollination. In many, like water hyacinth and water lily, the flowers emerge above the water surface and are pollinated by insects or wind, just like land plants.

Two main strategies in water-pollinated plants:

• Pollen on the water surface (e.g., Vallisneria):

  • Female flowers reach the water surface on a long stalk.
  • Male flowers or pollen grains are released onto the water surface.
  • They are carried passively by water currents until some reach the female flower and its stigma.

• Pollen underwater (e.g., seagrasses like Zostera):

  • Female flowers remain submerged.
  • Pollen grains are released directly into the water.
  • The pollen grains in many such species are long and ribbon-like, which helps them be carried passively in water currents.

A key adaptation: In most water-pollinated species, pollen grains are protected from getting wet by a mucilaginous (slimy) covering.

(See the in-text question on why wind/water-pollinated flowers lack colour and nectar, below.)

3. Animal Pollination (Zoophily)

The majority of flowering plants use animals as pollinators.

Common pollinating agents: Bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds, hummingbirds), and bats. Among animals, insects — especially bees — are the dominant biotic pollinators. Even larger animals like some primates (lemurs), tree-dwelling rodents, and reptiles (gecko lizards, garden lizards) have been reported as pollinators in some species.

Adaptations of animal-pollinated flowers:

• Appearance: Often large, colourful, and fragrant. If individual flowers are small, they are clustered into a showy inflorescence.
• Rewards: Flowers must provide something to attract and sustain animal visits. The usual rewards are nectar and pollen grains.
• Specialised scents: Flowers pollinated by flies and beetles often produce foul odours (like rotting meat) to attract these animals.
• Sticky pollen: Pollen grains in animal-pollinated flowers are generally sticky, so they adhere to the animal's body.
• Mutualism: The animal visits the flower for a reward. In the process, its body picks up pollen from the anthers. When it visits another flower, the pollen on its body contacts the stigma, achieving pollination.

A special case — floral rewards as egg-laying sites:

• Amorphophallus: The tallest flower (about 6 feet high) provides a safe place for insects to lay eggs.
• Yucca and Yucca moth: This is a classic example of obligate mutualism. The moth deposits its eggs in the locule of the ovary of the Yucca flower. In return, the flower gets pollinated by the moth. The moth larvae hatch and develop as the seeds begin to develop. Neither species can complete its life cycle without the other.

(See the pollinator-observation activity, below.)

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Outbreeding Devices: How Plants Avoid Self-Pollination

Most flowering plants produce hermaphrodite (bisexual) flowers, making self-pollination a constant risk. Continued self-pollination leads to inbreeding depression — a reduction in fitness and vigour of the offspring. To encourage cross-pollination, plants have evolved several devices to discourage self-pollination.

1. Non-synchrony of pollen release and stigma receptivity:

   • Either the pollen is released before the stigma becomes receptive (protandry).
   • Or the stigma becomes receptive before the pollen is released (protogyny).
   • This prevents autogamy.

2. Different positioning of anther and stigma:

   • The anther and stigma are placed at different positions in the flower so that pollen cannot easily fall onto the stigma of the same flower.
   • This prevents autogamy.

3. Self-incompatibility:

   • This is a genetic mechanism. The pistil can recognise its own pollen (or pollen from the same plant) and actively reject it.
   • Rejection occurs by inhibiting pollen germination on the stigma or by stopping pollen tube growth in the style. This prevents fertilisation by self-pollen.

4. Production of unisexual flowers:

   • Monoecious plants (e.g., castor, maize): Male and female flowers are on the same plant. This prevents autogamy but does not prevent geitonogamy (pollen can still move between flowers on the same plant).
   • Dioecious plants (e.g., papaya): Male and female flowers are on different plants. Each plant is either male or female. This prevents both autogamy and geitonogamy, forcing xenogamy.

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Pollen-Pistil Interaction: The Dialogue

Pollination is just the delivery of pollen. What happens next is a dynamic, chemical conversation between the pollen grain and the pistil.

• Recognition: The pistil has the ability to recognise whether the pollen is of the right type (compatible — same species, not self-incompatible) or the wrong type (incompatible — different species, or self-pollen in a self-incompatible plant).
• Acceptance or Rejection:
  • If the pollen is compatible, the pistil accepts it and promotes the events that lead to fertilisation.
  • If the pollen is incompatible, the pistil rejects it by preventing pollen germination on the stigma or by stopping pollen tube growth in the style.
• The mechanism: This recognition is the result of a continuous dialogue mediated by chemical components of the pollen interacting with those of the pistil. Botanists have only recently begun to identify these components.

The path of a compatible pollen tube:

1. The pollen grain lands on the stigma and germinates, producing a pollen tube that emerges through one of the germ pores.
2. The contents of the pollen grain move into the pollen tube.
3. The pollen tube grows through the tissues of the stigma and style, down into the ovary.
4. It enters the ovule through the micropyle.
5. It then enters one of the synergids (the two helper cells in the embryo sac) through a structure called the filiform apparatus. Recent studies show the filiform apparatus guides the pollen tube's entry. …