Self-Incompatibility in Plants

Pollination

• Pollination is the transfer of pollen grains from the anther to the stigma of a flower.
• There are two types of pollination: self-pollination and cross-pollination.

Self-Pollination

• Self-pollination is the transfer of pollen grains from the anther to the stigma of the same flower.
• It can occur in bisexual flowers or in unisexual flowers where both male and female flowers are borne on the same plant.
• Advantages of self-pollination:
  • It is a surer process in bisexual flowers where stamens and carpels mature at the same time.
  • Parental characters are preserved.
  • There is no wastage of pollen grains.
  • The flowers need not be large and showy.
• Disadvantages of self-pollination:
  • Continued self-pollination may lead to weakening of the variety or species.
  • The seeds produced are poor in quality and give less vigorous offspring.
  • It does not yield new varieties.

Cross-Pollination

• Cross-pollination is the transfer of pollen grains from the anther of one flower to the stigma of another flower of the same species.
• Advantages of cross-pollination:
  • The offspring are healthier.
  • The seeds produced are abundant.
  • New varieties may be produced by cross-pollination.
• Disadvantages of cross-pollination:
  • It requires an external agent (wind, water, insect) to be present.
  • The pollinating agent may not be available at the right time.
  • The pollen quantity may not be sufficient.

Agents of Cross-Pollination

• Insects: insects like bees and butterflies are common agents of cross-pollination.
• Wind: wind is another common agent of cross-pollination.
• Water: water-pollinated flowers are found in aquatic plants.
• Birds: some flowers are pollinated by birds.

Characteristics of Insect-Pollinated Flowers

• They have bright colors to attract insects.
• They produce nectar, which is food for insects.
• The pollen grains are sticky or spiny to enable them to be carried by insects.
• The stigma is often hidden and does not generally hang out of the flower.

Characteristics of Wind-Pollinated Flowers

• They are usually not brightly colored.
• They produce a large amount of pollen.
• The anthers are often versatile and loosely attached to the filaments.
• The pollen grains are light and smooth.
• The stigmas are often hairy and rough to trap the pollen grains.

Fertilization

• Fertilization occurs when a pollen grain reaches the stigma and germinates, producing a pollen tube.

• The pollen tube grows down through the style and into the ovary, where it fertilizes an egg cell.

• The fertilized egg cell develops into a zygote, which then grows into a seed.

• The process of fertilization is essential for the reproduction of flowering plants.### Differences between Wind-Pollinated and Insect-Pollinated Flowers

• Wind-pollinated flowers have dull colors, no fragrance, and produce large amounts of pollen grains that are light, smooth, and small.

• Insect-pollinated flowers are brightly colored, fragrant, and secrete nectar, with fewer pollen grains produced that are larger, sticky, and spiny.

Characteristics of Wind-Pollinated Flowers

• Unisexual flowers
• Stigma is long and hangs out of the petals
• Stamens are long and protrude above petals
• Examples: Maize, grass, rice, wheat, China rose, Salvia, pea, sunflower

Characteristics of Insect-Pollinated Flowers

• Stigma is often deep in the corolla and small
• Stamens may be within the corolla tube
• Examples: Sweet pea, flowers that require pollination by insects

Elephophily

• Elephophily is a type of pollination involving elephants
• Found in Rafflesia, a rare flower that grows at the feet of elephants
• Pollen grains get stuck to the elephant's feet and are carried to other similar flowers

Artificial Pollination

• Artificial pollination is the transfer of pollen to the stigma by humans
• Practiced by plant breeders and scientists to evolve new varieties
• Involves emasculation (removing anthers) and bagging (covering flowers with plastic bags)