D3.1.8 Sexual reproduction in flowering plants Notes

Sexual Reproduction in Flowering Plants Ensures Genetic Diversity

1. Flowering plants, or angiosperms, reproduce sexually to ensure genetic variation and adaptability.
2. This complex process involves gamete production, pollination, fertilization, and embryo development, even in hermaphroditic plants that have both male and female reproductive organs.

Flowers Are Composed of Male And Female Reproductive Parts

Male Reproductive Parts (Stamen)

1. Anther: Produces pollen grains containing male gametes.
2. Filament: Supports the anther and positions it for effective pollination.

Female Reproductive Parts (Carpel/Pistil)

1. Stigma: Receives pollen during pollination.
2. Style: A tube connecting the stigma to the ovary.
3. Ovary: Contains ovules where female gametes develop.

Gamete Production: Male and Female Contributions

Male Gametes Develop Pollen

1. Inside the Anther:
   1. Diploid cells undergo meiosis, producing haploid cells.
   2. Each haploid cell develops into a pollen grain, which contains:
      1. Two male gametes (sperm cells).
      2. A tube nucleus to guide pollen tube growth.

Female Gametes Develop The Ovule

1. Inside the Ovary:
   1. Diploid cells undergo meiosis, producing haploid nuclei.
   2. One nucleus becomes the egg cell (female gamete), while others assist in fertilization or embryonic development.

Pollination: Transferring Pollen to the Stigma

1. There are two key methods of pollination:
   1. Wind Pollination: Grasses and conifers rely on lightweight pollen carried by the wind.
   2. Animal Pollination: Insects, birds, and mammals transfer pollen as they feed on nectar.

Fertilization is The Union of Gametes

1. Once pollen lands on a compatible stigma:
   1. Pollen Tube Formation:
      1. The pollen grain germinates, forming a pollen tube that grows down the style toward the ovule.
      2. The tube nucleus directs this growth.
   2. Fusion of Gametes:
      1. One male gamete fuses with the egg cell, forming a diploid zygote.
      2. The second male gamete fuses with additional nuclei in the ovule, forming a triploid endosperm to nourish the developing embryo.

Embryo Development: From Zygote to Seed

1. After fertilization, the zygote undergoes mitosis, leading to:
   1. Embryo Formation: Develops structures like roots, shoots, and cotyledons.
   2. Seed Maturation:
      1. The ovule becomes a seed, containing:
         1. The embryo.
         2. The endosperm, a nutrient source.
         3. A protective seed coat.
2. The ovary develops into a fruit, aiding seed dispersal.

Many Plants Have Mechanisms to Prevent Self-Pollination And Ensure Cross-Pollination

1. This is meant to promote genetic diversity:
   1. Physical Separation: Male and female organs are positioned apart.
   2. Self-Incompatibility: A genetic mechanism prevents pollen from fertilizing ovules of the same plant.

Reflection and Review

1. Sexual reproduction in flowering plants is a remarkable process that ensures the survival and diversity of plant species.
2. By understanding the steps of gamete production, pollination, fertilization, and embryo development, you can appreciate the complexity and elegance of plant reproduction.