Comprehensive Study Notes on Flower Adaptation, Fertilization, and Seed Development

Adaptations of Flowers for Wind and Insect Pollination

  • General Characteristics of Wind-Pollinated Flowers

    • Petals: These flowers are usually small and inconspicuous. Petals are often absent altogether. If they are present, they are typically small, green, or dull-coloured. They lack scent, nectar, or honey guides because there are no pollinating agents (like animals) to attract.
    • Pollen Grains: Produced in large quantities because many are lost during the wind-dispersal process. The grains themselves are small, smooth, and light, allowing them to be easily carried by the wind.
    • Stamen (Anthers and Filaments): Anthers are large to facilitate the production of vast amounts of pollen. They are loosely attached to long, thin filaments and hang outside the flower so that pollen can be easily blown off by wind currents.
    • Stigma: The stigmas are long, branched, and feathery. They hang outside the flower to provide a large surface area for catching wind-borne pollen grains.
    • Examples: Guinea grass, maize, and sugar cane.

  • General Characteristics of Insect-Pollinated Flowers

    • Petals: These flowers are usually large and conspicuous. Petals are relatively large, brightly coloured, and scented. They contain nectaries and honey guides specifically designed to attract insects.
    • Pollen Grains: Produced in smaller quantities compared to wind-pollinated flowers because fewer grains are lost. They are relatively large, sticky, or spiky, which allows them to adhere to the bodies of insects.
    • Stamen (Anthers and Filaments): Anthers are firmly attached to short, stiff filaments. They are usually situated inside the flower so that an insect must brush against them to reach the nectar, picking up pollen without damaging the anthers.
    • Stigma: Stigmas are flat or lobed and sticky. They are situated inside the flower so that insects brush against them while seeking nectar, depositing pollen onto the sticky surface.
    • Examples: Pride of Barbados, flamboyant, and allamanda.

The Process of Fertilization in Flowering Plants

  • Initial Stages at the Stigma

    • Pollination occurs when pollen is transferred to the stigma of a flower.
    • Once a pollen grain lands on the stigma, it absorbs nutrients present on the surface, swells, and begins to develop a pollen tube.

  • Growth of the Pollen Tube

    • The pollen tube begins to grow downward through the style toward the ovary.
    • It carries three nuclei in its tip: one tube nucleus and two male nuclei (also known as male gametes).
    • The pollen tube navigates through the style by secreting digestive enzymes that digest a pathway through the tissue.

  • Entry into the Ovule

    • The pollen tube grows through the ovary wall and enters the ovule through a small opening called the micropyle.
    • Once the tube enters the ovule, it bursts.
    • The tube nucleus degenerates after serving its purpose of guiding the tube's growth.

  • Double Fertilization Events

    • First Fusion (Zygote Formation): One male nucleus fuses with the female sex cell (the female gamete or egg) in a process called fertilisation. This results in the formation of a zygote, which eventually develops into the embryo within the seed.
    • Second Fusion (Endosperm Formation): The second male nucleus fuses with two polar nuclei in the ovule (specifically within the embryo sac) to form the endosperm tissue. This tissue serves to store food for the developing zygote during the process of germination.

Seed Development Following Fertilization

  • Transformation of the Ovule

    • After fertilisation, each individual ovule develops into a seed.

  • Embryo Development

    • The zygote undergoes division by mitosis to form the embryo.
    • The embryo develops into three distinct parts:
      • The plumule: The embryonic shoot.
      • The radicle: The embryonic root.
      • Cotyledon(s): These are the seed leaves. Monocotyledons possess one cotyledon, while dicotyledons possess two cotyledons.

  • Endosperm vs. Non-Endospermic Seeds

    • Endospermic Seeds: In seeds such as maize and castor oil, the endosperm remains and serves as the primary food storage tissue.
    • Non-Endospermic Seeds: In seeds such as green bean and pigeon pea, the endosperm is absorbed by the cotyledons during development. In these cases, the cotyledons take over the role of storing food.

  • Structural Changes to the Seed

    • Testa: The integuments (the outer layers of the ovule) become dry and develop into the testa, which is the seed coat.
    • Micropyle: The micropyle survives the transformation and remains as a small opening in the testa.
    • Dormancy: Water is withdrawn from the seed, leading it to become dormant until conditions for germination are met.

Fruit Development and Floral Changes

  • Transformation of the Ovary

    • Following fertilisation, the ovary wall develops into the fruit.
    • A fruit contains one or more seeds. The specific number of seeds is determined by the number of ovules in the original ovary that were successfully fertilised.
    • The shape and structure of many fruits closely resemble the structure of the original ovary.

  • Fate of Other Flower Parts

    • Withering and Abscission: The stigma, style, stamens, and petals typically wither and drop off after their roles are completed.
    • Sepals: The sepals may drop off, though in some plants, such as the eggplant, they may remain attached to the fruit.