Flower and Fruit Development: Placentation and Floral Morphology

Seed and Fruit Development

  • Fertilized ovules develop into seeds.
  • Seed development triggers the surrounding ovary to develop into a fruit.
  • The ovary may develop into a fruit that is fleshy, fibrous, papery, and/or woody.
  • Seed and fruit development are coordinated processes.

Flower Characterization Based on Placentation Types

1. Marginal Placentation

  • Description: A megasporophyll (carpel) with ovules along its margins, which then rolls up to form a pistil. This results in a single "row" of ovules inside the ovary.
  • Structure: Characterized by:
    • 1 carpel
    • 1 locule
    • 1 "row" of ovules
  • Visual Features: When looking into the ovary from the top, you can observe a mid-vein and a suture where the opposite fused margins meet.
  • Example: Legume flower (e.g., pea, bean, redbud).
    • Components of a Legume Flower: Pedicel, Calyx, Sepals, Petals (Banner, Keel, Wing), Stamen (Free stamen, Fused stamens), Pistil (Stigma, Style, Ovary/Pod, Ovules that become seeds).

2. Axile Placentation

  • Description: This type involves three megasporophylls (carpels). Each sporophyll (carpel) rolls up separately with its ovules inside. These rolled sporophylls then fuse at their margins into one tri-carpellate pistil, with all fused margins facing into the middle of the ovary.
  • Structure: Characterized by:
    • >1 carpel
    • >1 locule
    • >1 "row" of ovules
    • Usually, the number of carpels, locules, and "rows" of ovules are equal.
  • Adaptation: The placental tissue can expand to produce more ovules in each "row".
  • Examples: Daffodil ovary (in cross-section), Bellflower family.

3. Parietal Placentation

  • Description: Three sporophylls (carpels) fuse margin-to-margin and roll into one tube, forming a multi-carpellate (compound) pistil. This arrangement results in three rows of ovules inside a single locule.
  • Structure: Characterized by:
    • >1 carpel
    • 1 locule
    • >1 "row" of ovules
    • The number of carpels is equal to the number of "rows" of ovules.
  • Adaptation: The placental tissue can expand to multiply the number of ovules in each "row".
  • Examples: Violet, Passion flower, Cucumber.

4. Basal Placentation

  • Description: A megasporophyll (carpel) has ovules on its margins, but only on the lower portion of the leaf. The rolled megasporophyll forms a pistil with the ovules located inside at its base.
  • Structure: Characterized by:
    • 1 or more carpels
    • 1 locule
    • 1 ovule or 1 cluster of ovules at the bottom.

Flower Characterization Based on Physical/Visual Position of Ovary

Superior Ovary / Hypogynous Flower

  • Superior Ovary: An ovary that visually sits atop (above) the receptacle.
  • Hypogynous Flower: A flower where the first three whorls (sepals, petals, and stamens) visually arise below the ovary (gynoecium).

Inferior Ovary / Epigynous Flower

  • Inferior Ovary: An ovary that is visually hidden within the receptacle.
  • Epigynous Flower: A flower where the first three whorls (sepals, petals, and stamens) visually arise from a position above the ovary.
  • Example: Malus (apple) flower, where the inferior ovary is enclosed by the hypanthium, and other floral parts appear to arise above it.

Perigynous Flower (For Your Information Only)

  • A type of flower where the ovary can be inferior, superior, or semi-superior. It is often conceptually grouped with inferior ovaries for simplicity in some classifications.

Flower Types Based on Whorls and Reproductive Structures

Complete Flowers

  • Definition: Flowers that possess all four floral whorls: receptacle, sepals, petals, stamens (microsporophylls), and pistil(s) (megasporophylls).
  • Example: Prunus sp. (e.g., cherry, plum).

Incomplete Flowers

  • Definition: Flowers that are missing any one or more of the four floral whorls.
  • Relationship to Imperfect Flowers: All imperfect flowers are incomplete, but not all incomplete flowers are imperfect (e.g., an incomplete flower can still be perfect if it's missing sepals or petals but has both stamens and pistils).
  • Examples:
    • Perfect, Apetalous Flower: A flower that has both male and female reproductive parts but is missing petals (e.g., elm flower).
    • Perfect, Aperiathous Flower: A flower that has both male and female reproductive parts but is missing both sepals and petals (e.g., grass flower).

Perfect Flowers

  • Definition: Also known as hermaphroditic flowers, these flowers possess both male (stamens/microsporophylls) and female (pistil(s)/megasporophylls) reproductive structures.
  • Floral Parts Present: Receptacle, sepals, petals, stamens, pistil(s).
  • Examples: Prunus sp., Fragaria sp. (strawberry).

Imperfect Flowers

  • Definition: Flowers that possess only the male or only the female reproductive structures.
  • Types of Imperfect Flowers:
    • Pistillate (Female/Carpellate) Flower: Contains a pistil(s) but lacks functional stamens.
    • Staminate (Male) Flower: Contains stamens but lacks a functional pistil(s).
  • Examples: Red Maple, Zucchini, Black Walnut.

Monoecious Angiosperms

  • Definition: Plants (sporophytes) that bear both pistillate (female) and staminate (male) imperfect flowers on the same individual plant.
  • Examples: Zucchini, Black Walnut, Corn.

Dioecious Angiosperms

  • Definition: Plants (sporophytes) that bear staminate (male) and pistillate (female) imperfect flowers on separate individual plants.
  • Examples: Campion (male plant with staminate flowers having non-functional pistils; female plant with pistillate flowers having non-functional stamens), Holly.

Composite Flowers

  • Family: Typically found in the Asteraceae family (e.g., sunflowers, daisies).
  • Description: "Composite flowers" are actually a type of flower cluster, or inflorescence, that is composed of many individual flowers. These individual flowers within the cluster are referred to as florets. Each floret contains structures such as a calyx and corolla.