Angiosperm Reproduction and Seed Development

Posted on May 16, 2026 in Biology

The study of the female reproductive cycle and the subsequent formation of seeds represents the final stages of plant reproduction.

1. Megasporangium (Ovule)

The megasporangium, commonly known as the ovule, is the site where the female gamete is formed.

Structure of the Ovule

  • Funiculus: The stalk attaching the ovule to the placenta.
  • Hilum: The junction where the body of the ovule meets the funiculus.
  • Integuments: Protective outer envelopes (usually one or two).
  • Nucellus: A mass of parenchymatous cells enclosed within the integuments that provides nutrition.
  • Micropyle: A small opening at the tip of the integuments where the pollen tube enters.
  • Chalaza: The basal part of the ovule, opposite the micropyle.

2. Megasporogenesis and Megagametogenesis

Megasporogenesis

This is the formation of megaspores. A single Megaspore Mother Cell (MMC) in the nucellus undergoes meiosis to produce a linear tetrad of four haploid megaspores. Usually, the three micropylar megaspores degenerate, and the one at the chalazal end becomes the functional megaspore.

Megagametogenesis

The functional megaspore undergoes three rounds of mitosis (without immediate cell wall formation) to produce the 8-nucleate, 7-celled embryo sac (female gametophyte).

Types of Female Gametophyte

  1. Monosporic (Polygonum type): Develops from one functional megaspore (most common).
  2. Bisporic (Allium type): Develops from two megaspore nuclei.
  3. Tetrasporic (Fritillaria type): All four megaspore nuclei participate in forming the embryo sac.

3. Double Fertilization

Unique to angiosperms, this process involves two fusion events:

  • Syngamy: One male gamete fuses with the egg cell to form a diploid zygote (2n).
  • Triple Fusion: The second male gamete fuses with the two polar nuclei (or the secondary nucleus) to form the triploid Primary Endosperm Nucleus (PEN) (3n).

4. Endosperm Types

The endosperm provides nourishment to the developing embryo.

  • Nuclear: The PEN undergoes repeated free-nuclear divisions without wall formation (e.g., coconut water).
  • Cellular: Every nuclear division is followed by cell wall formation (e.g., Petunia).
  • Helobial: A hybrid type where the first division is followed by a wall, but subsequent divisions are free-nuclear (common in monocots).

5. Embryogenesis, Polyembryony, and Apomixis

Embryogenesis

  • Dicots: The zygote develops into a pro-embryo, then a globular, heart-shaped, and finally a mature embryo with two cotyledons.
  • Monocots: The embryo has only one cotyledon, called the scutellum. The plumule is protected by a coleoptile and the radicle by a coleorhiza.

Specialized Reproduction

  • Polyembryony: The occurrence of more than one embryo in a seed (common in Citrus).
  • Apomixis: A form of asexual reproduction that mimics sexual reproduction, where seeds are formed without fertilization (e.g., in some grasses and Asteraceae).

6. Seed Structure and Dispersal

Dicot vs. Monocot Seed

  • Dicot Seed: Non-endospermic (usually), with two fleshy cotyledons (e.g., pea, bean).
  • Monocot Seed: Endospermic (usually), with a single scutellum and a prominent endosperm layer (e.g., maize).

Dispersal Mechanisms

  1. Anemochory (Wind): Seeds with wings (maple) or hairs (dandelion).
  2. Hydrochory (Water): Fibrous or buoyant coats (coconut).
  3. Zoochory (Animals): Hooks/spines (Xanthium) or edible fruits with resistant seeds (tomato).
  4. Autochory (Self): Explosive mechanisms that “shoot” seeds out (balsam).