2.1 Stamen, Microsporangium & Pollen Grain
A typical stamen consists of a long slender stalk called **filament** and a terminal bilateral structure called **anther**. The anther is typically bilobed and dithecous (two thecae per lobe), and contains four microsporangia (tetrasporangiate).
1. Structure of Microsporangium
A microsporangium is surrounded by four wall layers:
- Epidermis: Outer single protective layer.
- Endothecium: Fibrous layer helping in anther dehiscence.
- Middle layers: 1-3 layers that degenerate at maturity.
- Tapetum: Innermost layer. It has dense cytoplasm and is generally **multinucleate**. It nourishes the developing pollen grains.
At the center of each microsporangium is the **sporogenous tissue** in young anthers.
2. Microsporogenesis
The process of formation of microspores from a pollen mother cell (PMC) through meiosis. PMCs undergo meiosis to form a tetrad of microspores. As the anther matures and dehydrates, the microspores dissociate and develop into **pollen grains**.
3. Pollen Grain Structure
Represents the male gametophyte. It has a two-layered wall:
- Exine: Hard outer layer made of **sporopollenin** (one of the most resistant organic materials known; tolerates high temp, strong acids/alkali; no enzyme can degrade it). It has **germ pores** where sporopollenin is absent.
- Intine: Thin, continuous inner wall made of cellulose and pectin.
A mature pollen grain has two cells:
- Vegetative cell: Larger, rich food reserves, large irregular nucleus.
- Generative cell: Small, spindle-shaped, floats in vegetative cell cytoplasm. It divides mitotically to form **two male gametes**.
Pollen Shedding Stage: In over 60% of angiosperms, pollen is shed at the 2-celled stage (vegetative + generative). In the remaining species, the generative cell divides first, and pollen is shed at the 3-celled stage.
2.2 Pistil, Ovule & Megasporogenesis
The gynoecium represents the female reproductive part and consists of stigma, style, and ovary. Within the ovary is the ovarian cavity containing the placenta, from which megasporangia (commonly called **ovules**) arise.
1. Structure of Megasporangium (Ovule)
Key regions of a typical anatropous ovule:
- Funicle: Stalk attaching the ovule to the placenta.
- Hilum: Junction where the body of the ovule fuses with the funicle.
- Integuments: Protective envelopes (outer & inner) surrounding the nucellus.
- Micropyle: Small opening at the tip where integuments are absent.
- Chalaza: Basal part of the ovule, opposite the micropylar end.
- Nucellus: Central mass of parenchymatous cells containing abundant food reserves.
2. Megasporogenesis & Embryo Sac Development
The process of formation of megaspores from the Megaspore Mother Cell (MMC). A single MMC in the micropylar region of the nucellus undergoes meiosis to form a tetrad of four megaspores.
In most flowering plants, three megaspores degenerate, and only one remains functional. The functional megaspore develops into the female gametophyte (embryo sac) through mitotic divisions. This is called **monosporic development**.
3. The 7-Celled, 8-Nucleate Embryo Sac
A mature embryo sac has a unique distribution of 8 nuclei in 7 cells:
- Egg apparatus (at the micropylar end): Comprises **two synergids** and **one egg cell**. Synergids have finger-like **filiform apparatus** that guide the pollen tube.
- Antipodals (at the chalazal end): Three cells.
- Central cell: The largest single cell containing **two polar nuclei**.
2.3 Pollination & Outbreeding Devices
Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.
1. Kinds of Pollination
- Autogamy: Transfer of pollen from anther to stigma of the *same* flower. E.g., Viola, Oxalis, Commelina. These plants produce:
- Chasmogamous flowers: Exposed anthers and stigma.
- Cleistogamous flowers: Never open; assure seed-set even in the absence of pollinators; strictly autogamous.
- Geitonogamy: Transfer of pollen from anther of one flower to stigma of another flower on the *same* plant. Functionally cross-pollination, but genetically self-pollination.
- Xenogamy: Transfer of pollen from anther to stigma of a *different* plant. Brings genetically different pollens.
2. Agents of Pollination
- Wind (Anemophily): Pollen grains are light and non-sticky; stamens well-exposed; stigmas feathery. Commonly single ovule per ovary. E.g., Corn cob (tassels are stigma & style), grasses.
- Water (Hydrophily): Rare, mostly in monocots. E.g., Vallisneria, Hydrilla, and Zostera (seagrass). Pollen grains are long, ribbon-like, and protected by a mucilaginous coat.
- Biotic Agents (Animals/Insects): Most common biotic pollinating agents are bees. Flowers are large, colourful, fragrant, and rich in nectar. Floral rewards include nectar and pollen grains, or safe egg-laying sites (e.g., Amorphophallus, and the Yucca plant/moth relationship).
3. Outbreeding Devices
Mechanisms to discourage self-pollination and encourage cross-pollination (to avoid inbreeding depression):
- Pollen release and stigma receptivity are not synchronised.
- Anther and stigma are placed at different positions.
- Self-incompatibility: A genetic mechanism preventing self-pollen from fertilising the ovules by inhibiting pollen germination/tube growth.
- Production of unisexual flowers (e.g., castor, maize, papaya).
2.4 Pollen-Pistil Interaction & Double Fertilisation
1. Pollen-Pistil Interaction
A dynamic process involving pollen recognition followed by promotion or inhibition. The pistil checks compatibility. Compatible pollen germinates on the stigma to produce a pollen tube, which grows through the tissues of the style, reaches the ovary, enters the micropyle of the ovule, and enters a synergid through the **filiform apparatus**.
2. The Double Fertilisation Event
Unique to angiosperms. The pollen tube releases two male gametes into the cytoplasm of the synergid:
- Syngamy: One male gamete (n) moves towards the egg cell and fuses with its nucleus to form a diploid Zygote (2n), which develops into the embryo.
- Triple Fusion: The second male gamete (n) moves to the central cell and fuses with the two polar nuclei (2n) to form a triploid Primary Endosperm Nucleus (PEN, 3n).
Since two types of fusions (Syngamy and Triple Fusion) take place in an embryo sac, the phenomenon is termed **Double Fertilisation**. The central cell becomes the **Primary Endosperm Cell (PEC)** and develops into endosperm.
2.5 Post-Fertilisation Structures & Events
Post-fertilisation events include endosperm and embryo development, maturation of ovule into seed, and ovary into fruit.
1. Endosperm Development
Endosperm development **precedes** embryo development to ensure nutrition is available.
- Free-nuclear endosperm: The PEN undergoes successive nuclear divisions without cell wall formation. E.g., Coconut water (contains thousands of free nuclei).
- Cellular endosperm: Cell wall formation occurs subsequently. E.g., White coconut kernel (outer tissue).
2. Embryo Development (Embryogeny)
The zygote gives rise to the proembryo, then to globular, heart-shaped, and mature embryo stages.
- Dicot Embryo: Consists of an embryonal axis and two cotyledons. The portion of embryonal axis above cotyledons is the **epicotyl** (terminates in plumule). The portion below is the **hypocotyl** (terminates in radicle and root cap).
- Monocot Embryo: Possesses only **one cotyledon**, called **scutellum** (placed laterally). The embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called **coleorhiza**, and the shoot apex enclosed in a hollow foliar structure called **coleoptile**.
3. Seeds and Fruits
- Non-albuminous (Non-endospermous) seeds: Have no residual endosperm (completely consumed during development). E.g., Pea, groundnut.
- Albuminous (Endospermous) seeds: Retain part of the endosperm. E.g., Wheat, maize, barley, castor, sunflower.
- Perisperm: Persistent, residual nucellus in seeds. E.g., Black pepper, beet.
- False fruits: Thalamus contributes to fruit formation along with ovary. E.g., Apple, strawberry, cashew. True fruits develop solely from the ovary. **Parthenocarpic fruits** develop without fertilisation (e.g., Banana).
4. Apomixis & Polyembryony
- Apomixis: Production of seeds without fertilisation (a form of asexual reproduction mimicking sexual reproduction). E.g., Asteraceae, grasses.
- Polyembryony: Occurrence of more than one embryo in a seed. E.g., Citrus, Mango (nucellar cells protrude into embryo sac and develop into embryos).