Sexual Reproduction in Flowering Plants (Angiosperms)

FLOWER - A FASCINATING ORGAN OF ANGIOSPERMS

• Aesthetic and Cultural Significance: Human beings have maintained an intimate relationship with flowers since time immemorial. Flowers serve as objects of aesthetic, ornamental, social, religious, and cultural value. They are used as symbols for conveying deep human feelings including love, affection, happiness, grief, and mourning.
• Floriculture: This refers to the cultivation of flowers for sale or use in gardens and social/family celebrations.
• Biological Perspective: To a biologist, flowers are morphological and embryological marvels and the primary sites of sexual reproduction.
• Purpose of Floral Diversity: The immense diversity of structures in inflorescences, flowers, and floral parts (including scents, perfumes, and rich colors) represents adaptations that ensure the formation of fruits and seeds, the end products of sexual reproduction.
• Flower as a Modified Shoot: A handwritten note in the transcript identifies the flower as a "modified shoot."
• Major Floral Parts:
  • Androecium: The whorl of stamens, representing the male reproductive organ.
  • Gynoecium: The female reproductive organ.
  • Sterile/Accessory Parts: Petals (corolla) and sepals (calyx).
  • Supportive Structures: Thalamus (the base) and pedicel (stalk).

PRE-FERTILISATION: STRUCTURES AND EVENTS

• Initiation of Flowering: The decision to flower occurs long before the actual flower is visible. It involves several hormonal and structural changes initiated within the plant. These changes lead to the differentiation and development of the floral primordium.
• Developmental Sequence: Inflorescences are formed first, which then bear floral buds and eventually flowers.

STAMEN, MICROSPORANGIUM, AND POLLEN GRAIN

• The typical stamen (Male Reproductive Organ): Consists of two distinct parts:
  • Filament: A long and slender stalk. The proximal end of the filament is attached to the thalamus or a petal of the flower.
  • Anther: The terminal, usually bilobed structure.
• Variability: The number and length of stamens are highly variable among different species. Even within the same flower, the length of stamens can vary (examples noted in margin: Salvia and Mustard).
• Anther Structure:
  • Bilobed and Dithecous: A typical angiosperm anther has two lobes, each containing two theca (hence dithecous). A longitudinal groove often runs lengthwise, separating the theca.
  • Tetragonal Structure: In transverse section, the anther appears as a four-sided (tetragonal) structure.
  • Microsporangia: Located at the four corners of the anther, with two in each lobe. These develop further to become pollen sacs.
  • Pollen Sacs: These extend longitudinally through the entire length of the anther and are packed with pollen grains.
• Microsporangium Wall Layers: A typical microsporangium is circular in outline and surrounded by four wall layers:
  • Epidermis: The outermost protective layer.
  • Endothecium: The second layer; noted as being hygroscopic in nature to aid in dehiscence.
  • Middle Layers: Provide protection and aid in anther dehiscence.
  • Tapetum: The innermost layer. It nourishes the developing pollen grains. Tapetal cells possess dense cytoplasm and are generally bi-nucleate or multi-nucleate.
• Sporogenous Tissue: In a young anther, compactly arranged homogenous cells called sporogenous tissue occupy the center of each microsporangium.
• Microsporogenesis: The process of forming microspores from a pollen mother cell (PMC) through meiosis.
  • As the anther develops, sporogenous tissue cells undergo meiotic division to form microspore tetrads.
  • Every cell of the sporogenous tissue is a potential pollen or microspore mother cell.
  • Microspore Tetrad: The microspores are initially arranged in a cluster of four cells stuck together by callose.
  • Maturation: As anthers mature and dehydrate, the microspores dissociate from each other and develop into individual pollen grains.

POLLEN GRAIN: THE MALE GAMETOPHYTE

• Physical Characteristics:
  • Shape and Size: Generally spherical, measuring approximately $25-50\,\mu m$ in diameter.
  • Architecture: Exhibit a vast variety of sizes, shapes, colors, and designs due to the exine.
• Pollen Wall (Sporoderm):
  • Exine: The hard outer layer made of sporopollenin. Sporopollenin is one of the most resistant organic materials known; it withstands high temperatures, strong acids, and alkalis. No known enzyme degrades it.
  • Germ Pores: Apertures in the exine where sporopollenin is absent. These allow the pollen tube to emerge.
  • Intine: The thin, continuous inner wall made of cellulose and pectin.
• Internal Composition:
  • The cytoplasm is surrounded by a plasma membrane.
  • Vegetative Cell: The larger cell, containing abundant food reserves and a large, irregularly shaped nucleus.
  • Generative Cell: A small cell that floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus.
• Shedding Stages:
  • Two-celled stage: Occurs in over $60\%$ of angiosperms.
  • Three-celled stage: In the remaining species, the generative cell divides mitotically to produce two male gametes before the pollen is shed.
• Impact on Human Health:
  • Pollen causes severe allergies and bronchial afflictions (asthma, bronchitis).
  • Parthenium (Carrot Grass): Came to India as a contaminant with imported wheat; it is ubiquitous and a major cause of pollen allergy.
• Pollen as Nutrients: Pollen grains are rich in nutrients. Pollen products (tablets and syrups) are used as food supplements. Claims suggest they increase the performance of athletes and race horses.
• Pollen Viability: The period pollen remains functional varies. It is influenced by temperature and humidity.
  • Short Viability: Rice and wheat pollen lose viability within $30\,\text{minutes}$.
  • Long Viability: Members of Rosaceae, Leguminoseae, and Solanaceae maintain viability for months.
  • Storage: Pollen can be stored for years in liquid nitrogen at $-196^\circ C$. These are used in pollen banks for crop breeding.

THE PISTIL, MEGASPORANGIUM (OVULE), AND EMBRYO SAC

• Gynoecium: The female reproductive part of the flower.
  • Monocarpellary: Consisting of a single pistil.
  • Multicarpellary: Consisting of more than one pistil.
  • Syncarpous: Pistils fused together (e.g., Papaver).
  • Apocarpous: Pistils are free (e.g., Michelia).
• Parts of a Pistil:
  • Stigma: Landing platform for pollen grains.
  • Style: Elongated slender part beneath the stigma.
  • Ovary: Basal bulged part containing the ovarian cavity (locule). Inside the locule is the placenta.
• The Megasporangium (Ovule):
  • Funicle: The stalk attaching the ovule to the placenta.
  • Hilum: The junction where the body of the ovule fuses with the funicle.
  • Integuments: One or two protective envelopes (which later become the testa and tegmen).
  • Micropyle: A small opening at the tip of the integuments for entry of oxygen/water/pollen tube.
  • Chalaza: The basal part of the ovule, opposite the micropylar end.
  • Nucellus: A mass of cells enclosed by integuments containing abundant reserve food.
  • Quantity: Ovaries may contain one ovule (wheat, paddy, mango) or many (papaya, watermelon, orchids).
• Megasporogenesis: The process of forming megaspores from the megaspore mother cell (MMC).
  • The MMC is a large cell in the micropylar region of the nucellus with dense cytoplasm and a prominent nucleus.
  • The MMC undergoes meiosis to produce four megaspores ($n$).
• Female Gametophyte (Embryo Sac):
  • Monosporic Development: In most flowering plants, only one megaspore is functional, while the other three degenerate. The functional megaspore develops into the embryo sac.
  • Phases of Development: The nucleus of the functional megaspore undergoes three sequential mitotic nuclear divisions (strictly free nuclear, no immediate cell wall formation) to form a $2$-nucleate, $4$-nucleate, and finally an $8$-nucleate embryo sac.
  • Cellular Organization: At maturity, cell walls are laid down. The typical angiosperm embryo sac is $8$-nucleate but $7$-celled.
    • Egg Apparatus: Three cells at the micropylar end ($2$ synergids and $1$ egg cell).
    • Filiform Apparatus: Cellular thickenings in synergids that guide the pollen tube.
    • Antipodals: Three cells at the chalazal end.
    • Central Cell: A large cell containing two polar nuclei situated below the egg apparatus.

POLLINATION

• Definition: The transfer of pollen grains from the anther to the stigma of a pistil. Since both male and female gametes are non-motile in flowering plants, pollination is required for fertilization.
• Kinds of Pollination:
  1. Autogamy: Pollination within the same flower. Requires synchrony in pollen release and stigma receptivity.
     • Chasmogamous flowers: Open flowers with exposed anthers and stigma.
     • Cleistogamous flowers: Flowers that never open (e.g., Viola, Oxalis, Commelina). They are invariably autogamous and ensure seed-set even without pollinators but lack genetic variation.
  2. Geitonogamy: Transfer of pollen to the stigma of another flower on the same plant. Functionally cross-pollination (requires an agent) but genetically autogamy.
  3. Xenogamy: Transfer of pollen to the stigma of a different plant. This brings genetically different types of pollen to the stigma.
• Agents of Pollination:
  • Abiotic Agents:
    • Wind (Anemophily): Most common abiotic mode. Requires light, non-sticky pollen, well-exposed stamens, and feathery stigmas. Often many flowers are packed into an inflorescence (e.g., corn cob tassels). Common in grasses.
    • Water (Hydrophily): Rare (limited to about $30$ genera, mostly monocots like Vallisneria, Hydrilla, Zostera). Pollen is often ribbon-like and protected by a mucilaginous covering. Note: Not all aquatic plants use water; water lily and hyacinth are pollinated by insects/wind.
  • Biotic Agents:
    • Dominant Agents: Bees are the most common. Others include butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds/hummingbirds), and bats.
    • Specific Adaptations: Large colorful flowers, fragrance, and nectar reward. Small flowers cluster into inflorescences for visibility. Foul odors attract flies and beetles.
    • Floral Rewards: Nectar and pollen. Some provide safe egg-laying sites (e.g., Amorphophallus at $6\,\text{feet}$ tall, and the symbiotic relationship between the moth and the Yucca plant).
    • Nectar/Pollen Robbers: Insects that consume rewards without aiding pollination.

OUTBREEDING DEVICES AND POLLEN-PISTIL INTERACTION

• Inbreeding Depression: Continued self-pollination leads to reduced vigor.
• Devices to Discourage Self-Pollination:
  1. Dichogamy: Non-synchronization of pollen release and stigma receptivity.
  2. Heterostyly: Different positions of anthers and stigma.
  3. Self-incompatibility: A genetic mechanism inhibiting self-pollen germination or tube growth.
  4. Dicliny (Unisexual flowers): If monoecious (castor, maize), it prevents autogamy; if dioecious (papaya), it prevents both autogamy and geitonogamy.
• Pollen-Pistil Interaction:
  • The pistil recognizes compatible vs. incompatible pollen through a chemical dialogue.
  • If compatible, the pollen germinates. The pollen tube grows through the style, carrying either two cells (vegetative/generative) or three cells (vegetative/two male gametes).
  • The pollen tube enter the ovule through the micropyle and is guided into a synergid by the filiform apparatus.
• Artificial Hybridization:
  • Emasculation: Removal of anthers from bisexual flower buds before dehiscence.
  • Bagging: Covering the emasculated stigma with butter paper to prevent unwanted pollination.

DOUBLE FERTILISATION

• Process: This unique event in angiosperms involves two types of fusions:
  1. Syngamy: One male gamete ($n$) + Egg cell ($n$) $\rightarrow$ Zygote ($2n$). The zygote develops into the embryo.
  2. Triple Fusion: Second male gamete ($n$) + Two polar nuclei ($n+n$ in central cell) $\rightarrow$ Primary Endosperm Nucleus (PEN, $3n$). The central cell becomes the Primary Endosperm Cell (PEC) and develops into the endosperm.
• Significance: Double fertilization ensures that the nutrient tissue (endosperm) develops only if fertilization is successful.

POST-FERTILISATION: STRUCTURES AND EVENTS

• Endosperm Development: Occurs before embryo development to ensure nutrition.
  • Free-Nuclear Endosperm: Successive nuclear divisions without cell wall formation (e.g., coconut water is made of thousands of nuclei).
  • Cellular Endosperm: Cell walls are formed later (e.g., the white kernel of coconut).
  • Fate:
    • Non-albuminous (Ex-albuminous): Completely consumed during development (e.g., pea, groundnut, beans).
    • Albuminous: Persists in the mature seed (e.g., castor, coconut, wheat, maize).
• Embryogeny: The zygote develops into a proembryo, then a globular embryo, then a heart-shaped embryo, and finally a mature embryo.
  • Dicot Embryo: Consists of an embryonal axis and two cotyledons.
    • Epicotyl: Above cotyledons, ends in plumule (shoot tip).
    • Hypocotyl: Below cotyledons, ends in radicle (root tip) covered by a root cap.
  • Monocot Embryo: Only one cotyledon called the scutellum (lateral).
    • Coleorrhiza: Sheath protecting radicle.
    • Coleoptile: Hollow foliar structure protecting the shoot apex.
• The Seed: Final product of sexual reproduction (fertilized ovule).
  • Perisperm: Residual persistent nucellus found in black pepper and beet.
  • Dormancy: A state of metabolic inactivity when water content drops to $10-15\%$ and conditions are unfavorable.
  • Old Records:
    • Lupinus arcticus (Arctic Tundra) - $10,000$ years dormancy.
    • Phoenix dactylifera (King Herod’s palace) - $2,000$ years old.
• The Fruit: Developed from the ovary.
  • Pericarp: The wall of the fruit.
  • True Fruit: Develops only from ovary.
  • False Fruit: Other parts like the thalamus contribute (e.g., apple, strawberry, cashew).
  • Parthenocarpic Fruit: Develops without fertilization (e.g., banana). Can be induced by growth hormones.

APOMIXIS AND POLYEMBRYONY

• Apomixis: Seed production without fertilization (asexual reproduction mimicking sexual). Common in Asteraceae and grasses.
  • Mechanisms: Formation of diploid egg without meiosis or protrusion of nucellar cells into the embryo sac.
• Polyembryony: Presence of more than one embryo in a seed (common in Citrus and Mango).
• Importance in Agriculture: Apomixis prevents the segregation of characters in hybrid varieties, allowing farmers to reuse hybrid seeds year after year without buying expensive new stocks.