Learning Objectives
- Understand the structure of a flower and its reproductive parts
- Learn the processes of microsporogenesis and megasporogenesis
- Study pollination types and mechanisms
- Understand double fertilization and post-fertilization events
Key Concepts
Structure of Flower
Flower is the reproductive unit of angiosperms. Parts: Calyx (sepals), Corolla (petals), Androecium (male — stamens), Gynoecium (female — carpels/pistils). A stamen consists of filament + anther (bilobed, dithecous, tetrasporangiate). A pistil consists of stigma + style + ovary.
Microsporogenesis and Male Gametophyte
Microsporogenesis: Formation of microspores (pollen grains) in pollen sacs of anther. Microspore mother cells (2n) undergo meiosis → 4 microspores (n) in a tetrad. Each microspore develops into a pollen grain (male gametophyte). Pollen grain has two layers: Exine (outer, made of sporopollenin — most resistant organic material, has germ pores) and Intine (inner, made of cellulose and pectin). Mature pollen grain: 2-celled (vegetative cell + generative cell) or 3-celled (vegetative cell + 2 male gametes). Vegetative cell has tube nucleus; generative cell divides to form 2 male gametes.
Megasporogenesis and Female Gametophyte
Ovule (Megasporangium): Contains nucellus, integuments, micropyle, chalaza, hilum, funicle. Megasporogenesis: Megaspore mother cell (2n) undergoes meiosis → 4 megaspores (n). Only one is functional (other 3 degenerate). Functional megaspore develops into embryo sac (female gametophyte).
Embryo sac (7-celled, 8-nucleate): 3 mitotic divisions of functional megaspore → 8 nuclei. Organization: Micropylar end: 1 egg cell + 2 synergids (with filiform apparatus — guides pollen tube). Chalazal end: 3 antipodal cells. Central cell: 2 polar nuclei (fuse to form secondary nucleus — diploid). This is the Polygonum type (most common) embryo sac.
Pollination
Self-pollination (Autogamy): Pollen from anther to stigma of same flower. Geitonogamy: Pollen transfer between different flowers of same plant (genetically self-pollination). Cross-pollination (Xenogamy): Pollen from one plant to stigma of another plant — brings genetic variation.
Agents of pollination: Wind (anemophily — light pollen, feathery stigma, e.g., grasses, maize), Water (hydrophily — rare, e.g., Vallisneria, Zostera), Insects (entomophily — colourful, scented, nectar, e.g., Rafflesia), Birds (ornithophily), Bats (chiropterophily).
Outbreeding devices: Self-incompatibility (pollen rejected on same plant), dioecy, dichogamy (anther and stigma mature at different times), herkogamy (mechanical barrier). Pollen-pistil interaction: Compatible pollen is accepted; pollen tube grows through style to ovule.
Double Fertilization
Unique to angiosperms. Discovered by S.G. Nawaschin. Pollen tube enters ovule through micropyle (porogamy). Releases 2 male gametes: (1) One male gamete + egg cell → Zygote (2n) — syngamy. (2) Other male gamete + polar nuclei/secondary nucleus → Primary Endosperm Nucleus (3n) — triple fusion. This is double fertilization. PEN develops into endosperm (3n) — triploid, nourishes embryo.
Post-Fertilization Events
Endosperm development: Precedes embryo development. Nuclear type (free nuclear divisions, then cell wall formation — coconut water is free nuclear endosperm), Cellular type (cell wall after each division), Helobial type (intermediate). Embryo development: Zygote → proembryo → globular → heart-shaped → torpedo → mature embryo. Dicot embryo: Embryonal axis with epicotyl (plumule) and hypocotyl (radicle), 2 cotyledons. Monocot embryo: Scutellum (single cotyledon), coleoptile (covers plumule), coleorhiza (covers radicle).
Seed: Mature ovule. Fruit: Mature ovary. Apomixis: Production of seeds without fertilization (genetic clone of parent). Example: some grasses, Asteraceae. Polyembryony: More than one embryo in a seed (e.g., Citrus — nucellar embryos).
Summary
Sexual reproduction in angiosperms involves microsporogenesis, megasporogenesis, pollination, double fertilization, and seed/fruit development. The embryo sac is 7-celled, 8-nucleate. Double fertilization produces both zygote and endosperm. Seeds develop from ovules and fruits from ovaries.
Important Terms
- Sporopollenin: Most resistant biological material; forms exine of pollen grain
- Double fertilization: Syngamy + Triple fusion, unique to angiosperms
- Embryo sac: Female gametophyte; 7-celled, 8-nucleate (Polygonum type)
- Filiform apparatus: Finger-like projections of synergids that guide pollen tube
- Apomixis: Seed formation without fertilization
- Polyembryony: Multiple embryos in one seed
- Autogamy: Self-pollination within same flower
- Xenogamy: Cross-pollination between different plants
Quick Revision
- Pollen grain: exine (sporopollenin) + intine (cellulose); germ pores in exine
- Embryo sac: egg (1) + synergids (2) + antipodals (3) + central cell (2 polar nuclei) = 7 cells, 8 nuclei
- Double fertilization: sperm + egg → zygote (2n); sperm + polar nuclei → PEN (3n)
- Discovered by Nawaschin
- Wind pollination: light pollen, feathery stigma, no colour/scent
- Outbreeding devices: self-incompatibility, dichogamy, dioecy
- Seed = mature ovule; Fruit = mature ovary
- Apomixis = seeds without fertilization; Polyembryony = multiple embryos (Citrus)