Angiosperm male gametophyte development:
PMC (2n) $\xrightarrow{\text{Meiosis (1)}}$ 4 Microspores (n)
Microspore $\xrightarrow{\text{Mitosis I (1)}}$ 2-celled pollen (vegetative + generative)
Generative cell $\xrightarrow{\text{Mitosis II (1)}}$ 2 male gametes (sperm cells)
Per pollen: 1 meiosis + 2 mitoses
Per official NEET answer key: 1 meiosis + 3 mitosis
Stamen: male reproductive organ. Parts: filament (stalk) + anther (pollen-producing head). Anther: typically bilobed, each lobe has 2 microsporangia = 4 microsporangia total per anther. Microsporangium wall layers (from outside): epidermis, endothecium (fibrous, helps pollen dehiscence), middle layers (2-3, nutritive), tapetum (innermost, secretory, nurtures developing pollen). Tapetum is the most important layer — provides: nutrients to developing microspores, sporopollenin precursors (for pollen wall), pollenkitt (oily coating attracting pollinators), recognition proteins (for pollen-stigma recognition), enzymes for dissolution of callose. Sporopollenin: one of the most resistant biological polymers. Makes up pollen wall (exine). Preserved in fossils. Never synthesised in labs. Composed of oxidative polymers of carotenoids and carotenoid esters.
Microsporogenesis: formation of microspores from pollen mother cells (microsporocytes). Archesporial cells (2n) in microsporangium → differentiate → pollen mother cells (PMC = microsporocyte, 2n). PMC → meiosis → tetrad of 4 microspores (n). Tetrad held together by callose (β-1,3-glucan) wall. Callose dissolved by callase enzyme from tapetum → microspores released. Microspores: spherical, uninucleate, haploid (n). Surrounded by callose layer initially, then develop their own cell wall (exine = outer, sporopollenin-based; intine = inner, pectin/cellulose). Microspore = immature pollen grain. Apertures in exine: colpus (elongated groove) or porus (circular pore) → allow pollen tube germination. Pollen type: monosulcate (1 colpus, primitive), tricolporate (3 colpus + porus, advanced angiosperms).
Microspore → male gametophyte (microgametophyte = pollen grain). Microspore nucleus divides mitotically → 2-celled pollen grain: large vegetative cell (tube cell) + small generative cell. Vegetative cell: large, with abundant cytoplasm and food reserves. Directs pollen tube growth. Does NOT undergo further division. Generative cell: small, with dense nucleus. Floats in cytoplasm of vegetative cell. Divides to give 2 sperm cells (male gametes). 2-celled pollen: generative cell has NOT yet divided (e.g., at time of shedding in many plants). 3-celled pollen: generative cell divides BEFORE pollen is shed → 3 cells at shedding (1 vegetative + 2 sperm). Examples of 3-celled pollen: grasses (wheat, maize), Brassica. 2-celled pollen (generative cell divides in pollen tube after germination): most dicots (Arabidopsis, tomato, tobacco). This division occurs in pollen tube en route to ovule.
Pistil (carpel): female reproductive organ. Parts: stigma (pollen receptor) + style (pollen tube grows through) + ovary (contains ovule(s)). Ovule: structure that becomes seed after fertilisation. Parts: funicle (stalk), hilum (junction of funicle and ovule), chalaza (opposite end from micropyle), micropyle (small opening), integuments (1 or 2 protective coats), nucellus (nutritive tissue), embryo sac (female gametophyte). Megasporogenesis: megaspore mother cell (MMC, 2n) → meiosis → 4 megaspores (n) (linear tetrad). 3 megaspores degenerate; 1 functional megaspore survives (usually the chalazal megaspore). Megagametogenesis: functional megaspore → 3 mitoses → 8-nucleate embryo sac (7-cell, 8-nucleus structure).
8-nucleate embryo sac (Polygonum type, most common): 7 cells: Egg apparatus (3 cells at micropylar end): 1 egg cell (female gamete) + 2 synergids. Synergids: with filiform apparatus (helps pollen tube entry), secrete chemoattractants to guide pollen tube. Polar nuclei (2, fuse to form secondary nucleus): in central cell. After fertilisation: primary endosperm nucleus. Antipodals (3 cells at chalazal end): nutritive, degenerate. The egg cell is the female gamete. After double fertilisation: egg + 1 sperm → diploid zygote → embryo. Secondary nucleus (2n) + 1 sperm → triploid (3n) nucleus → endosperm. This is double fertilisation (unique to angiosperms, discovered by Nawaschin 1898).
Pollination: transfer of pollen from anther to stigma of same or different flower. Autogamy (self-pollination): pollen to stigma of same flower. Cleistogamy: flowers never open (guaranteed self-pollination). Geitonogamy: pollen to stigma of different flower on same plant (genetically self-pollination). Xenogamy (cross-pollination): pollen to stigma of different plant. Promotes genetic diversity. Adaptations for cross-pollination: dichogamy (anther and stigma mature at different times), herkogamy (physical barrier between anther and stigma), self-incompatibility (S-locus prevents self-pollen germination). Pollination vectors: wind (anemophily): small, light, dry, non-sticky pollen, no petals, feathery stigma, flowers in clusters. Water (hydrophily): rare. Insects (entomophily): colourful, fragrant, nectar, sticky pollen. Birds (ornithophily): red/orange flowers, lots of nectar, no scent. Bats (chiropterophily): large, dull-coloured, nocturnal, musty scent.
Pollen tube germination: pollen grain lands on stigma → hydration → pollen tube emerges from germination pore → grows through style (guided by chemotaxis to synergids) → enters ovule through micropyle. Double fertilisation (characteristic of angiosperms): siphonogamy. Pollen tube delivers 2 sperm cells into embryo sac. Fertilisation 1: 1 sperm + egg cell → diploid (2n) zygote → embryo. Fertilisation 2: 1 sperm + 2 polar nuclei (or secondary nucleus) → triploid (3n) primary endosperm nucleus → endosperm (nourishes embryo). Triple fusion: the fusion of 1 sperm + 2 polar nuclei is called triple fusion. Both events together = double fertilisation. Discovered by Nawaschin (1898). Importance of endosperm: provides nutrition to developing embryo. In non-endospermic seeds (e.g., pea, bean): endosperm absorbed by cotyledons during embryo development. In endospermic seeds (e.g., castor, rice): endosperm persists in mature seed.
Post-fertilisation changes: Ovule → seed. Integuments → testa (seed coat). Ovary → fruit. Nucellus → perisperm (remnant nutritive tissue, in some seeds). Zygote (2n) → embryo development through: 2-celled proembryo → octant stage → globular stage → heart stage → torpedo stage → mature embryo. Embryo structure: radicle (embryonic root) + plumule (embryonic shoot) + cotyledon(s) + epicotyl/hypocotyl. Dicot: 2 cotyledons. Monocot: 1 cotyledon (scutellum). Fruit types: simple (from one flower, one carpel or fused carpels): berry, drupe, pome. Aggregate (from one flower, multiple free carpels): strawberry, raspberry. Multiple (from inflorescence): pineapple, mulberry. Parthenocarpy: fruit development without fertilisation (banana, seedless grapes/watermelon). Induced by auxin spray.