Angiosperm Reproduction: Pollination and Double Fertilization

Flower: Reproductive structure producing gametes (ovule, pollen), protecting/nourishing megaspore, capturing pollen, nourishing embryos, and developing seeds/fruits.
Seed: Embryo and nutrient source in a protective coat.
Fruit: Enlarged ovary containing seeds.
Flower Organs: Sepals, Petals, Stamens (filament + anther for pollen), Carpels (ovary, style, stigma for pollen reception).

Problem: Plants are not mobile for reproduction.
Methods: Animals (biotic - 80%), Wind/Water (abiotic - 20%).
Animal Pollination: Dominant, involving insects (e.g., bees, butterflies, beetles, moths) and vertebrates (e.g., birds, bats).
Attraction: Flower color, nectar, odor, deception/mimicry.
Rewards: Pollen (protein, sugar, fat, starch), Nectar (sugar, amino acids, lipid).
Pollination Modes: Flowers and pollen adapt to specific vectors (e.g., musty odors for beetles, red tubular flowers for birds, night-opening musty flowers for bats).
Co-evolution: Plants and pollinators evolve together (e.g., Darwin's Star Orchid and Xanthopan morganii praedicta moth).
Wind Pollination: Flowers are small, colorless, odorless, lack petals, positioned for wind, produce abundant, smaller, smoother pollen ( $500$ to $2.5$ million grains/ovule).

Adult phase is sporophyte ( $2n$ ).
Heterosporous: microspores (male gametophytes/pollen) and megaspores (female gametophytes/ovule).
Pollen Development: Microsporocytes ( $2n$ ) in anther undergo meiosis to form haploid microspores ( $n$ ). Microspores undergo mitosis to form two-celled pollen grains (tube cell, generative cell).
Pollen Tube Growth: Pollen grain lands on stigma, germinates pollen tube, grows down style, enters ovule through micropyle.
Generative cell: Divides by mitosis to form two sperm cells ( $n$ ).
Double Fertilization: Unique to angiosperms.
1. One sperm ( $n$ ) fuses with the egg cell ( $n$ ) to form a diploid zygote ( $2n$ ), which develops into the embryo.
2. The other sperm ( $n$ ) fuses with two polar nuclei ( $n$ ) of the central cell to form a triploid nucleus ( $3n$ ), which develops into the endosperm (reserve tissue).