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Reproductive Structures of Flowering Plants AP Biology Spring 2011 Flowers form on a modified branch of the sporophyte when master genes in the apical meristem are turned on Flower parts are arranged in whorls of modified leaves on the end of the receptacle Sepals: outermost green, leaf-like parts arranged in a ring called the calyx Petals: the coloured parts located between the reproductive structures and the sepals, arranged in a ring called the corolla Stamens: male parts, slender stalk (filament) capped with an anther, inside which pollen sacs enclose pollen grains Carpels: female parts, vessel shaped structures with an expanded lower ovary (with ovules), slender column (style), and an upper surface (stigma) for pollen landing Meiosis of cells within flowers produces haploid spores • Microspores in the anther • Megaspore in the ovule They then undergo mitosis to produce the haploid gametophytes that ultimately become the egg or sperm The egg and the sperm fuse to form the diploid zygote As the zygote develops into a plant, the seed is formed Complete flowers: have all four whorls (sepals, petals, stamens, and carpels) Incomplete flowers: are missing one or more whorls Perfect flowers: have both stamens and carpel (both sexes on one flower) Imperfect flowers: produce either stamens or carpels and are known as either “male” or “female” flowers