MODIFIED FLOWERS
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Flowers II Modified Flowers and Sexual Reproduction Bracts Additional floral structures outside the calyx May be leaflike or petal-like The showy white or pink "petals" of dogwood are bracts Tepals Sepals are brightly colored and identical to the petals Complete and Incomplete flowers Flowers containing all four floral appendages are known as complete flowers Flowers lacking any of the four floral structures are known as incomplete flowers Flowers of grasses possess neither sepals nor petals Perfect and Imperfect Flowers Perfect flowers have both stamens and carpels Imperfect flowers lack either stamens or carpels *Unisexual flowers *Male flowers called staminate *Female flowers called pistillate or carpellate Plants with Unisexual Flowers Monoecious plants have both male and female flowers on a single individual Dioecious plants are either male or female with only unisexual flowers on a single individual Ovary Position Superior ovary - sepals, petals, and stamens are inserted beneath the ovary Inferior ovary - sepals, petals and stamens are inserted above the ovary Flower symmetry Regular flower displays radial symmetry Irregular flowers display bilateral symmetry Inflorescence Flowers grouped in clusters Sometimes what appears as a single flower is actually an inflorescence Sunflower, daisies, and dogwood flower common examples The arrangement of flowers in the inflorescence varied with many patterns possible: spike, umbel, head, and catkin Inflorescence Types Spike Umbel Catkin Head Sexual Reproduction in Flowers Meiosis occurs: Stamens in pollen chambers of anther Carpels in developing ovules in ovary Pollen development -1 Microspore mother cells become distinct in the pollen chambers Each MMC undergoes meiosis to produce 4 microspores Each microspore develops into a pollen grain, the male gametophyte Pollen chamber in anther Pollen development - 2 Microspore nucleus undergoes mitosis to produce generative nucleus tube nucleus Microspore wall modified into pollen wall When mature pollen are released from the anthers Pollen wall Intine - inner layer Exine - outer layer Exine may be ornamented with spines, ridges, or pores Pollen - Male gametophyte Exine Tube nucleus Intine Generative nucleus Pollen Types Ragweed Thistle Oak Ovule One or more ovules develop within the ovary Ovule is surrounded by integuments Opening in integuments is known as the micropyle Ovule development - 1 One cell becomes distinct as a megaspore mother cell The megaspore mother cell undergoes meiosis to produce four megaspores Three degenerate leaving one surviving megaspore Ovule development - 2 Surviving megaspore undergoes three mitotic divisions to producing 8 nuclei These 8 nuclei are distributed with 3 near the micropyle end of the ovule, 3 at the opposite end and 2 (polar nuclei) in the center One of the nuclei at the micropyle end is the egg This mature female gametophyte is often called the embryo sac Ovule - Female Gametophyte Polar nuclei Integuments Egg Micropyle Pollination Transfer of pollen from the anther to the stigma Self-pollination - same flower Cross- pollination - from one flower to another Pollen transfer occurs mainly by animals or wind Animal Pollinated Flowers Flowers brightly colored and fragrant Essential oils attract Nectar produced Color patterns may be nectar guides Pollen larger, sticky, and not abundant Wind Pollinated Flowers Flowers small and inconspicuous often an inflorescence Often lacking sepals and petals; no nectar Stigma Pollen small, dry, light, and abundant Ovary One ragweed plant can release one billion pollen grains (1 million tons/yr in NA) POLLEN Study of pollen called palynology has applications in many diverse fields: petroleum geology, anthropology, archeology, criminology, and medicine When pollen is released by windpollinated plants, only a very tiny percentage reaches the stigma remainder settles back to earth. Pollen tube growth Pollen grain germinates on compatible stigma Pollen tube begins growing down into the style towards the ovary Generative nucleus divides mitotically producing two non-motile sperm. Pollen tube continues to grow until it reaches the micropyle of an ovule Pollen tube growth Pollen Pollen tube Ovary Ovule Pollen tube growth Pollen Sperm * * Pollen tube Ovary Ovule Double fertilization A distinctive feature of angiosperms Both sperm involved in fertilization. One sperm fertilizes the egg to produce a zygote Second sperm fuses with the two polar nuclei producing the primary endosperm nucleus which develops into endosperm Fertilization Polar nuclei Egg Pollen tube Sperm * * Double fertilization Sperm + Egg -----> Zygote Sperm + 2 polar nuclei ------> Primary Endosperm Nucleus Endosperm A nutritive tissue for the developing embryo. Major food source for the human population Endosperm reserves in wheat, rice, and corn are especially important food sources Following fertilization Sepals, petals, and stamens drop off Ovary greatly expands becoming a fruit Each fertilized ovule becomes a seed Integuments of the ovule develop into the seed coat Summary 1. Meiosis reduces the number of chromosomes from diploid to haploid 2. The flower is the unique reproductive structure of angiosperms 3. Pollination is the transfer of pollen from anther to stigma occurring through the action of wind or animals 4. In angiosperms reproduction is accomplished through the process of double fertilization.
