CHAPTER 44 Plant Reproduction and Development Sexual versus Asexual Reproduction Many plant species can reproduce both sexually and asexually Asexual reproduction can occur via fragmentation Fragmentation can be an artificial process cuttings or a natural process The parent plant sends out horizontal stems runners that can lead to completely new plants Asexual reproduction is natural cloning where all the offspring are genetically identical to the parent plant Sexual offspring combine the genes from two parents and are genetically different from their parents Plant Life Cycles Alternation of generations revisited see fig 44 1 2 distinct multicellular adult forms sporophyte diploid plant which produces haploid spores gametophyte haploid plant which produces gametes In mosses and ferns the gametophyte is small and independent Water is essential for sexual reproduction In seed plants gymnosperms and angiosperms the gametophyte is very small and relies on the sporophyte it s part of the sporophyte Water no longer needed for transporting the gametes Two types of spores are seen in the flowering plants They are heterosporous Megaspores gives rise to the female gametophyte Microspores gives rise to the male gametophyte The two different gametophytes produce two types of gametes female gametophyte is an embryo sac containing an egg cell male gametophyte is a pollen grain sperm After fertilization the zygote develops into the seed plant embryo which will grow into a new sporophyte The Evolution of Flowers Gymnosperms were the earliest seed plants The gymnosperms and some angiosperms rely on wind pollination for fertilization see fig E44 1 and 44 6 Advantage don t need any biological help Disadvantage it s an inefficient process Insects began carrying pollen grains from the male cone to the female cones about 150 million years ago Why The plants enticed the insects with food Pollen is a protein rich food source and the female cones produce sugar rich secretions Angiosperms flowering plants developed about 130 million years ago The flowers attracted the pollinators The Flower Structure The parts of the flower evolved from leaves Complete flowers have 4 main parts see fig 44 2 Sepals Petals Stamens Carpel The stamens are the male reproductive part and consist of the filament or stalk which hold the anther The anther contains the pollen grains male gametophyte The carpel is the female reproductive part and is a vase shaped structure made up of the stigma the style and the ovary The stigma is the top of the vase It is a sticky structure designed to catch the pollen The style is the neck of the vase The pollen must grow through the style to reach the egg The ovary is the bottom of the vase The ovary contains the ovules which become seeds The outer layers of the ovary become the fruit Incomplete flowers lack one or more of the 4 floral parts Imperfect flowers are single sex and therefore are missing one set of sex parts either stamens or carpel see fig 44 3 Gametophyte Development in Flowering Plants The gametophytes are haploid and develop within sporophyte flowers They are very small compared to the sporophyte and cannot live on their own Pollen develops within the pollen sacs of anthers see fig 44 4 1 Microspore mother cell a k a microsporocyte develops inside the pollen sacs containing 2 Diploid microspore mother cells divide by meiosis to form 4 haploid microspores from each microspore mother cell 3 Each microspore divides by mitosis to make an immature pollen grain a tube cell b generative cell inside the tube cell 4 The generative cell goes through mitosis to form two sperm cells This is now the mature pollen grain see fig 44 5 Embryo sac development see fig 44 7 The outer layer of the ovule is the integument The tissue of the ovule is diploid The series of events leading to the female gametophyte are 1 The megaspore mother cell a k a megasporocyte develops within the ovule that is within the ovary of the carpel 2 The megaspore mother cell divides by meiosis to form four haploid megaspores 3 Three of the four megaspores degrade 4 The remaining one of the four goes through 3 rounds of mitosis BUT NOT CYTOKINESIS which results in 8 nuclei in one cell 5 The nuclei are distributed 3 on each end of the megaspore and 2 in the center At this point cytokinesis occurs forming 7 NOT 8 cells 3 cells on each end of the embryo sac one is the egg one larger central cell with 2 polar nuclei The larger center cell becomes the primary endosperm cell Pollination and Fertilization see fig 44 8 1 Pollination starts when pollen from an anther lands on a stigma 2 The pollen grain grow a tube down through the style towards the ovary 3 The 2 sperm cells from the generative cell move down the tube to the ovary where a double fertilization occurs Double Fertilization One sperm fuses with the egg cell to form the zygote The other sperm fuses with the polar nuclei in the primary endosperm cell making this triploid endosperm 3 sets of chromosomes tissue Development of Seeds and Fruits see fig 44 9 The seed develops from the ovule The integuments become the seed coat The zygote becomes the embryo The primary endosperm becomes the endosperm that acts as food for the new plant The walls of the ovary turn into the flesh of the fruit The plant embryo contains the cotyledons seed leaves see fig 44 10 In dicots there are 2 cotyledons in the embryo and monocots have only 1 cotyledon In the monocots the cotyledon is protected by a tough sheath called the coleoptile see fig 44 11 Coevolution of Flowers and Pollinators Animal pollinated flowers have several modifications in order to Attract animal pollinators Frustrate undesirable visitors Ensure cross fertilization Bee pollinated flowers Brightly colored white yellow or blue with ultraviolet patterns see fig 44 13 These flower are tubular and produce their nectar the food enticement at the bottom of a short tube see fig 44 14 Butterfly pollinated flowers Also brightly colored with have long tubular flowers Moth pollinated flowers Light colored and sweet smelling so they re easier to find in the dark Typically night flowering plants Fly pollinated flowers see case study page 865 Smell like rotting flesh or dung Hummingbird pollinated flowers Very deep tubular flowers of red or orange colors that produce large amounts of nectar These flowers usually lack fragrance birds attracted by color not scent and don t have a landing zone Copulatory pollinated flowers Sex is the
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