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Plant Reproduction and Development Chapter 43 Asexual Reproduction • Part of a single plant divides by mitosis to give rise to a new plant – Spreading of runners (strawberries) – Production of bulbs (daffodils) – Sprouting of rhizomes (irises) • Offspring are genetically identical to parent plant • Advantageous in a stable environment Sexual Reproduction • Two parent plants give rise to genetically variable offspring • Involves meiosis, gamete formation, and fertilization • Allows offspring to cope with a changing environment or invade different habitats Alternation of Generations • Plant life cycles are characterized by alteration of generations • Multicellular diploid plants (sporophytes) and multicellular haploid plants (gametophytes) take turns producing each other Alternation of Generations • In seedless plants (mosses and ferns) – Gametophytes are independent plants – Mobile sperm swim through a thin film of water to reach egg – Reproduction restricted to moist habitats Fern Life Cycle • Diploid sporophyte (spore-forming plant) produces haploid spores by meiosis • Spore germinates (begins to grow and develop); divides repeatedly by mitosis forming a haploid gametophyte (gameteforming plant) • Gametophyte produces haploid sperm and eggs by mitosis (gametes are produced at different times to prevent self-fertilization) Fern Life Cycle • Sperm and egg fuse to form a diploid zygote (fertilized egg) • Zygote undergoes repeated mitosis to form a new diploid sporophyte plant Reproduction in Seed Plants • Seed plants (gymnosperms and angiosperms) – Gametophytes develop within sporophytes – Reproduction can occur in dry habitats • Male gametophyte is surrounded by a protective coat called a pollen grain • The pollen grain encloses sperm cells in a watertight packet that can be easily transported to another plant • Egg-producing female gametophyte remains moist and protected within the sporophyte, and the pollen grain ensures that the sperm are delivered directly to the egg Reproduction in Seed Plants • The fertilized egg becomes enclosed in a drought-resistant seed – Consists of an embryonic plant and a food reserve encased within a protective outer coating – May lie dormant up to years waiting for conditions favorable for germination and growth Reproduction in Seed Plants • Non-flowering gymnosperms were the earliest seed plants • Gymnosperms bear male and female gametophytes on separate cones • Male cones release pollen grains that travel via wind to female cones Animal Pollinators • Wind pollination is inefficient because most pollen grains fail to reach their target • Flowering plants (angiosperms) evolved from gymnosperms • Flowers produce pollen and nectar that attract animal pollinators (bees, moths, butterflies, hummingbirds) • Flowers enhance a plant’s reproductive success because animal pollinators transport pollen from plant to plant as they feed Flowers • Flowers are the reproductive structures of angiosperms, produced by the sporophyte generation • Angiosperms bear male and female gametophytes on flowers • Within each flower, meiosis gives rise to two types of haploid spores – Megaspore divides by mitosis to form female gametophyte (embryo sac) that is retained within flower – Microspore divides by mitosis to form male gametophyte (pollen grain) Flowers • Gametophytes form haploid gametes by mitosis • Sperm are liberated from pollen grain when it lands on female structure of another plant • Sperm burrow to egg and fuse to form a diploid zygote that becomes encased in a seed • Seed germinates to form a new sporophyte Complete Flowers • Complete flowers have four major parts • Sepals – located at base of flower; surround and protect the bud – Dicot sepals are green and leaf-like – Monocot sepals resemble petals • Petals – located above sepals; usually brightly colored and fragrant (attract pollinators) Complete Flowers • Stamens (male reproductive structures) – attached above petals – Each consists of a filament (stalk) and anther (produces pollen) • Carpel (female reproductive structure) – centrally located – Each consists of a sticky stigma (catches pollen), an elongate style, and a bulbous ovary containing one or more ovules • Ovules develop into seeds • Ovary develops into a fruit Incomplete Flowers • Incomplete flowers lack one or more of the four major floral parts – Grass flowers lack both petals and sepals Imperfect Flowers • Imperfect flowers are incomplete flowers lacking either stamens (female flowers) or carpels (male flowers) • American holly bears separate male and female flowers on separate plants; fruits develop on female plants • Zucchini bears separate male and female flowers on the same plant; fruits develop from female flowers The Male Gametophyte • Pollen contains the male gametophyte • Pollen develops within anthers of diploid sporophyte plants • Pollen grains have tough protective coats that exhibit species-specific shapes and patterns • Each anther consists of four pollen sacs that contain microspore mother cells • Each diploid microspore mother cell divides by meiosis to form four haploid microspores The Male Gametophyte • Each microspore divides by mitosis to form an immature male gametophyte consisting of two cells – Tube cell (will form a pollen tube after pollination) – Generative cell (will divide by mitosis to form two sperm after pollination) • Pollen sacs of anthers split open when male gametophytes are mature • Pollen is dispersed by either animal pollinators or wind currents The Female Gametophyte • Nucleus of remaining megaspore divides by mitosis three times (without cytokinesis) to form a single cell with 8 haploid nuclei The Female Gametophyte • Plasma membranes partition the cell into a seven-celled embryo sac – Three small cells at either end; one is the egg – One large central cell containing two polar nuclei Pollination • Pollination occurs when a pollen grain lands on the stigma of a compatible plant • Pollen grain absorbs water and germinates – Tube cell extends, forming a pollen tube that grows down the style – Generative cell divides (mitosis) to form two sperm cells • Sperm cells follow pollen tube to the ovule Double Fertilization • Double fertilization is the process by which two sperm fuse with two cells of the embryo sac – One sperm fuses with the egg to form the diploid (2n) zygote – Second sperm fuses with both polar nuclei to form the triploid (3n) endosperm cell – Double fertilization is unique to flowering plants Fruits and Seeds • The female gametophyte and the surrounding integuments of the ovule develop into a seed • The seed is surrounded by the ovary, which develops into a fruit • Petals and stamens shrivel and fall away as the fruit enlarges From Ovary to Fruit • A fruit is a ripened ovary • Examples: apples, berries, peaches, oranges, bananas, avocados, zucchini, tomatoes, and peppers • Fruits help disperse seeds away from the parent plant – Fleshy, edible fruits attract animals – Spiked fruits attach to animal fur – Winged fruits promote wind dispersal From Ovule to Seed • Integuments of ovule develop into the seed coat (outer covering of seed) • Inside the ovule – Triploid endosperm cell divides to form endosperm that absorbs nutrients from parent plant – Zygote develops into an embryo The Embryo • The embryo consists of – An embryonic root – An embryonic shoot • Hypocotyl - region below cotyledons, but above root • Epicotyl - region above cotyledons; tip is apical meristem – One or two cotyledons (seed leaves) Cotyledons • Dicot seeds have two cotyledons – Absorb nutrients from endosperm during seed development and store for future use – Transfer stored nutrients to other parts of embryo during seed germination • Monocot seeds have one cotyledon – Absorbs and transfers most nutrients during seed germination Seed Dormancy • As seeds mature, they enter a period of dormancy – Suspended growth and development – Lowered metabolic rate – Resistance to adverse environmental conditions – Dormancy ensures that germination occurs at a time and place most advantageous to the seed Seed Dormancy • Conditions that break dormancy vary between plant species – Some germinate as soon as the environment is suitable (adequate moisture and temperature) • Conditions that break dormancy vary between plant species – Others have additional requirements • Initial drying (seeds within fleshy fruits) • Exposure to prolonged cold (seeds of temperate and arctic plants) • Disruption of seed coat Seed Germination • Germination (resumption of growth) occurs when the seed absorbs enough water to rupture the seed coat • Emergence of the embryonic root is followed by emergence of the embryonic shoot • Emerging roots and shoots must push through the soil without abrading the apical meristems at their tips Seed Germination • Root apical meristem is protected by a root cap • Shoot apical meristem is protected by either – A coleoptile (sheath enclosing the shoot tip of monocots) – An epicotyl or hypocotyl hook that pulls the shoot tip of dicots out of the soil Cotyledons • Food stored in the seed (as endosperm or within cotyledons) provides the energy for sprouting • Cotyledons of monocots and dicots with epicotyl hooks – Remain below ground – Digest endosperm and transfer nutrients to seedling Cotyledons • Cotyledons of dicots with hypocotyl hooks are carried out of the soil – Become photosynthetic – Transfer previously stored nutrients and newly synthesized sugars to the seedling – Wither and fall off when food reserves have been consumed Coevolution • Some plants and their pollinators have coevolved – Each has acted as an agent of natural selection on the other • Animal-pollinated flowers must – Attract useful pollinators able to locate flowers and extract pollen or nectar – Frustrate undesirable visitors who might eat nectar or pollen without fertilizing the flower Coevolution • Animal-pollinated flowers are grouped into three categories based on how they attract pollinators – Food – Sex deceptions – A nursery Food • Many animals forage on flowers, distributing pollen from flower to flower in the process • Examples: beetles, bees, moths, butterflies, hummingbirds • Bees are attracted to flowers by sweet odors and bright colors • Bees do not see the same range of colors that humans do • Bee-pollinated flowers are typically white, blue, yellow, or orange, with markings that reflect UV light pointing toward the center Food • Bee-pollinated flowers have structural adaptations to ensure pollen transfer • Example: Scotch broom flower • When a bee visits a young plant, the stamens emerge from a crevice between petals, brushing pollen onto her back as her weight deflects the petals downward • In older flowers, the sticky stigma of the carpel protrudes from the crevice, becoming pollinated by pollen-coated, nectar-foraging bees Food • Other adaptations for drawing pollinators – Nectar-containing tubes to accommodate the long tongues of moths and butterflies – Tubular flower shape to match the long bills and tongues of hummingbirds – Flowers that heat up to broadcast their scents • Example: skunk cabbage Sex Deceptions • Some plants capitalize on the mating drive and behaviors of male wasps • Example: some orchid flowers mimic female wasps or bees in scent and shape – The males attempt to copulate, becoming covered with pollen in the process – Pollen is transferred as they repeat the process with other orchids Nurseries • Some insects fertilize a flower and then lay their eggs in the flower’s ovary • Example: yuccas and yucca moths – A female moth collects pollen from one flower and transfers it to another, smearing it over the stigma and laying her eggs inside the ovary – The developing seeds provide nutrition for the developing catepillars Fruits Disperse Seeds • Mechanisms to disperse seeds away from the parent plant include – Explosive fruits that eject their seeds – Lightweight fruits with large wind-catching surfaces Fruits Disperse Seeds • Floating fruits for water dispersal • Clingy or tasty fruits that allow animal dispersal