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Plant Reproduction and Development
Chapter 44 Plant Reproduction and Development
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How do plants reproduce?
Asexually Existing plant uses mitosis – identical Lilac bushes that sprout new trunks from the root Strawberries and runners Tulips and other bulbs grow new, smaller bulbs Sexually Fusion of gametes from parents
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Alternation of generations
Plant sexual life cycles alternate between two multicellular stages, haploid and diploid
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Sporophyte – multicellular diploid
Sexual Life Cycle Sporophyte – multicellular diploid Garden plants, produce flowers Produces specialized reproductive cells that undergo meiosis to form haploid spores Spores undergo mitosis to form multicellular haploid gametophyte Angiosperms and Gymnosperms produce separate male and female gametophyte stages
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Sexual Life Cycle of a Flowering Plant
Diploid mother cells develop in anthers (male) or ovaries (female) Meiosis produces haploid spores Mitosis of the spores male & female gametophytes sperm & egg Pollen carries sperm to flower, sperm travel in tube to female gametophyte Fertilization diploid zygote Zygote develops into embryo, seedling, mature sporophyte
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Alternation of Generations
mother cell spores flower MEIOTIC CELL DIVISION stigma male gametophyte (pollen grain) anther In the flower, diploid mother cells develop in the reproductive structures: anthers (male) and ovaries (female) 1 Meiotic cell division of mother cells in the sporophyte produces haploid spores 2 Mitotic cell division of the spores forms male gametophytes (pollen), which produce sperm, and female gametophytes, which produce eggs 3 ovule sperm ovary Pollen carries the sperm to the female reproductive structure of a flower; sperm travel within a pollen tube to the female gametophyte 4 A sperm fertilizes an egg within the female gametophyte, producing a diploid zygote 5 mother cell pollen tube MEIOTIC CELL DIVISION ovule mature sporophyte sperm nuclei spores The zygote develops into an embryo, a seedling, and eventually, a new mature sporophyte 6 female gametophyte egg female gametophyte FERTILIZATION seedling seed haploid (n) diploid (2n) seed fruit embryo
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Sexual Life Cycle Varies between Plants
Size, complexity and lifespan of sporophyte and gametophyte varies Mosses, liverworts – gametophyte is independent Resulting sporophyte grows on gametophyte Ferns – sperm fertilize eggs in independent gametophyte, zygote begins growing on gametophyte but sporophyte develops its own roots and leaves – becomes dominant stage
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Angiosperms and Gymnosperms
Differ from mosses, liverworts, ferns Diploid sporophyte is the dominant stage In angiosperms and gymnosperms, sperm is transported within pollen grain. In mosses, liverwort and fern all require water for fertilization (sperm swim to eggs) Gametophytes are very, very small
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Flower – reproductive structure of angiosperm
Flower Structure Flower – reproductive structure of angiosperm Complete flower – has 4 sets of modified leaves Sepals, petals, stamens, carpels Petunia, rose, lily
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Complete Flower Structure
Sepal – at base of flower In monocots, resemble petals In dicots – green and leafy Surround and protect flower bud Petals – brightly colored, advertise for pollinators Stamens – attached above petals Filament with anther, pollen Carpel – vase shaped, sticky stigma on elongated style, bulbous ovary at base of carpel – one or more ovules where female gametophyte develops Fertilized ovule becomes seed and dev. into fruit (encloses)
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A Complete Flower anther petal stamen filament stigma style carpel
sepal ovary ovules (a) A representative dicot flower
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Incomplete Flower Structure
Lack one or more of 4 floral components Grass (lack petals, sepals) Also described as imperfect Produce separate male and female flowers, often on a single plant (zucchini) American holly, female produces red berries
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Zucchini Flowers – male and female
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Animation: Pollen Development
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Pollen is the Male Gametophyte
Develop within anthers of the sporophyte Microspore mother cells develop within pollen sacs of the anther Meiosis produces 4 haploid microspores Each produces an immature male gametophyte (pollen grain)by mitosis, contains the generative cell Tube cell + generative cell in the pollen cell The generative cell undergoes mitosis to form 2 sperm cells.
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Male Gametophyte Development
pollen sacs microspore mother cell anther Microspore mother cells develop within the pollen sacs of the anther of a flower 1 MEIOTIC CELL DIVISION sporophyte microspores 2 tube cell nucleus Meiotic cell division produces four haploid microspores mature pollen grain Immature pollen grain tube cell cytoplasm sperm cells stigma generative cell The generative cell produces two sperm cells by mitotic cell division; the male gametophyte is now mature 4 Each microspore produces an immature male gametophyte (a pollen grain) by mitotic cell division 3 tube cell nucleus haploid (n) diploid (2n)
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Tough, waterproof outercoat Characteristic of the plant species
Pollen Tough, waterproof outercoat Characteristic of the plant species Used to identify climate in fossils
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Wind-Pollinated Flowers
Anther, pollen
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Species vary – one to several dozen ovules
Female Gametophyte Forms in ovule Species vary – one to several dozen ovules Megapore mother cell develops within ovule Meiosis produces 4 haploid megaspores, 3 degenerate Remaining megaspore form 8 nuclei by mitosis (3X mitosis) Plasma membranes form, 7 cells – 3 at one end (1 N each), one is the egg
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Female Gametophyte Development
ovule megaspore mother cell A megaspore mother cell develops within each ovule of the ovaries of a flower 1 ovary integuments MEIOTIC CELL DIVISION Cytoplasmic division produces the seven cells of the mature female gametophyte 4 Meiotic cell division produces four haploid megaspores; three degenerate 2 megaspores central cell with two nuclei female gametophyte egg cell The single remaining megaspore forms eight nuclei by mitosis 3 haploid (n) diploid (2n)
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Animation: Ovule Development
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Pollination and Fertilization
Pollen grain lands on stigma Absorbs water, breaks out of coat and elongates through stigma Pollen tube reaches ovule Double fertilization – both sperm fuse with cells of the female gametophyte One sperm fertilizes egg zygote One sperm fertilizes central cell, mitosis produces endosperm
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Pollination and Fertilization of a Flower
pollen grain A pollen tube grows down through the style of the carpel to the ovary; the tube cell nucleus travels at the tip of the tube, and the two sperm follow close behind 2 tube cell nucleus sperm pollen tube Pollination occurs when a pollen grain lands on the stigma of a carpel 1 sperm tube cell nucleus Double fertilization: 3 One sperm fuses with the central cell ovule ovary central cell One sperm fuses with the egg cell egg
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Animation: Pollination and Fertilization
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Fruit and Seed Development
Female gametophyte and integuments become seeds Ovary becomes fruit Petals, pollen, stamens dry up and fall off
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Development of Fruit and Seeds in a Pepper
ripening sepal ovary wall “flesh” of pepper ovary pepper fruit petal ovule seed pepper flower pepper fruits
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Three processes transform ovule into seed
Seed Development Three processes transform ovule into seed Integuments become seed coat Triploid central cell divides to form endosperm Zygote develops into the embryo As seed matures, embryo differentiates into shoot and root Shoot includes 1 or 2 cotyledons – absorb food from endosperm Monocot – most of endosperm stays in seed until germination Dicot – cotyledons absorb most of the endosperm, so the mature seed is full of embryo
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Monocot Dicot Seed Structures Shoot
Coleoptile – sheath that surrounds embryonic leaves Dicot Hypocotyl Epicotyl
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Seed Development integuments (diploid) seed coat central
cell (triploid) endosperm zygote (diploid) embryo fertilized ovule seed (a) Early development of the seed seed coat embryonic root endosperm cotyledon embryonic leaves shoot coleoptile hypocotyl shoot embryonic leaves seed coat embryonic root cotyledons (b) Corn seed (monocot) (c) Bean seed (dicot)
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Animation: Embryo and Endosperm Development
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Germination – sprouting of seed
Embryo grows and breaks out of seed Forms seedling Warmth and moisture are necessary
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Some seeds have a period of dormancy
Resist adverse environmental conditions Dormancy solves 2 problems Prevents seeds from germinating within moist fruit Environmental conditions optimal for germination may not coincide with conditions that will allow seedling to survive and mature Seeds mature in fall – in temperate climate, it isn’t a good time to germinate In moist, tropical regions dormancy is less common
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Additional Requirements for Germination
Necessary to break dormancy Drying – often dispersed by fruit eating animals, excreted and dry our Cold – prolonged sub freezing temp. – ensures that seeds released in temperate weather do not germinate Seed coat disruption – weathered or partially digested before germination can occur Desert plants have seeds that are water soluble
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Cotyledons Nourish the Developing Plant
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Germination Embryo absorbs water, seed coat bursts Root emerges first and grows, absorbing water and minerals Shoot cells elongate and push upward Monocots - energy comes from endosperm, digested by cotyledons and transferred to embryo Dicots – cotyledons have already absorbed endosperm so they transfer energy to embryo
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Seeds are buried in soil and must be protected
Germination, part 2 Seeds are buried in soil and must be protected Root tip protected by root cap Monocot – coleoptile encloses shoot tip to protect Dicot – shoot forms a hook, as grows clears a path for downward pointing apical meristem Cotyledons are carried out of the soil, become green and photosynthetic, transfer stored and new food to shoot True leaves take over photosynthesis, cotelydons die back
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Seed Germination true leaves coleoptile root (a) Corn (monocot) true
cotyledon epicotyl seed coat hypocotyl hook cotyledons withered cotyledons hypocotyl root (b) Bean (dicot)
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Plants and their Pollinators
Coevolution – each as acted as an agent of natural selection on the other Some flowers provide food Beetles, moths, butterflies, hummingbirds Animals distribute pollen Flower colors have coevolved to match the color vision of the animal Bees see UV light so flowers are white, blue, yellow, orange Marking s that point to the center of the flower Structural adaptations - nectar containing tubes, stamens, smell, etc.
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UV Patterns Guide Bees to Nectar
far- red near UV red orange yellow green blue violet human bee 700 600 500 400 wavelength (nm) (a ) A comparison of color vision in humans and bees human vision bee vision (b ) Flower color patterns seen by humans and bees
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“Pollinating” a Pollinator
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Vertebrate Pollinators
Hummingbirds need a lot of energy so the flowers they pollinate produce more nectar than flowers that are pollinated by insects.
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Mating Decoys Particularly orchids Mimic female wasps, bees or flies in smell and shape. Males attempt to copulate but only pick up pollen packet which transfers to the next flower
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Nurseries for Pollinators
Some insects pollinate the flower, then lay their eggs in the flower’s ovary Milkweed and milkweed bugs Yucca and yucca moth Visit – collect – visit and drill hole, lay eggs – pollinate stigma with pollen Neither can reproduce without the other
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Fruit helps disperse seeds
Disperse seeds far away so there is no competition Adult plants can withstand more damage than seedlings Species will be more successful if they disperse their seeds a distance Many different types of dispersal Seed dispersion methods
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Water-Dispersed Fruit
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Wind-Dispersed Fruits
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Clingy Fruits
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Colored fruit attracts animals
Blackberries, raspberries, strawberries, tomatoes, peppers - small seeds that animals swallow Eventually excreted unharmed Some seed coats must be scraped or weakened by an animal’s digestive tract before germination Transported away from its parent plant and ends up with is own fertilizer! Seed dispersal video
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