Transcript Slide 1
The Flower and Sexual Reproduction Chapter 13 Significance of the Flower • Flowers and fruit least affected by environment • Appearance of flowers and fruits important to understanding evolutionary relationships among angiosperms Function of Flowers • To facilitate the important events of gamete formation and fusion Steps in Sexual Cycle • Production of special reproductive cells after meiosis • Pollination • Fertilization • Seed and fruit development • Seed and fruit dissemination • Seed germination Flower Parts • Four whorls of modified leaves – Sepals – Petals – Stamens – Carpels Flower Parts Collective Term Part Description Function Sepals Usually green, encloses other flower parts Calyx Protect reproductive parts inside flower Corolla Petals Colored, attractive flower parts Catch attention of pollinators Androecium Produces pollen Stamens Just inside corolla, male flower part, made up of anther and filament Gynoecium Contains ovules Carpels (pistil) Modified leaves folded over and fused to protect ovules, usually in center of flower, made up of stigma, style, and ovary Flower Parts • Perianth – Collective term for calyx and corolla – Protects stamens and pistil(s) – Attracts and guides movements of some pollinators Androecium • Whorl of stamens – Consists of • Filament • Anther – Made up of four elongated lobes called pollen sacs Androecium • Pollen sac – Contains microsporocytes – Each microsporocyte • Divides by meiosis to produce four haploid microspores • Each microspore nucleus divides mitotically to form two-celled pollen grain (male gametophyte) Pollen • Contains tube cell and generative cell • Elaborate cell wall – wall pattern genetically determined – Varies among plants – Contains sporopollenin • Resists decay • Reason pollen grains make good fossils Mature Pollen • • • • • Anther wall splits Releases pollen Pollen transported to stigma (pollination) Pollen absorbs water Secretes proteins – Some involved in pollen recognition and compatibility reactions • Pollen grain germinates Gynoecium • Female organs • Simple pistil – Single folded carpel • Compound pistil – Several separate carpels or a group of fused carpels • Ovary – Chambers called locules Gynoecium • Placenta – Tissue within ovary to which ovule is attached • Types of placentation – Parietal • On ovary wall – Axile • On axis of ovary – Central placentation • Ovules form on central column Gynoecium • Style – Often withers after pollination • Stigma – May have hairs that help hold pollen grains – Sometimes secretes sticky fluid that stimulates pollen growth Gynoecium • Ovule – Structure that eventually becomes the seed – As it matures, forms 1 or 2 outer protective layers called integuments • Micropyle – small opening in integuments where pollen tube enters – Consists of 1 or 2 outer protective integuments, micropyle, megasporocyte, and nucellus – Megasporocyte • Enlarges in preparation for meiosis • Embedded in tissue called nucellus Gynoecium • Embryo sac – Female gametophyte plant (haploid) • Megasporocyte – Undergoes meiosis – Produces 4 megaspores (1n) • 3 megaspores nearest micropyle disintegrate • 1 remaining megaspore develops into mature embryo sac Gynoecium • Stages in embryo sac development – Series of 3 mitotic divisions form 8 nucleate embryo sac – Nuclei migrate – Cell wall forms around nuclei Gynoecium • Within embryo sac – At micropylar end of embryo sac • Egg cell and 2 synergic cells – All 3 of the above cells sometimes called egg apparatus – Center • Polar nuclei lie in center of central cell – Opposite end • 3 antipodal cells Double Fertilization • Generative cell within pollen grain divides by mitosis to form 2 sperm cells – 1 sperm cell fuses with egg to form diploid (2n) zygote – 1 sperm fuses with the 2 polar nuclei • Forms triploid (3n) primary endosperm nucleus – Divides to become food reserve tissue called endosperm Double Fertilization • Double fertilization actually refers to – Fusion of egg and sperm – Fusion of sperm with polar nuclei Flower Development • Shoot apex transformed into floral apex – Broadening of apical dome – General increase in RNA and protein synthesis – Increase in rate of cell division in apical dome • Bracts – 1st organs to form from floral apex • Flower itself is really a shortened and modified stem. Flower Types • Complete flower – Has all four sets of floral whorls (sepals, petals, stamens, carpels) • Incomplete flower – Lacks one or more of the sets of floral whorls Flower Types • Perfect flower – Bisexual flowers – Have both male and female flower parts • Imperfect flower – Unisexual flowers – Flowers will be either • Staminate (stamen bearing) male • Pistillate (pistil bearing) female Flower Types • Monoecious – Plant with staminate and pistillate flowers on one individual plant • Dioecious – Staminate and pistillate flowers on separate individual plants Flower Symmetry • Regular symmetry – Any line drawn through center of flower divides flower into two similar halves • Irregular symmetry – Only one line can divide flower into two similar halves Fusion of Flower Parts • Connation – Union of parts of same whorl • Adnation – Union of flower parts from different whorls Ovary Position • Superior ovary – Ovary located above the points of origin of the perianth and androecium • Inferior ovary – Ovary located below the points of attachment of the perianth and stamens Inflorescences • Clusters or groups of flowers • Types – Raceme – Spike – Umbel – Head – Cyme Types of Inflorescences Type Description Simple type of inflorescence, main axis has short Raceme branches called pedicels, panicle → branched raceme Example Radish Spike Main axis elongated, no pedicels, catkin → spike that Walnut, usually bears only pistillate or staminate flowers willow Umbel Short floral axis, flowers arise umbrella-like from approximately same level Onion, carrot Head Flowers lack pedicels, crowded together on short axis Sunflower Cyme Main axis produces flower that involves entire apical meristem so axis does not elongate, other flowers arise on lateral branches farther down axis Chickweed Self-Pollination and CrossPollination • Joseph Koelreuter – 1760s – 1st to demonstrate importance of pollen to plant reproduction • Christian Sprengel – Correctly distinguished between selfpollinating and cross-pollinating species – Described role of wind and insects as pollen vectors Self-Pollination and CrossPollination • Koelreuter and Sprengel – Founders of study called pollination ecology Self-Pollination and CrossPollination • Two types of pollination – Self-pollination (selfing) – Cross-pollination (outcrossing) Selfpollination or selfing No genetic recombination Only one plant involved Crosspollination or outcrossing Genetic recombination Transfer of pollen from one plant to stigma of another plant Self-Pollination and CrossPollination • Outcrossing or cross-pollination – Insured by separation of sexes into different individual plants • Self-pollination prevented by – Different maturation times for stigma and anther of same plant – Inhibition of pollen tube growth through style – Inhibition of zygote formation Self-Pollination and CrossPollination • Advantages of self-pollination – Means of reproduction for scattered populations in extreme habitats – Common among plants in disturbed habitats – Saves pollen and the metabolic energy to produce it – Increases probability that pollen will reach stigma because distance traveled and travel time are short Apomixis • Sexual reproduction in which no fusion of sperm and egg occurs – Parthenogenesis • Embryo develops from unfertilized egg – Adventitious • Embryo arises from diploid tissue surrounding the embryo sac Pollination Syndrome • Unique set of pollen traits that adapt a plant for pollination Flower Trait Beetle Fly Bee Butterfly Dull white or green Pale and dull to dark brown or purple; sometimes flecked with translucent patches Bright white, red, yellow, blue, or ultraviolet Bright including red and purple Absent Absent Present Present Odor None to strongly fruity or fetid Putrid Fresh, mild, pleasant Faint but fresh Usually absent Nectar Sometimes present; not hidden Usually present; somewhat hidden Ample; deeply hidden Ample Modest in amount Limited; often sticky and scented Limited Large, regular dish-like; erect Funnel-like or a complex trap Regular or irregular; often tubular with a lip; erect Regular; tubular with a lip; erect Tulip tree, magnolia. dogwood Skunk cabbage, philodendron Larkspur, snapdragon, violet Phlox Color Nectar guides Pollen Flower shape Examples Trait Moth Bird Bat Wind Pale and dull red, purple, pink, or white Scarlet, orange, red, or white Dull white, green, or purple Dull green, brown, or colorless; petals may be absent or reduced Absent Absent Absent Absent None Strong and musty; emitted at night None Odor Strong and sweet; emitted at night Abundant; deeply hidden Abundant; somewhat hidden None Nectar Abundant; deeply hidden Limited Modest Ample Abundant; small, smooth, and not sticky Regular; tubular without a lip; closed by day; pendant or horizontal Regular or irregular; tubular without a lip; pendant or horizontal Regular; trumpet-like; closed by day; pendant or borne on trunk Regular; small; anthers and stigmas exserted Tobacco, Easter lily, some cacti Fuchsia, hibiscus Banana, agave, sausage tree, Color Nectar guides Pollen Flower shape Examples Walnut, grasses Pollinators • Animals – Visit flowers for some reward – Incidentally transfer pollen – Rewards include • Pollen • Nectar Pollinators • Pollen – Excellent food for animals • Contains – – – – – 15-30% protein 15% sugar 3-13% fat 1-7% starch Trace amounts of vitamins, essential elements, secondary substances – Highly noticeable – Distinctive odor Pollinators • Nectar – Sugary water transported by phloem into secretory structures called nectaries – Contains • 15-75% sugar • Minor amounts of amino acids – All 13 essential amino acids needed for insects are present Biotic Pollen Vectors • Beetles – Among oldest insect groups – Flowers pollinated by beetles typically have primitive traits • • • • Regular symmetry Large, simple flowers Bowl shaped architecture Floral parts not fused – Many beetle-pollinated species are tropical Biotic Pollen Vectors • Flies – No single syndrome of floral traits for fly pollination • Bees and butterflies – Active by day – Need landing platform – Harvest nectar as reward Biotic Pollen Vectors • Moths – Active by night or at dawn and dusk – Harvest nectar as reward – Moth pollinated flowers • • • • • • • White or faintly colored Emit heavy odors Fringed blossom rim Are pendant or horizontal Have no nectar guides Often closed during day Have long, narrow tubes with pools of nectar at their base Biotic Pollen Vectors • Butterflies – Flowers pollinated by butterflies • • • • • Vividly colored Emit faint odors Have broad blossom rim Are erect Exhibit prominent nectar guides Biotic Pollen Vectors • Birds – Not recognized by botanists as pollinators until relatively recently – Bird pollinated flowers • • • • • • Scarlet to red to orange in color Generally lack nectar guides Deep tubes usually without a landing platform Are pendant or horizontal Have abundant nectar Emit no odor Biotic Pollen Vectors • Bats – Bat pollinated flowers • Open at night • Positioned below foliage of parent tree hanging pendant or attached to trunk or low limbs • Drab white, green, or purple • Strong musty odor at night • Large, tough flowers • Lots of pollen and nectar Abiotic Pollen Vectors • Wind-pollinated flowers – – – – – – – Small Colorless Odorless Nectarless Petals often lacking or reduced to small scales Positioned to dangle or wave in open Stigmas enlarged and elaborate and often extend outside of flower Abiotic Pollen Vectors • Pollen from wind-pollinated flowers – Generally smoother, smaller, drier than animal-pollinated species – Often changes shape from spherical to Frisbee shape on release to dry air – More pollen grains/ovule than animalpollinated flowers Aquatic Plants • Many aquatic plants produce flowers that project above water surface – Vectors are usually wind and insects • Some produce flowers at water surface – Pollen floats from anther to stigma Seeds and Fruits Chapter 14 Fruits and Seeds • Fruits – Packaging structure for seeds of flowering plants • Seeds – Mature ovules – Contain embryonic plant • Fruits and seeds – Most important source of food for people and animals Seed – Mature Ovule • Fertilization occurs • Zygote develops into embryo • Primary endosperm nucleus develops into endosperm – Suspensor supports embryo in endosperm – Endosperm is nutrient-rich storage tissue – Endosperm persists in many monocots and only in a few dicots Seed – Mature Ovule • Integuments of ovule develop into seed coat – Seed coat acts as protective shell around embryo – Sometimes contains chemical substance that inhibits seed from germinating until conditions are right for germination Common bean Castor bean Grasses Onion Monocot or dicot Dicot Dicot Monocot Monocot External features of seed Hilum, micropyle, raphe Caruncle – covers hilum and micropyle, raphe runs length of seed Micropyle Micropyle Endosperm Not present Massive amounts Yes Yes, small amount Cotyledons 2 fleshy cotyledons 2 thin cotyledons 1 cotyledon 1 cotyledon Embryo Embryonic root (radicle) at one end, shoot – epicotyl at other end, hypocotyl – just below cotyledons Short hypocotyl, small epicotyl, small radicle Shoot apex and several rudimentary leaves ensheathed in coleoptile, radicle surrounded by coleorhiza, scutellum – secretes enzymes that digest food stored in endosperm Simple embryo, radicle, and simple cotyledon are prominent, shoot apex close to midpoint of axis and appears as notch, embryo coiled, radicle usually points toward micropyle Germination Hypocotyl elongates, raises cotyledons and shoot apex toward light Cotyledons first function as absorbing organs, cotyledons emerge from seed coat, become green, photosyntesize, wither, die Primary root pushes through coleorhiza, adventitious roots develop, coleoptile elongates and emerges aboveground, uppermost leaf pushes through coleoptile and becomes part of the photosynthesizing shoot Slightly bent cotyledon breaks soil surface, straightens out, base of cotyledon encloses shoot apex, first leaf emerges through opening at base of cotyledon Seeds • Key terms – Hilum • Large oval scar left when seed breaks away from placental connection (funiculus) – Micropyle • Small opening in seed coat at one end of hilum • Opening through which pollen tube enters ovule Seeds – Raphe • Ridge at end of hilum opposite the micropyle • At base of the funiculus – Caruncle • Spongy outgrowth of outer seed coat • Absorbs water needed during germination Germination • 1st step in growth of embryo • Begins with imbibition (uptake of water) – Water activates enzymes that digest food stored in cytoplasmic organelles called protein bodies, lipid bodies, and amyloplasts • 1st indication germination has begun – Swelling of radicle Germination • Two types of germination – Epigeal germination • Straightening of hypocotyl raises cotyledons and shoot apex toward light – Hypogeal germination • Cotyledons remain belowground • Only apex and 1st leaf are raised upward Dormancy of Seeds • Seeds remain viable for long periods • Many viable seeds will not germinate even when conditions are right – In state of dormancy – Factors that break dormancy • Light – some lettuce species • Scarring or breaking through seed coat – legumes • Exposure to temperatures close to freezing – gooseberry • Exposure to high temperature of fire – some pines Fruits • Ripened ovary • Commonly refers to a juicy and edible structure • Functions – Protect seeds – Aid in dispersal of seeds – May be factor in timing of germination of seeds Fruits • Play important role in classification of angiosperms • Examples of fruits – Apple, plum, peach, grapes, string beans, eggplant, squash, tomato, cucumber, corn, oats Fruits • Fruit wall (pericarp) has three layers – Exocarp – Mesocarp – Endocarp • Accessory – Tissues other than ovary wall that form part of a fruit Main Categories of Fruits • Simple – Derived from single ovary – Dry or fleshy – Dehiscent (splits open) or indehiscent • Compound – Composed of more than one fruit Main Categories of Fruits – Two types of compound fruits • Aggregate – Derived from many separate ovaries of a single flower – Example: strawberry • Multiple – Enlarged ovaries of several flowers grown more or less together into a single mass – Example: pineapple Criteria for Classifying Fruits • • • • • • • Structure of flower from which fruit develops Number of ovaries involved in fruit formation Number of carpels in each ovary Nature of mature pericarp (dry or fleshy) Whether pericarp splits (dehisces) at maturity If pericarp dehisces, manner of its splitting Role accessory tissues play in formation of mature fruit Simple Fruits – Dry and Dehiscent • Legume or pod – Arises from single carpel – At maturity usually dehisces along two sides – Example: pea • Shell – pericarp • Pea - seed Simple Fruits – Dry and Dehiscent • Follicle – Develops from a single carpel – Opens only along one side – Example: magnolia • Capsules – Simple fruits derived from compound ovaries – Dehisces in various ways along top surface – Example: poppy Simple Fruits – Dry and Dehiscent • Silique – Dry fruit derived from superior ovary consisting of two locules – Dry pericarp separates into 3 portions • Seed attached to central, persistent portion – Example: members of mustard family Simple Fruits – Dry and Indehiscent • Achene – Dry, one seeded fruit – Pericarp easily separated from seed coat – Example: sunflower • Caryopsis or grain – Fruit of grass family – Dry, one seeded indehiscent fruit – Pericarp and seed coat firmly united all around embryo Simple Fruits – Dry and Indehiscent • Samara – Outgrowths of ovary wall form wing-like structure that aids in seed dispersal • One seeded simple fruit – Example: elm • Two seeded simple fruit – Example: maple Simple Fruits – Dry and Indehiscent • Schizocarp – Two carpels that split when mature along midline into two one-seeded indehiscent halves – Example: celery • Nut – One seeded, indehiscent dry fruit with hard or stony pericarp (shell) – Example: walnut Fleshy Pericarp • Popular for food • Fleshy fruit wall – Attractive to animals – Seeds tend to have hard seed coat not broken down as it passes through animal Fleshy Pericarp • Drupes – One seeded – Derived from single carpel – Hard endocarp – Thin exocarp – Fleshy mesocarp – Examples: cherry, almond, peach, apricot Fleshy Pericarp • Berry – Derived from compound ovary – Many seeds embedded in flesh – Types of berries • Hesperidium – Exocarp and mesocarp – rind with numerous oil cavities – Endocarp – thick, juicy pulp segments composed of wedge-shaped locules – Juice forms in juice sacs or vesicles » Outgrowths of endocarp wall – Examples: lemons, oranges, limes, grapefruit Fleshy Pericarp • Pepo – Rind consists mainly of receptacle tissue that surrounds it and is fused with exocarp – Flesh of fruit » Mainly mesocarp and endocarp – Examples: watermelon, cucumber, squash Fleshy Pericarp • Pomes – Fruit derived from flower with inferior ovary – Flesh • Enlarged hypanthium (fleshy floral tube) – Core • From ovary – Example: apple Compound Fruits • Aggregate fruits – Formed from numerous carpels of one individual flower – Many simple fruits attached to a fleshy receptacle – Example: blackberry Compound Fruits • Multiple fruit – Formed from individual ovaries of several flowers all grouped together – Fruit • Enlarged fleshy receptacle – Example: fig (drupes) – Example: pineapple (berries) Partheocarpy • Parthenocarpic fruits – Develop without fertilization – Seedless fruits – Regularly produced in cultivated plants • Eggplant, navel orange, banana, pineapple – In orchids • Placing dead pollen or water extract of pollen on stigma may start fruit development Parthenocarpy – Commercially induced in some plants • Spray blossoms with dilute aqueous solution of growth substance such as auxin Role of Fruit • Aid in dispersal of seeds inside • Deter inappropriate seed-dispersing animals from taking fruit or seed • To protect seed from herbivores who consume seeds but do not disperse them Role of Fruit • No nutritional relationship between fruit and seeds within it – Stored food in fruit cannot be used by dormant seeds or by germinating seedlings – Only stored food available to seedlings is in endosperm and cotyledons within seed coat Role of Fruit and Seeds • Fruits and seed are rich in chemical resources – Sugar, starch, protein, lipid, amino acids, variety of secondary compounds – Caloric value is approximately 5,100 kcal/gram dry weight Abiotic Mechanisms for Seed Dispersal • Wind – Winged and plumed fruits common adaptations for dispersal – Seeds ballistically exploded by violent dehiscence of pericarp • Water – Seeds float, germinate when washed ashore – Flash floods spread seeds Biotic Vectors for Seed Dispersal • Ants, birds, bats, rodents, fish, ruminants, primates – Attracted to fruit by color, position, season availability, odor, taste Biotic Vectors for Seed Dispersal • Biotic vector – May eat fruit and discard seeds • True of some primates – Swallow seeds unchewed • Seeds pass unharmed through gut • Excreted some distance away • Often case with birds Biotic Vectors for Seed Dispersal • May eat some seeds and cache others – Seedlings later emerge from cached seeds – Squirrels, jays • May harvest seeds and deposit them in granaries below ground – Ants Biotic Vectors for Seed Dispersal • May eat elaiosomes (food bodies) at one end of seed and then discard seed – ants Biotic Vectors for Seed Dispersal • Sometimes animals transfer seeds in a more parasitic fashion – Seeds of some aquatic and marsh plants stick to feet of birds in mud and are carried long distances – Birds carry sticky mistletoe seeds on their feet to new host trees – Seeds with beards, spines, hooks, or barbs adhere to animal hair and human clothing and are carried to new sites Antiherbivore Mechanisms • Mechanisms that discourage herbivores include – Reducing the time of fruit availability – Making the fruit or seed coat physically hard – Making the fruit or endosperm chemically repellent Antiherbivore Mechanisms • Reducing the time of fruit availability – Some species produce fruit and seed abundantly only during mast years – Low amount of seeds produced in off years keeps number of seed eaters in check – Seed-eating populations not large enough to consume all seeds available during mast year – Some seeds escape consumption and germinate Antiherbivore Mechanisms • Making the fruit or seed coat physically hard – Prevents seed from being damaged by grinding action in the crop of birds or the mouths of chewing mammals – Legume seed coats are hard and often pass through animal guts unharmed Antiherbivore Mechanisms • Making fruit or endosperm chemically repellent – Effect is negative and often toxic • • • • • • Lectins – cause red blood cells to clump Enzyme inhibitors Cyanogens – release cyanide (potent nerve toxin) Saponins - a detergent Alkaloids – opium Unusual amino acids Distant Dispersal of Seeds • Benefit of fruit and seed dispersal – Spread species far from its parent – Many fruits and seeds wasted because eaten or deposited in places inappropriate for germination – In stressful habitats – Advantageous to prevent or limit dispersal away from parents Distant Dispersal of Seeds • Method of limiting dispersal – Self-planting • Grasses produce bent awns (slender bristles) that drive grain into soil – Peanut • Fruits become buried as they mature • Seeds never leave immediate proximity of parent – Sea rocket • Bipartie fruit – Top half carried by ocean currents, bottom half attached to parent