The Flower and Sexual Reproduction

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

Part DescriptionCollective Term

Function

SepalsUsually green, encloses other flower parts

Calyx Protect reproductive parts inside flower

PetalsColored, attractive flower parts Corolla Catch attention of

pollinators

StamensJust inside corolla, male flower part, made up of anther and filament

Androecium Produces pollen

Carpels (pistil)

Modified leaves folded over and fused to protect ovules, usually in center of flower, made up of stigma, style, and ovary

Gynoecium Contains ovules

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 Example

RacemeSimple type of inflorescence, main axis has short branches called pedicels, panicle → branched raceme

Radish

SpikeMain axis elongated, no pedicels, catkin → spike that usually bears only pistillate or staminate flowers

Walnut, willow

UmbelShort floral axis, flowers arise umbrella-like from approximately same level

Onion, carrot

HeadFlowers 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 Cross-Pollination

• Joseph Koelreuter– 1760s– 1st to demonstrate importance of pollen to

plant reproduction

• Christian Sprengel– Correctly distinguished between self-

pollinating and cross-pollinating species– Described role of wind and insects as pollen

vectors


• Koelreuter and Sprengel – Founders of study called pollination ecology


• Two types of pollination– Self-pollination

(selfing)– Cross-pollination

(outcrossing)

Self-pollination or selfing

No genetic recombination

Only one plant involved

Cross-pollination or outcrossing

Genetic recombination

Transfer of pollen from one plant to stigma of another plant


• 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


• 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

Color

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

Nectar guides

Absent Absent Present Present

OdorNone to strongly fruity or fetid

Putrid Fresh, mild, pleasant

Faint but fresh

NectarSometimes present; not hidden

Usually absent Usually present; somewhat hidden

Ample; deeply hidden

PollenAmple Modest in amount Limited; often

sticky and scentedLimited

Flower shape

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

ExamplesTulip tree, magnolia. dogwood

Skunk cabbage, philodendron

Larkspur, snapdragon, violet

Phlox

Trait Moth Bird Bat Wind

Color

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

Nectar guides

Absent Absent Absent Absent

OdorStrong and sweet; emitted at night

None Strong and musty; emitted at night

None

NectarAbundant; deeply hidden

Abundant; deeply hidden

Abundant; somewhat hidden

None

PollenLimited Modest Ample Abundant; small,

smooth, and not sticky

Flower shape

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

ExamplesTobacco, Easter lily, some cacti

Fuchsia, hibiscus Banana, agave, sausage tree,

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


• Flies– No single syndrome of floral traits for fly

pollination

• Bees and butterflies– Active by day– Need landing platform– Harvest nectar as reward


• 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


• Butterflies– Flowers pollinated by butterflies

• Vividly colored• Emit faint odors• Have broad blossom rim• Are erect • Exhibit prominent nectar guides


• 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


• 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 animal-

pollinated 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

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The Flower and Sexual Reproduction