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The Flower and Sexual Reproduction
Chapter 13
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Significance of the Flower
• Flowers and fruit least affected by environment
• Appearance of flowers and fruits important to understanding evolutionary relationships among angiosperms
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Function of Flowers
• To facilitate the important events of gamete formation and fusion
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Steps in Sexual Cycle
• Production of special reproductive cells after meiosis
• Pollination
• Fertilization
• Seed and fruit development
• Seed and fruit dissemination
• Seed germination
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Flower Parts
• Four whorls of modified leaves– Sepals– Petals– Stamens– Carpels
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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
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Flower Parts
• Perianth– Collective term for calyx and corolla– Protects stamens and pistil(s)– Attracts and guides movements of some
pollinators
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Androecium
• Whorl of stamens– Consists of
• Filament• Anther
– Made up of four elongated lobes called pollen sacs
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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)
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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
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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
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Gynoecium
• Female organs
• Simple pistil– Single folded carpel
• Compound pistil– Several separate carpels or a group of fused
carpels
• Ovary– Chambers called locules
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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
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Gynoecium
• Style– Often withers after pollination
• Stigma – May have hairs that help hold pollen grains– Sometimes secretes sticky fluid that
stimulates pollen growth
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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
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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
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Gynoecium
• Stages in embryo sac development– Series of 3 mitotic divisions form 8 nucleate
embryo sac– Nuclei migrate– Cell wall forms around nuclei
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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
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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
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Double Fertilization
• Double fertilization actually refers to– Fusion of egg and sperm– Fusion of sperm with polar nuclei
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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.
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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
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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
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Flower Types
• Monoecious– Plant with staminate and pistillate flowers on
one individual plant
• Dioecious – Staminate and pistillate flowers on separate
individual plants
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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
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Fusion of Flower Parts
• Connation– Union of parts of same whorl
• Adnation – Union of flower parts from different whorls
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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
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Inflorescences
• Clusters or groups of flowers
• Types– Raceme– Spike– Umbel– Head– Cyme
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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
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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
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Self-Pollination and Cross-Pollination
• Koelreuter and Sprengel – Founders of study called pollination ecology
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Self-Pollination and Cross-Pollination
• 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
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Self-Pollination and Cross-Pollination
• 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
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Self-Pollination and Cross-Pollination
• 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
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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
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Pollination Syndrome
• Unique set of pollen traits that adapt a plant for pollination
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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
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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
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Pollinators
• Animals – Visit flowers for some reward– Incidentally transfer pollen– Rewards include
• Pollen • Nectar
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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
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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
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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
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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
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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
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Biotic Pollen Vectors
• Butterflies– Flowers pollinated by butterflies
• Vividly colored• Emit faint odors• Have broad blossom rim• Are erect • Exhibit prominent nectar guides
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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
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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
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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
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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
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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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Seeds and Fruits
Chapter 14
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Fruits
• Play important role in classification of angiosperms
• Examples of fruits– Apple, plum, peach, grapes, string beans,
eggplant, squash, tomato, cucumber, corn, oats
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Fruits
• Fruit wall (pericarp) has three layers– Exocarp– Mesocarp– Endocarp
• Accessory – Tissues other than ovary wall that form part of
a fruit
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Main Categories of Fruits
• Simple– Derived from single ovary– Dry or fleshy– Dehiscent (splits open) or indehiscent
• Compound – Composed of more than one fruit
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Fleshy Pericarp
• Drupes– One seeded– Derived from single carpel– Hard endocarp– Thin exocarp– Fleshy mesocarp– Examples: cherry, almond, peach, apricot
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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
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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
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Fleshy Pericarp
• Pomes – Fruit derived from flower with inferior ovary– Flesh
• Enlarged hypanthium (fleshy floral tube)
– Core• From ovary
– Example: apple
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Compound Fruits
• Aggregate fruits– Formed from numerous carpels of one
individual flower– Many simple fruits attached to a fleshy
receptacle– Example: blackberry
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Compound Fruits
• Multiple fruit– Formed from individual ovaries of several
flowers all grouped together– Fruit
• Enlarged fleshy receptacle
– Example: fig (drupes)– Example: pineapple (berries)
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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
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Parthenocarpy
– Commercially induced in some plants• Spray blossoms with dilute aqueous solution of
growth substance such as auxin
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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
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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
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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
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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
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Biotic Vectors for Seed Dispersal
• Ants, birds, bats, rodents, fish, ruminants, primates– Attracted to fruit by color, position, season
availability, odor, taste
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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
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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
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Biotic Vectors for Seed Dispersal
• May eat elaiosomes (food bodies) at one end of seed and then discard seed– ants
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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
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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
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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
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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
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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
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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
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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