The%20Flower%20and%20Sexual%20Reproduction

1
The Flower and Sexual Reproduction

• Chapter 13

2
Significance of the Flower

• Flowers and fruit least affected by environment
• Appearance of flowers and fruits important to
  understanding evolutionary relationships among
  angiosperms

3
Function of Flowers

• To facilitate the important events of gamete
  formation and fusion

4
Steps in Sexual Cycle

• Production of special reproductive cells after
  meiosis
• Pollination
• Fertilization
• Seed and fruit development
• Seed and fruit dissemination
• Seed germination

5
Flower Parts

• Four whorls of modified leaves
• Sepals
• Petals
• Stamens
• Carpels

6
Flower Parts
Part Description Collective Term Function
Sepals Usually green, encloses other flower parts Calyx Protect reproductive parts inside flower
Petals Colored, attractive flower parts Corolla Catch attention of pollinators
Stamens Just 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
7
Flower Parts

• Perianth
• Collective term for calyx and corolla
• Protects stamens and pistil(s)
• Attracts and guides movements of some pollinators

8
Androecium

• Whorl of stamens
• Consists of
• Filament
• Anther
• Made up of four elongated lobes called pollen
  sacs

9
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)

10
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

11
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

12
Gynoecium

• Female organs
• Simple pistil
• Single folded carpel
• Compound pistil
• Several separate carpels or a group of fused
  carpels
• Ovary
• Chambers called locules

13
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

14
Gynoecium

• Style
• Often withers after pollination
• Stigma
• May have hairs that help hold pollen grains
• Sometimes secretes sticky fluid that stimulates
  pollen growth

15
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

16
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

17
Gynoecium

• Stages in embryo sac development
• Series of 3 mitotic divisions form 8 nucleate
  embryo sac
• Nuclei migrate
• Cell wall forms around nuclei

18
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

19
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

20
Double Fertilization

• Double fertilization actually refers to
• Fusion of egg and sperm
• Fusion of sperm with polar nuclei

21
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.

22
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

23
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

24
Flower Types

• Monoecious
• Plant with staminate and pistillate flowers on
  one individual plant
• Dioecious
• Staminate and pistillate flowers on separate
  individual plants

25
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

26
Fusion of Flower Parts

• Connation
• Union of parts of same whorl
• Adnation
• Union of flower parts from different whorls

27
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

28
Inflorescences

• Clusters or groups of flowers
• Types
• Raceme
• Spike
• Umbel
• Head
• Cyme

29
Types of Inflorescences
Type Description Example
Raceme Simple type of inflorescence, main axis has short branches called pedicels, panicle ? branched raceme Radish
Spike Main axis elongated, no pedicels, catkin ? spike that usually bears only pistillate or staminate flowers Walnut, 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
30
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

31
Self-Pollination and Cross-Pollination

• Koelreuter and Sprengel
• Founders of study called pollination ecology

32
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
33
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

34
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

35
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

36
Pollination Syndrome

• Unique set of pollen traits that adapt a plant
  for pollination

37
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
Odor None to strongly fruity or fetid Putrid Fresh, mild, pleasant Faint but fresh
Nectar Sometimes present not hidden Usually absent Usually present somewhat hidden Ample deeply hidden
Pollen Ample Modest in amount Limited often sticky and scented Limited
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
Examples Tulip 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
Odor Strong and sweet emitted at night None Strong and musty emitted at night None
Nectar Abundant deeply hidden Abundant deeply hidden Abundant somewhat hidden None
Pollen Limited 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
Examples Tobacco, Easter lily, some cacti Fuchsia, hibiscus Banana, agave, sausage tree, Walnut, grasses
39
Pollinators

• Animals
• Visit flowers for some reward
• Incidentally transfer pollen
• Rewards include
• Pollen
• Nectar

40
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

41
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

42
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

43
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

44
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

45
Biotic Pollen Vectors

• Butterflies
• Flowers pollinated by butterflies
• Vividly colored
• Emit faint odors
• Have broad blossom rim
• Are erect
• Exhibit prominent nectar guides

46
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

47
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

48
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

49
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

50
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

51
Seeds and Fruits

• Chapter 14

52
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

53
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

54
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

55
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
56
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

57
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

58
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

59
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

60
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

61
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

62
Fruits

• Play important role in classification of
  angiosperms
• Examples of fruits
• Apple, plum, peach, grapes, string beans,
  eggplant, squash, tomato, cucumber, corn, oats

63
Fruits

• Fruit wall (pericarp) has three layers
• Exocarp
• Mesocarp
• Endocarp
• Accessory
• Tissues other than ovary wall that form part of a
  fruit

64
Main Categories of Fruits

• Simple
• Derived from single ovary
• Dry or fleshy
• Dehiscent (splits open) or indehiscent
• Compound
• Composed of more than one fruit

65
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

66
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

67
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

68
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

69
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

70
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

71
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

72
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

73
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

74
Fleshy Pericarp

• Drupes
• One seeded
• Derived from single carpel
• Hard endocarp
• Thin exocarp
• Fleshy mesocarp
• Examples cherry, almond, peach, apricot

75
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

76
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

77
Fleshy Pericarp

• Pomes
• Fruit derived from flower with inferior ovary
• Flesh
• Enlarged hypanthium (fleshy floral tube)
• Core
• From ovary
• Example apple

78
Compound Fruits

• Aggregate fruits
• Formed from numerous carpels of one individual
  flower
• Many simple fruits attached to a fleshy
  receptacle
• Example blackberry

79
Compound Fruits

• Multiple fruit
• Formed from individual ovaries of several flowers
  all grouped together
• Fruit
• Enlarged fleshy receptacle
• Example fig (drupes)
• Example pineapple (berries)

80
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

81
Parthenocarpy

• Commercially induced in some plants
• Spray blossoms with dilute aqueous solution of
  growth substance such as auxin

82
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

83
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

84
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

85
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

86
Biotic Vectors for Seed Dispersal

• Ants, birds, bats, rodents, fish, ruminants,
  primates
• Attracted to fruit by color, position, season
  availability, odor, taste

87
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

88
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

89
Biotic Vectors for Seed Dispersal

• May eat elaiosomes (food bodies) at one end of
  seed and then discard seed
• ants

90
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

91
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

92
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

93
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

94
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

95
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

96
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