Chapter 44
Plant Reproduction and
Development
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
Alternation of generations
• Plant sexual life cycles alternate between two
multicellular stages, haploid and 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
Sexual Life Cycle of a Flowering Plant
1. Diploid mother cells develop in anthers
(male) or ovaries (female)
2. Meiosis produces haploid spores
3. Mitosis of the spores  male & female
gametophytes  sperm & egg
4. Pollen carries sperm to flower, sperm travel
in tube to female gametophyte
5. Fertilization  diploid zygote
6. Zygote develops into embryo, seedling,
mature sporophyte
Alternation of Generations
mother cell
spores
MEIOTIC CELL
DIVISION
flower
stigma
male gametophyte
(pollen grain)
anther
ovule
ovary
1 In the flower,
diploid mother
cells develop in
the reproductive
structures:
anthers (male)
and ovaries
(female)
2 Meiotic
cell division
of mother
cells in the
sporophyte
produces
haploid
spores
mother
cell
MEIOTIC CELL
DIVISION
ovule
mature
sporophyte
seedling
3 Mitotic cell
division of the
spores forms male
gametophytes
(pollen), which
produce sperm,
and female
gametophytes,
which produce
eggs
sperm
4 Pollen
carries the
sperm to the
female
reproductive
structure of a
flower; sperm
travel within a
pollen tube to
the female
gametophyte
sperm
nuclei
spores
6 The zygote
develops into an
embryo, a seedling,
and eventually, a
new mature
sporophyte
female
gametophyte
egg
female
gametophyte
FERTILIZATION
seed
haploid (n)
diploid (2n)
pollen
tube
seed
fruit
embryo
5 A sperm
fertilizes an
egg within the
female
gametophyte,
producing a
diploid zygote
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
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
Flower Structure
• Flower – reproductive structure of angiosperm
• Complete flower – has 4 sets of modified
leaves
– Sepals, petals, stamens, carpels
– Petunia, rose, lily
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)
A Complete Flower
anther
petal
filament
stamen
stigma
style
sepal
ovary
ovules
(a) A representative dicot flower
carpel
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
Zucchini Flowers – male and female
Animation: Pollen Development
Pollen is the Male Gametophyte
• Develop within anthers of the sporophyte
1. Microspore mother cells develop within pollen
sacs of the anther
2. Meiosis produces 4 haploid microspores
3. Each produces an immature male gametophyte
(pollen grain)by mitosis, contains the generative
cell
– Tube cell + generative cell in the pollen cell
4. The generative cell undergoes mitosis to form 2
sperm cells.
Male Gametophyte Development
pollen
sacs
microspore
mother cell
anther
1 Microspore
mother cells develop
within the pollen
sacs of the anther
of a flower
sporophyte
tube cell
nucleus
mature
pollen
grain
sperm cells
MEIOTIC CELL
DIVISION
2 Meiotic cell division
produces four haploid
microspores
microspores
Immature
pollen grain
tube cell
cytoplasm
stigma
generative cell
4 The generative
cell produces two
sperm cells by mitotic
cell division; the male
gametophyte is now
mature
haploid (n)
diploid (2n)
3 Each microspore
produces an
immature male
gametophyte (a
pollen grain) by
mitotic cell division
tube cell
nucleus
Pollen
• Tough, waterproof
outercoat
• Characteristic of the plant
species
• Used to identify climate in
fossils
Wind-Pollinated Flowers
Anther, pollen
Female Gametophyte
• Forms in ovule
• Species vary – one to several dozen ovules
1. Megapore mother cell develops within ovule
2. Meiosis produces 4 haploid megaspores, 3
degenerate
3. Remaining megaspore form 8 nuclei by
mitosis (3X mitosis)
4. Plasma membranes form, 7 cells – 3 at one
end (1 N each), one is the egg
Female Gametophyte Development
ovule
megaspore mother cell
1 A megaspore mother cell
develops within each ovule
of the ovaries of a flower
ovary
integuments
MEIOTIC CELL
DIVISION
4 Cytoplasmic division
produces the seven cells of the
mature female gametophyte
megaspores
female
gametophyte
egg cell
haploid (n)
diploid (2n)
central
cell with
two nuclei
2 Meiotic
cell division
produces four
haploid
megaspores;
three
degenerate
3 The single remaining
megaspore forms eight
nuclei by mitosis
Animation: Ovule Development
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
Pollination and Fertilization of a Flower
pollen
grain
sperm
1 Pollination
occurs when
a pollen grain
lands on the
stigma of a
carpel
tube cell
nucleus
2 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
pollen tube
sperm
tube cell nucleus
3 Double
fertilization:
ovule
ovary
central
cell
egg
One sperm
fuses with the
central cell
One sperm
fuses with the
egg cell
Animation: Pollination and Fertilization
Fruit and Seed Development
• Female gametophyte and integuments
become seeds
• Ovary becomes fruit
• Petals, pollen, stamens dry up and fall off
Development of Fruit and Seeds in a Pepper
ripening
sepal
ovary
wall
ovary
petal
ovule
pepper flower
“flesh” of
pepper
pepper
fruit
seed
pepper fruits
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
Seed Structures
• Monocot
– Shoot
• Coleoptile – sheath that surrounds embryonic leaves
• Dicot
– Shoot
• Hypocotyl
• Epicotyl
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
shoot
coleoptile
embryonic
leaves
shoot
embryonic
leaves
hypocotyl
seed coat
embryonic
root
(b) Corn seed (monocot)
cotyledons
(c) Bean seed (dicot)
Animation: Embryo and Endosperm Development
Germination
• Germination – sprouting of seed
– Embryo grows and breaks out of seed
– Forms seedling
• Warmth and moisture are necessary
Dormancy
• 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
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
Cotyledons Nourish the Developing Plant
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
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
Seed Germination
true
leaves
coleoptile
root
(a) Corn (monocot)
true
leaves
cotyledon
hypocotyl
hook
seed
coat
epicotyl
cotyledons
hypocotyl
root
(b) Bean (dicot)
withered
cotyledons
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.
UV Patterns Guide Bees to Nectar
farred
red
orange yellow
green
blue
near
violet UV
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
“Pollinating” a Pollinator
Vertebrate Pollinators
Hummingbirds need a lot of energy so the
flowers they pollinate produce more nectar
than flowers that are pollinated by insects.
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
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
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
Water-Dispersed Fruit
Wind-Dispersed Fruits
Clingy Fruits
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