Download Plant Reproduction and Development

Plant Reproduction
and Development
Chapter 43
Asexual Reproduction
• Part of a single plant divides by mitosis to
give rise to a new plant
– Spreading of runners (strawberries)
– Production of bulbs (daffodils)
– Sprouting of rhizomes (irises)
• Offspring are genetically identical to parent
plant
• Advantageous in a stable environment
Sexual Reproduction
• Two parent plants give rise to genetically
variable offspring
• Involves meiosis, gamete formation, and
fertilization
• Allows offspring to cope with a changing
environment or invade different habitats
Alternation of Generations
• Plant life cycles are characterized by
alteration of generations
• Multicellular diploid plants (sporophytes)
and multicellular haploid plants
(gametophytes) take turns producing each
other
Alternation of Generations
• In seedless plants (mosses and ferns)
– Gametophytes are independent plants
– Mobile sperm swim through a thin film of
water to reach egg
– Reproduction restricted to moist habitats
Fern Life Cycle
• Diploid sporophyte (spore-forming plant)
produces haploid spores by meiosis
• Spore germinates (begins to grow and
develop); divides repeatedly by mitosis
forming a haploid gametophyte (gameteforming plant)
• Gametophyte produces haploid sperm and
eggs by mitosis (gametes are produced at
different times to prevent self-fertilization)
Fern Life Cycle
• Sperm and egg fuse to form a diploid
zygote (fertilized egg)
• Zygote undergoes repeated mitosis to form
a new diploid sporophyte plant
Reproduction in Seed Plants
• Seed plants (gymnosperms and angiosperms)
– Gametophytes develop within sporophytes
– Reproduction can occur in dry habitats
• Male gametophyte is surrounded by a protective
coat called a pollen grain
• The pollen grain encloses sperm cells in a
watertight packet that can be easily transported
to another plant
• Egg-producing female gametophyte remains
moist and protected within the sporophyte, and
the pollen grain ensures that the sperm are
delivered directly to the egg
Reproduction in Seed Plants
• The fertilized egg becomes enclosed in a
drought-resistant seed
– Consists of an embryonic plant and a food
reserve encased within a protective outer
coating
– May lie dormant up to years waiting for
conditions favorable for germination and
growth
Reproduction in Seed Plants
• Non-flowering gymnosperms were the
earliest seed plants
• Gymnosperms bear male and female
gametophytes on separate cones
• Male cones release pollen grains that
travel via wind to female cones
Animal Pollinators
• Wind pollination is inefficient because most
pollen grains fail to reach their target
• Flowering plants (angiosperms) evolved from
gymnosperms
• Flowers produce pollen and nectar that attract
animal pollinators (bees, moths, butterflies,
hummingbirds)
• Flowers enhance a plant’s reproductive success
because animal pollinators transport pollen from
plant to plant as they feed
Flowers
• Flowers are the reproductive structures of
angiosperms, produced by the sporophyte
generation
• Angiosperms bear male and female
gametophytes on flowers
• Within each flower, meiosis gives rise to two
types of haploid spores
– Megaspore divides by mitosis to form female
gametophyte (embryo sac) that is retained within
flower
– Microspore divides by mitosis to form male
gametophyte (pollen grain)
Flowers
• Gametophytes form haploid gametes by
mitosis
• Sperm are liberated from pollen grain when
it lands on female structure of another plant
• Sperm burrow to egg and fuse to form a
diploid zygote that becomes encased in a
seed
• Seed germinates to form a new sporophyte
Complete Flowers
• Complete flowers have four major parts
• Sepals – located at base of flower;
surround and protect the bud
– Dicot sepals are green and leaf-like
– Monocot sepals resemble petals
• Petals – located above sepals; usually
brightly colored and fragrant (attract
pollinators)
Complete Flowers
• Stamens (male reproductive structures) –
attached above petals
– Each consists of a filament (stalk) and anther
(produces pollen)
• Carpel (female reproductive structure) –
centrally located
– Each consists of a sticky stigma (catches pollen), an
elongate style, and a bulbous ovary containing one
or more ovules
• Ovules develop into seeds
• Ovary develops into a fruit
Incomplete Flowers
• Incomplete flowers lack one or more of
the four major floral parts
– Grass flowers lack both petals and sepals
Imperfect Flowers
• Imperfect flowers are incomplete flowers
lacking either stamens (female flowers) or
carpels (male flowers)
• American holly bears separate male and
female flowers on separate plants; fruits
develop on female plants
• Zucchini bears separate male and female
flowers on the same plant; fruits develop
from female flowers
The Male Gametophyte
• Pollen contains the male gametophyte
• Pollen develops within anthers of diploid
sporophyte plants
• Pollen grains have tough protective coats that
exhibit species-specific shapes and patterns
• Each anther consists of four pollen sacs that
contain microspore mother cells
• Each diploid microspore mother cell divides by
meiosis to form four haploid microspores
The Male Gametophyte
• Each microspore divides by mitosis to form an
immature male gametophyte consisting of two
cells
– Tube cell (will form a pollen tube after pollination)
– Generative cell (will divide by mitosis to form two
sperm after pollination)
• Pollen sacs of anthers split open when male
gametophytes are mature
• Pollen is dispersed by either animal pollinators
or wind currents
The Female Gametophyte
• Nucleus of remaining megaspore divides
by mitosis three times (without cytokinesis)
to form a single cell with 8 haploid nuclei
The Female Gametophyte
• Plasma membranes partition the cell into a
seven-celled embryo sac
– Three small cells at either end; one is the egg
– One large central cell containing two polar
nuclei
Pollination
• Pollination occurs when a pollen grain
lands on the stigma of a compatible plant
• Pollen grain absorbs water and germinates
– Tube cell extends, forming a pollen tube that
grows down the style
– Generative cell divides (mitosis) to form two
sperm cells
• Sperm cells follow pollen tube to the ovule
Double Fertilization
• Double fertilization is the process by
which two sperm fuse with two cells of the
embryo sac
– One sperm fuses with the egg to form the
diploid (2n) zygote
– Second sperm fuses with both polar nuclei to
form the triploid (3n) endosperm cell
– Double fertilization is unique to flowering
plants
Fruits and Seeds
• The female gametophyte and the
surrounding integuments of the ovule
develop into a seed
• The seed is surrounded by the ovary,
which develops into a fruit
• Petals and stamens shrivel and fall away
as the fruit enlarges
From Ovary to Fruit
• A fruit is a ripened ovary
• Examples: apples, berries, peaches,
oranges, bananas, avocados, zucchini,
tomatoes, and peppers
• Fruits help disperse seeds away from the
parent plant
– Fleshy, edible fruits attract animals
– Spiked fruits attach to animal fur
– Winged fruits promote wind dispersal
From Ovule to Seed
• Integuments of ovule develop into the
seed coat (outer covering of seed)
• Inside the ovule
– Triploid endosperm cell divides to form
endosperm that absorbs nutrients from parent
plant
– Zygote develops into an embryo
The Embryo
• The embryo consists of
– An embryonic root
– An embryonic shoot
• Hypocotyl - region below cotyledons, but above
root
• Epicotyl - region above cotyledons; tip is apical
meristem
– One or two cotyledons (seed leaves)
Cotyledons
• Dicot seeds have two cotyledons
– Absorb nutrients from endosperm during seed
development and store for future use
– Transfer stored nutrients to other parts of
embryo during seed germination
• Monocot seeds have one cotyledon
– Absorbs and transfers most nutrients during
seed germination
Seed Dormancy
• As seeds mature, they enter a period of
dormancy
– Suspended growth and development
– Lowered metabolic rate
– Resistance to adverse environmental
conditions
– Dormancy ensures that germination occurs at
a time and place most advantageous to the
seed
Seed Dormancy
• Conditions that break dormancy vary between
plant species
– Some germinate as soon as the environment is
suitable (adequate moisture and temperature)
• Conditions that break dormancy vary between
plant species
– Others have additional requirements
• Initial drying (seeds within fleshy fruits)
• Exposure to prolonged cold (seeds of temperate
and arctic plants)
• Disruption of seed coat
Seed Germination
• Germination (resumption of growth) occurs
when the seed absorbs enough water to
rupture the seed coat
• Emergence of the embryonic root is
followed by emergence of the embryonic
shoot
• Emerging roots and shoots must push
through the soil without abrading the
apical meristems at their tips
Seed Germination
• Root apical meristem is protected by a
root cap
• Shoot apical meristem is protected by
either
– A coleoptile (sheath enclosing the shoot tip
of monocots)
– An epicotyl or hypocotyl hook that pulls the
shoot tip of dicots out of the soil
Cotyledons
• Food stored in the seed (as endosperm or
within cotyledons) provides the energy for
sprouting
• Cotyledons of monocots and dicots with
epicotyl hooks
– Remain below ground
– Digest endosperm and transfer nutrients to
seedling
Cotyledons
• Cotyledons of dicots with hypocotyl hooks
are carried out of the soil
– Become photosynthetic
– Transfer previously stored nutrients and newly
synthesized sugars to the seedling
– Wither and fall off when food reserves have
been consumed
Coevolution
• Some plants and their pollinators have
coevolved
– Each has acted as an agent of natural
selection on the other
• Animal-pollinated flowers must
– Attract useful pollinators able to locate flowers
and extract pollen or nectar
– Frustrate undesirable visitors who might eat
nectar or pollen without fertilizing the flower
Coevolution
• Animal-pollinated flowers are grouped into
three categories based on how they attract
pollinators
– Food
– Sex deceptions
– A nursery
Food
• Many animals forage on flowers, distributing
pollen from flower to flower in the process
• Examples: beetles, bees, moths, butterflies,
hummingbirds
• Bees are attracted to flowers by sweet odors
and bright colors
• Bees do not see the same range of colors that
humans do
• Bee-pollinated flowers are typically white, blue,
yellow, or orange, with markings that reflect UV
light pointing toward the center
Food
• Bee-pollinated flowers have structural
adaptations to ensure pollen transfer
• Example: Scotch broom flower
• When a bee visits a young plant, the stamens
emerge from a crevice between petals, brushing
pollen onto her back as her weight deflects the
petals downward
• In older flowers, the sticky stigma of the carpel
protrudes from the crevice, becoming pollinated
by pollen-coated, nectar-foraging bees
Food
• Other adaptations for drawing pollinators
– Nectar-containing tubes to accommodate the
long tongues of moths and butterflies
– Tubular flower shape to match the long bills
and tongues of hummingbirds
– Flowers that heat up to broadcast their scents
• Example: skunk cabbage
Sex Deceptions
• Some plants capitalize on the mating drive
and behaviors of male wasps
• Example: some orchid flowers mimic
female wasps or bees in scent and shape
– The males attempt to copulate, becoming
covered with pollen in the process
– Pollen is transferred as they repeat the
process with other orchids
Nurseries
• Some insects fertilize a flower and then lay
their eggs in the flower’s ovary
• Example: yuccas and yucca moths
– A female moth collects pollen from one flower
and transfers it to another, smearing it over
the stigma and laying her eggs inside the
ovary
– The developing seeds provide nutrition for the
developing catepillars
Fruits Disperse Seeds
• Mechanisms to disperse seeds away from
the parent plant include
– Explosive fruits that eject their seeds
– Lightweight fruits with large wind-catching
surfaces
Fruits Disperse Seeds
• Floating fruits for water dispersal
• Clingy or tasty fruits that allow animal
dispersal