Download - Plant Reproduction and Development
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 spores3. Mitosis of the spores male & female
gametophytes sperm & egg4. Pollen carries sperm to flower, sperm travel in
tube to female gametophyte5. Fertilization diploid zygote 6. Zygote develops into embryo, seedling, mature
sporophyte
femalegametophyte
male gametophyte(pollen grain)
pollentube
spermnuclei
sperm
femalegametophyte
fruithaploid (n)diploid (2n)
The zygote develops into an embryo, a seedling, and eventually, a new mature sporophyte
6
A sperm fertilizes an egg within the female gametophyte, producing a diploid zygote
5
In the flower, diploid mother cells develop in the reproductive structures: anthers (male) and ovaries (female)
1 Meioticcell division of mother cells in the sporophyte produces haploid spores
2 Mitotic cell division of the spores forms malegametophytes (pollen), which produce sperm, and female gametophytes,which produce eggs
3
flowerstigma
anther
ovaryovule
mothercell
mother cell spores
ovule
maturesporophyte
seedling
embryoseed
egg
spores
seed
FERTILIZATION
MEIOTIC CELLDIVISION
MEIOTIC CELLDIVISION
Pollen carries the sperm to the female reproductive structure of a flower; sperm travel within a pollen tube to the female gametophyte
4
Alternation of Generations
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)
sepal
ovules
petalfilament
style
stigma
anther
ovary
carpel
stamen
(a) A representative dicot flower
A Complete Flower
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 microspores3. 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 microsporemother cell
pollen sacs
microspores
anther
sporophyte
sperm cells
stigma
tube cellnucleus
mature pollengrain
Immaturepollen grain
tube cellcytoplasm
generative cell
tube cellnucleus
Microspore mother cells develop within the pollen sacs of the antherof a flower
1
Meiotic cell divisionproduces four haploidmicrospores
2
Each microspore produces an immature male gametophyte (a pollen grain) by mitotic cell division
3
MEIOTIC CELLDIVISION
The generative cell produces two sperm cells by mitotic cell division; the male gametophyte is now mature
4
haploid (n)diploid (2n)
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 ovule2. Meiosis produces 4 haploid megaspores, 3
degenerate3. 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
A megaspore mother celldevelops within each ovule of the ovaries of a flower
1
Cytoplasmic divisionproduces the seven cells of the mature female gametophyte
4
Meiotic cell division produces four haploidmegaspores;three degenerate
2
The single remaining megaspore forms eight nuclei by mitosis
3
MEIOTIC CELLDIVISION
ovule megaspore mother cell
integuments
megasporescentralcell withtwo nucleifemale
gametophyte
haploid (n)diploid (2n)
egg cell
ovary
Female Gametophyte Development
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 tube
tube cellnucleus
sperm
sperm
tube cell nucleus
ovary centralcellegg
ovule
pollengrain
Pollination occurs whena pollen grain lands on the stigma of a carpel
1
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
2
Doublefertilization:
One sperm fuses with the central cell
One sperm fuses with the egg cell
3
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
sepal
ovary
petal
pepper flower pepper fruits
ovary wall
ovule
pepperfruit
“flesh” ofpepper
seed
ripening
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
integuments(diploid)
seed coat
centralcell (triploid)
endosperm
(a) Early development of the seed
(b) Corn seed (monocot) (c) Bean seed (dicot)
fertilized ovule seed
zygote(diploid)
embryo
embryonicleaves
embryonicroot
shoot
endosperm
seed coat
cotyledon
cotyledons
embryonic root
embryonicleaves
coleoptileshoot
hypocotyl
seed coat
Seed Development
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
root
hypocotylhook
root
seedcoat
cotyledons
cotyledon
hypocotyl
epicotylwitheredcotyledons
trueleaves
coleoptile
trueleaves
(b) Bean (dicot)
(a) Corn (monocot)
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 nearUV
400
bee visionhuman vision
violetbluegreenyelloworangeredfar-red
wavelength (nm)700 600 500
human
bee
(a ) A comparison of color vision in humans and bees
(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 nectarthan 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