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