AP Biology 2005-2006

Chapter 4.

Plant Reproduction

KEY CONCEPT All plants alternate between two phases in their life cycles.

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Plant life cycles alternate between producing spores and gametes. A two-phase life cycle is called alternation of

generations. haploid phase diploid phase alternates between

the two fertilizationfertilization

meiosismeiosis

SPOROPHYTE PHASE

GAMETOPHYTE PHASE

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Animal vs. Plant life cycle

multicellular2n

multicellularsporophyte

2n

multicellulargametophyte

1n

unicellulargametes

1n

spores2n

gametes1n

Animal Plant

alternation of generations

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diploid

Alternation of generations

haploid

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Evolutionary trends

Alternation of generations dominant haploid plant

bryophytes - mosses dominant diploid plant

pteridophytes - ferns gymnosperm - conifers angiosperm - flowering plants

Evolutionary advantage? reduction of gametophyte protects delicate

egg & embryo in protective sporophyte

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Gametophytes of seed plants

male gametophyte pollen in male cone

female gametophyte develops in female

cone

seed naked in cone

male gametophyte pollen in anthers of

flower

female gametophyte develops in ovaries

of flower

seed protected in ovary ovary wall can

develop into fruit

Gymnosperm Angiosperm

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The sporophyte is the dominant phase for seed plants.

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Gymnosperm life cyclefemale

gametophytein cone

malegametophyte

in pollen

sporophytein seed

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Angiosperm life cycle

femalegametophyte

in ovary

malegametophyte

in pollen

sporophytein seed

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Reproduction in angiosperm Sporophyte plant produces unique reproductive

structure = the flower male gametophyte = pollen grain

develop within anthers of flower female gametophyte = embryo sac

develop within ovaries of flower pollination by wind or animals brings pollen grain to

female gametophyte fertilization takes place within ovary

double fertilization = embryo & endosperm seeds contain sporophyte embryo

development of seeds in ovary ovary develops into fruit around the seed

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Flowers

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Flower Modified shoot

with 4 rings of modified leaves sepals petals stamens

male carpals

female

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Male & female parts of flower

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Parts of flower Male

stamens = male reproductive organs stamens have stalks (filament) &

terminal anthers which carry pollen sacs

pollen sacs produce pollen pollen grain = gametophyte

sperm-producing structure

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Parts of flower

Female carpels = female reproductive organs ovary at the base slender neck = style within the ovary are 1 or more ovules within ovules are embryo sacs female gametophyte = embryo sac

egg-producing structure

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Fertilization in Angiosperms

When pollen grain lands on stigma it begins to grow pollen tube.

Nucleus within pollen grain divides and forms 2 sperm nuclei

Pollen tube contains tube nucleus and 2 sperm nuclei

Pollen tube grows into style and eventually reaches ovary and enters ovule

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Fertilization in Angiosperms

Inside embryo sac, two fertilizations occur One sperm nuclei fuses with egg nucleus

to produce diploid zygote – grows into plant embryo

Second sperm nucleus fuses with two polar nuclei to form triploid (3N) cell – food tissue known as endosperm.

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stigma

pollen tube

ovary

ovule micropyle

zygote (2n)

endosperm (3n)

embryo sac

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Fertilization (recap)

Pollination pollen released from anthers is carried by wind

or animals to land on stigma pollen grain produces a pollen tube

pollen tube grows down style into ovary & discharges 2 sperm into the embryo sac

1 sperm fertilizes egg = zygote zygote develops into embryo

ovule develops into a seed ovary develops into a fruit containing

1 or more seeds

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Pollination “syndromes”Generalist HummingbirdButterfly

Hawk mothBee

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Self-pollinationWhy?

Guarantees seeds (no need for mates or pollinators)

May save resources if flowers are smaller

Maximizes transmission of own genes

Why not?Inbreeding depression - short-term cost

Loss of genetic diversity - long-term cost

An evolutionary dead end? - very long-term cost

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Preventing self-pollination

Various mechanismsstamens & carpels may mature at different times arranged so that animal pollinator won’t transfer

pollen from anthers to stigma of same flowerbiochemical self-incompatibility = block pollen

tube growth

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Fertilization takes place within the flower.

Male gametophytes, or pollen grains, are produced in the anthers. – male spores produced in

anthers by meiosis– each spore divides by

mitosis to form twohaploid cells

– two cells form asingle pollen grain

pollen grain

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four female spores produced in ovule by meiosis

one spore develops into female gametophyte

female gametophyte contains seven cells one cell has two nuclei, or polar nuclei one cell will develop into an egg

One female gametophyte can form in each ovule of a flower’s ovary.

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Pollination occurs when a pollen grain lands on a stigma.

pollen tubepollen tube

spermsperm

stigmastigma

– one cell from pollen grain forms pollen tube– other cell forms two sperm that travel down tube

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Flowering plants go through the process of double fertilization.

femalegametophyte

ovule

egg

sperm

polar nuclei

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endosperm

seed coat

embryo

– one sperm fertilizes the egg

– other sperm unites with polar nuclei, forming endosperm

– endosperm provides food supply for embryo

Flowering plants go through the process of double fertilization.

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Each ovule becomes a seed.

The surrounding ovary grows into a fruit.

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Fertilization in flowering plants

Double fertilization 2 sperm from pollen

1 sperm fertilizes egg = diploid zygote

1 sperm fuses with 2 polar nuclei to form 3n endosperm

endosperm = food tissue in seed

coconut milk grains

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Fertilization in flowering plants

Development of the new sporophyte

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Plant embryo

endosperm

cotyledons

embryo

seed coat

ovary wall

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Fruit Fruit is a mature ovary

seeds develop from ovules wall of ovary thickens to form fruit fruits protect dormant seeds &

aid in their dispersal

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Seed dispersal

• Why disperse?

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Seed dispersal

• Why disperse?

• sample more (better?) sites for germination/growth

• avoid predation or disease

• avoid competition

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Seed dispersal

Plants produce enormous numbers of seeds to compensate for low survival rate

a lot of genetic variation for natural selection to screen

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Seeds and Seed Germination

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Seed structure

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Seed Dormancy

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Dormancy

Metabolism falls Number of organelles per cell falls Dehydration – water content falls Vacuoles in cells deflate Food reserves become dense

crystalline bodies

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Maintaining dormancy

Physical barriers The seed coat (testa) is waxy = waterproof and impermeable to oxygen

Physical state – dehydrated Chemical inhibitors present e.g. salts,

mustard oils, organic acids, alkaloids Growth promoters absent

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Seed viability Viability: When a seed is capable of

germinating after all the necessary environmental conditions are met.

Average life span of a seed 10 to 15 years. Some are very short-lived e.g. willow (< 1

week) Some are very long-lived e.g. mimosa 221

years Conditions are very important for longevity Cold, dry, anaerobic conditions These are the conditions which are

maintained in seed banks

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Germination: The breaking of dormancy

The growth of the embryo and its penetration of the seed coat

Break down of barriersAbrasion of seed coat

(soil particles)Decomposition of seed coat (soil microbes, gut

enzymes)Cracking of seed coat

(fire)Change in physical state - rehydration

Destruction and dilution of inhibitors

Light, temperature, waterProduction of growth

promoters

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Seeds begin to grow when environmental conditions are favorable. Seed dormancy is a state in which the

embryo has stopped growing.

– Dormancy may end when conditions

are favorable.– While dormant,

embryo can withstand extreme

conditions.

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Seed Germination

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Germination begins the growth of an embryo into a seedling.

– water causes seed to swell and crack coat– embryonic root, radicle, is first to emerge– water activates enzymes that help send sugars to

embryo

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Germination begins the growth of an embryo into a seedling. water causes seed to swell and crack coat embryonic root, radicle, is first to emerge water activates enzymes that help send

sugars to embryo

– embryonic shoot, plumule, emerges next

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Germination begins the growth of an embryo into a seedling. water causes seed to swell and crack coat embryonic root, radicle, is first to emerge water activates enzymes that help send

sugars to embryo embryonic shoot, plumule, emerges next

– leaves emerge last

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Once photosynthesis begins, the plant is called a seedling.

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GerminationSTAGE EVENTS

PREGERMINATION (a) Rehydration – imbibition of water.(b) RNA & protein synthesis stimulated.(c) Increased metabolism – increased respiration.(d) Hydrolysis (digestion) of food reserves by

enzymes.(e) Changes in cell ultra structure.(f) Induction of cell division & cell growth.

GERMINATION (a) Rupture of seed coat.(b) Emergence of seedling, usually radicle first.

POST GERMINATION (a) Controlled growth of root and shoot axis.(b) Controlled transport of materials from food stores

to growing axis.(c) Senescence (aging) of food storage tissues.

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Any Questions??

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Aaaaah…Structure-Function

yet again!