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CHAPTER 34
PLANT REPRODUCTION AND
GROWTH
ANGIOSPERM REPRODUCTION
• Reproduction in flowering plants, the angiosperms, can be asexual or sexual.• Asexual reproduction is common in stable
environments.• This vegetative reproduction results when
new individuals are simply cloned from parts of the parent.
• Asexual reproduction allows individuals to reproduce with a lower investment of energy than sexual reproduction.
ANGIOSPERM REPRODUCTION
• There are many forms of vegetative reproduction: • Runners are slender stems that grow along the
soil surface.• Rhizomes are underground horizontal stems
that create a network, giving rise to new shoots.• Suckers are produced by roots and give rise to
new plants.• Adventitious plantlets arise from
meristematic tissue located in the notches of leaves.
VEGETATIVE REPRODUCTION
RunnerRhizome
ANGIOSPERM REPRODUCTION
• Sexual reproduction in plants involves an alternation of generations.• The diploid sporophyte generation gives rise to
a haploid gametophyte generation, which is enclosed within the tissues of the sporophyte.
Parentsporophyte tissue(main plant bodyand outer whorlsof flowers)
Femalegametophyte(embryo sac)is inside baseof flower
Malegametophytes(pollen grains)are on thesefilaments
ANGIOSPERM REPRODUCTION
STRUCTURE OF THE FLOWER
• Most flowers contain male and female parts.• The male parts are called stamens.• The female part is called the carpel.
• Flowers that contain only male or only female parts are known as imperfect.
THE STRUCTURE OF A FLOWER
Stigma
Style
Ovary
Anther
Stamen
(a)
Carpel
(c)
(b)
b: © David Sieren/Visuals Unlimited; c: © Barbara Gerlach/Visuals Unlimited
STRUCTURE OF THE FLOWER
• Pollen formation occurs in the anthers.• Inside the anthers are pollen sacs, which
contain microspore mother cells.• Each microspore mother cell undergoes meiosis
to form four haploid microspores.• These microspores then undergo mitosis to
form pollen grains that contain a generative cell and a tube cell nucleus.• The generative cell will later divide to form
two sperm cells.
STRUCTURE OF THE FLOWER
• Egg formation occurs in the ovary, which contains the ovule.– Each ovule contains a megaspore
mother cell that undergoes meiosis.• Only one megaspore survives to undergo
repeated mitotic divisions that produce eight haploid nuclei.
– These nuclei are enclosed in an embryo sac, where the nuclei are precisely arranged.
Formation of pollen and egg
1
2 3
12
34
Pollen grains (n)
Tube cellnucleus
Mitosis
Microspores (n)Microsporemother cell (2n)
Generativecell
Meiosis
Pollen sacs
Anther
Stamen
Carpel
Ovary
Antipodals
Eggcell
8-nucleate embryo sac (n)Degeneratedmegaspores
Survivingmegaspore
Megaspores (n)Megasporemother cell (2n)
Ovule
Meiosis
Polarnuclei
Mitosis
Synergids
2 3
pollen tube
Tube cell
Sperm cells
Tube cell nucleusPollen tube
Growth of
Pollination
Pollen grain
Stigma
Style
Ovary
Carpel
Embryosac
1
Tubecell
Ovule
Generativecell
4 5
Antipodals
Egg cell
Synergids
Polar nuclei
Release of sperm cells Double fertilization
Zygote(2n)
Endosperm(3n)
GAMETES COMBINE WITHIN THE FLOWER
• Pollination is the process by which pollen is transferred from the anther to the stigma.
FERTILIZATION
• Pollen grains adhere to the sticky surface of the stigma and begin to grow a pollen tube.
• The pollen tube pierces the style and grows until it reaches the ovule in the ovary.
• When the pollen tube reaches the entry to the embryo sac, it releases two sperm cells.• One sperm fertilizes the egg while the other sperm
fuses with the polar nuclei to form endosperm.• This process of using two sperm cells in fertilization
is called double fertilization.
GAMETES COMBINE WITHIN THE FLOWER
• Many angiosperms use animals to carry pollen grains from flower to flower.• These pollinators may
be rewarded for their efforts with food (e.g., nectar).
• Coevolution has occurred between plants and pollinators.
GAMETES COMBINE WITHIN THE FLOWER
• In some angiosperms and in all gymnosperms, pollen is dispersed by wind and reaches the stigmas passively.• The individuals of a given
plant species must grow where there is ample wind and grow relatively close together.
SEEDS
• Development is the entire series of events that occurs between fertilization and maturity.• The first stage of development is active cell
division to form an organized mass of cells, the embryo.
2 3 4
567
8 Root apicalmeristem
Root apex (radicle)
Endosperm
Basal cell
Suspensor
Endosperm (3n)
Embryo
Cotyledons
Protoderm
Cotyledon
Shoot apicalmeristem
Procambium
Groundmeristem
Hypocotyl
Cotyledons
Triploid endospermmother cell
First celldivision
Pollentube
Zygote(2n)
1Polarnuclei
Egg(n)
Micropyle
Sperm (n)
DEVELOPMENT IN AN ANGIOSPERM EMBRYO
SEEDS
• The integuments that form the outermost covering of the ovule develop into a seed coat.• This layer is relatively impermeable and
encloses the dormant embryo within the seed, together with a source of food.
• Germination cannot take place until water and oxygen reach the embryo.• This assures that the seed will germinate when
conditions are favorable for a plant’s survival.
FRUIT
• During seed formation, the flower ovary begins to develop into fruit.• Fruits form in many ways and exhibit a wide
array of modes of specialization.• Fleshy fruits are normally dispersed by birds
and other vertebrates.• Some fruits are dispersed by wind or by
attaching themselves to the fur of mammals or the feathers of birds.
• Some fruits are dispersed by water.
TYPES OF FRUITS AND COMMON MODES OF DISPERSION
GERMINATION
• When a seed encounters conditions suitable for its germination:• It first absorbs water.• Once the seed coat ruptures, aerobic
respiration begins.• The roots emerge first.• Cotyledons emerge, in dicots, from
underground along with the stem.
DEVELOPMENT OF ANGIOSPERMS
DICOT
First leaf
Cotyledon
Hypocotyl
Seedcoat
Primaryroots
Secondaryroots Primary
root
Radicle
CotyledonColeoptile
Witheredcotyledons
MONOCOT
Adventitiousroot
Firstleaf
GROWTH AND NUTRITION
• Plants require a number of nutrients.• Macronutrients are needed in large amounts.• Micronutrients are needed in trace amounts.• These nutrients are called essential because
the plant cannot manufacture them.
PLANT HORMONES
• Plant hormones control the expression of some plant genes.• Hormones in plants are produced in tissues that
are not specialized for that purpose and carry out many other functions.
PLANT HORMONES
• At least five major kinds of hormones are found in plants
• auxin• gibberellins• cytokinins• ethylene• abscisic acid
AUXIN
• Phototropism is the growth of plants toward light.• Charles Darwin and his son Francis performed
experiments that suggested that a substance caused the plant to bend if exposed to light.
• The substance was later identified to be auxin.
THE DARWINS’ EXPERIMENT WITH PHOTOTROPISM
4
1 2
3
LightLight
Charles and Francis Darwin found that a young grassseedling normally bent towards the light.
If the tip of the seedling was covered with a light proofcap, the seedling did not bend toward the light.
Light Light
When the tip of the seedling was covered with atransparent cap, the bending did occur.
When the Darwins placed a lightproof collar below thetip, the seedling bent toward the light.
AUXIN
• Frits Went worked out how auxin controls plant growth.• Auxin causes the tissues
on the side of the seedling into which it flows to grow more than those on the opposite side.
• The side of the plant in the shade has more auxin and divides more, causing the plant to bend towards the light.
Shaded sideof seedlingLight
Lighted sideof seedling
PHOTOPERIODISM AND DORMANCY
• Photoperiodism is a mechanism by which organisms measure seasonal changes in relative day and night length.• Plants’ flowering responses fall into three basic
categories in relation to day length:• Long-day plants flower when days become
longer in the summer.• Short-day plants flower when days become
shorter in the fall.• Day-neutral plants produce flowers without
regard to day length.
PHOTOPERIODISM AND DORMANCY
• Plants have the ability to stop growing altogether when conditions are not favorable.• This is called dormancy.• In temperate zones, dormancy is generally
associated with winter when low temperature and the freezing of water make it impossible for plant growth.
TROPISMS
• Tropisms are directional and irreversible growth responses to external stimuli• Gravitropism causes
stems to grow upward and roots to grow downward.
• Thigmotropism is the response of plants to touch.