PLANT REPRODUCTION
AND GROWTH
CHAPTER 34
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
Runner
Rhizome
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.
ANGIOSPERM REPRODUCTION
Female
gametophyte
(embryo sac)
is inside base
of flower
Male
gametophytes
(pollen grains)
are on these
filaments
Parent
sporophyte tissue
(main plant body
and outer whorls
of flowers)
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
Anther
Stamen
Stigma
Carpel
Style
Ovary
(b)
(a)
(c)
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
Pollen sacs
Pollen grains (n)
1
Anther
3
2
Meiosis
Mitosis
Microspore
Microspores (n)
mother cell (2n)
Tube cell
nucleus
Generative
cell
Stamen
Carpel
Ovary
Ovule
Megaspore
mother cell (2n)
Megaspores (n)
Surviving
megaspore
Antipodals
4
2
3
1
Meiosis
Polar
nuclei
Mitosis
Synergids
Degenerated
megaspores
Egg
cell
8-nucleate embryo sac (n)
GAMETES COMBINE WITHIN THE
FLOWER
• Pollination
is the
process by
which
pollen is
transferred
from the
anther to
the stigma.
Pollen grain
Generative
Tube cell
cell
Stigma
Tube cell
Sperm cells
Style
Tube cell nucleus
Carpel
Ovary
Embryo
sac
Pollen tube
Growth of
Ovule
pollen tube
1 Pollination
2
3
Endosperm
(3n)
Antipodals
Zygote
(2n)
Polar nuclei
Egg cell
Synergids
4 Release of sperm cells
5 Double fertilization
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.
DEVELOPMENT IN AN ANGIOSPERM
EMBRYO
Polar
nuclei
1
Triploid endosperm
mother cell
2
3
4
Endosperm (3n)
Suspensor
Egg
(n)
Basal cell
Pollen
tube
Micropyle
Zygote
(2n)
First cell
division
Sperm (n)
Hypocotyl
Shoot apical
meristem
Procambium
Cotyledons
Ground
meristem
Cotyledon
Embryo
Protoderm
Endosperm
7
Cotyledons Root apex (radicle)
8
Root apical
meristem
6
5
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
MONOCOT
First leaf
First
leaf
Cotyledon
Withered
cotyledons
Hypocotyl
Coleoptile
Cotyledon
Radicle
Seed
coat
Primary
roots
Secondary
roots
Primary
root
Adventitious
root
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
1
2
Light
Charles and Francis Darwin found that a young grass
seedling normally bent towards the light.
Light
If the tip of the seedling was covered with a light proof
cap, the seedling did not bend toward the light.
4
3
Light
When the tip of the seedling was covered with a
transparent cap, the bending did occur.
Light
When the Darwins placed a lightproof collar below the
tip, the seedling bent toward the light.
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.