Download The Living World - Chapter 20 - McGraw Hill Higher Education

Essentials of
The Living World
First Edition
GEORGE B. JOHNSON
27
Plant Reproduction
and Growth
PowerPoint® Lectures prepared by Johnny El-Rady
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27.1 Angiosperm Reproduction
In asexual reproduction, an individual inherits all of
its chromosomes from a single parent
Offspring and parent are genetically identical
In a stable environment, asexual reproduction is
more advantageous than sexual reproduction
It has a lower investment of energy
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Vegetative reproduction occurs when new
individuals are cloned from parts of the parent
Runners
Long, slender stems that grow along the soil surface
Rhizomes
Underground horizontal stems that create a network
underground
“Maternity plant”
Suckers
Sprouts produced by
roots give rise to new
plants
Adventitious plantlets
New plants arise from
notches along the leaves
Fig. 27.1
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Sexual reproduction in plants is characterized by an
alteration of generations
Diploid sporophyte  haploid gametophyte
Male gametophytes are pollen grains
Develop from microspores
Female gametophyte is the embryo sac
Develops from a megaspore
Angiosperms have different structures for producing
male and female gametes
These are not permanent parts of the adult
individual
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Flowers contain male parts (stamens) and female
parts (carpels)
Often flowers contain both parts, but there are
exceptions
Dioecious plants
Contain flowers that
produce only ovules
or only pollen
Monoecious plants
Contain male and
female parts in
separate flowers, but
in the same plant
Fig. 27.17a
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Pollen grains develop from microspores formed in pollen sacs
located in the anther
Eggs develop in ovules, each of which contains a megaspore
mother cell
Fig. 27.2
20.2
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Pollination
The process by which pollen is transferred to stigma
Self-pollination occurs when a flower’s anther
pollinates the same flower’s stigma
This can lead to self-fertilization
Other plants are adapted to outcrossing
Crossing between two different plants
Some plants exhibit self-incompatibility
Genetic relatedness blocks flower fertilization
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Yellow flowers
attract bees
In angiosperms, pollen
is carried from flower to
flower by insects and
Fig. 27.3
other animals
These pollinators are
drawn to the flower’s
nectar
Long proboscis gets
deep nectar supply
In certain angiosperms and all gymnosperms, pollen
is wind-blown and reaches the stigma passively
Wind-pollinated plants grow in dense strands
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Fertilization
Pollen adheres to stigma and begins to grow a
pollen tube
Pollen tube pierces the style and eventually
reaches the ovule
Two sperm cells are released
One fertilizes the egg
cell to form the zygote
(2n)
Double
fertilization
The other fuses with
two polar nuclei to form
the endosperm (3n)
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27.2 Seeds
Development is the entire series of events that occur
between fertilization and maturity
The first stage is the formation of the embryo
Early in development, the angiosperm embryo stops
developing and becomes dormant because of drying
Outermost covering of ovule develops into seed coat
enclosing dormant embryo and a stored food source
Most of embryo’s metabolic activities cease
Germination is the resumption of metabolic activities
leading to growth of the mature plant
Occurs when conditions are favorable for plant’s survival
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Fig. 27.4 Development in an angiosperm embryo
Polar
nuclei
Egg
(n)
Micropyle
Globular
proembryo
Triploid endosperm
mother cell
Endosperm (3n)
Basal
cell
Pollen
tube Zygote
(2n)
Sperm
(n)
Cotyledon
Procambium
Ground
meristem
Protoderm
Root apex (radicle)
First cell Suspensor
division
Shoot
apical
meristem
Hypocotyl
Cotyledons
Endosperm
Root apical
meristem
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Cotyledons
27.3 Fruit
During seed formation, the flower ovary begins to
develop into fruit
Fruits can be fleshy or dry and hard
There are three main kinds of fleshy fruit
Berries
Drupes
Pomes
Fig. 27.5
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Fleshy fruit are normally dispersed by animals
Animals eat the fruit and excrete the seeds as
solid wastes
Dry fruits are dispersed by several mechanisms
By wind
By water
Mangroves
Coconuts
By attaching
to animals
Burgrass
Dandelion
Fig. 27.5e
Fig. 27.5f
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27.4 Germination
Germination is the resumption of a seed’s growth
and reproduction
It is triggered by water
The seed coat ruptures and the plant begins to send
out roots and shoots
Oxygen is required for active growth
Endosperm or cotyledons provide the food source
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Fig. 27.6 Development of angiosperms
Cotyledons emerge
from the underground
Dicot: Soybean
Monocot: Corn
Cotyledon
stays
underground
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27.5 Plant Hormones
Differentiation in plants,
unlike that in animals,
is largely reversible
In the 1950s, F.C.
Steward was able to
regenerate a fertile
carrot plant from bits
of phloem tissue
Fig. 27.7
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Following germination, further plant development
depends on the activity of meristematic tissues
And the interaction with the environment
Fig. 27.8 Stages of plant differentiation
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Differentiation results from the activation or
suppression of key genes
This gene expression is controlled by hormones
Plant hormones are produced in non-specialized
tissues
Five major types of hormones
Auxins
Gibberellins
Cytokinins
Ethylene
Abscisic acid
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TABLE 27.1
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TABLE 27.1
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27.6 Auxin
Charles Darwin and his son Francis published a
book called The Power of Movement in Plants (1881)
In it, they describe the phenomenon of phototropism
Growing plants bending toward light
The Darwins concluded that, in response to light, an
“influence” arises at the tip of the shoot
It is then transmitted downward causing the shoot
to bend
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Fig. 27.9 The Darwins’ experiment with phototropism
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In 1926, Frits Went identified the hormone involved
in phototropism
He called it auxin (from the Gr. word, “to increase”)
Fig. 27.10
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Phototropism can be explained as such
Auxin contents on the
two sides of shoot differ
The side that is in the
shade has more auxin
Cells elongate
more than those on
the lighted side
Auxin appears to act by
increasing the stretchability
of the plant cell wall
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Fig. 27.11
Synthetic auxins are routinely used to control weeds
When treated, the weeds literally “grow to death”
2,4-dichlorophenoxyaceticacid
Known as 2,4-D
Affects only broad-leaved dicots
2,4,5-trichlorophenoxyaceticacid
Known as 2,4,5-T
Kills woody seedlings and weeds
Notorious as the Agent Orange of the Vietnam War
Easily contaminated with dioxin
An endocrine disruptor
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27.7 Other Plant Hormones
Gibberellins
A large class of over
100 hormones
Play major role in
stem elongation
Defective in
gibberellin
production
Promote elongation
between the node
regions
Also hasten seed
germination
Fig. 27.12
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27.7 Other Plant Hormones
Cytokinins
Stimulate cell division
Determine the course
of differentiation
Promote growth of
lateral buds
Inhibit formation of
lateral roots
Fig. 27.13
Apical meristem
intact
Auxin inhibits
lateral buds
Cytokinins stimulate
lateral buds
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27.7 Other Plant Hormones
Ethylene
Gas that hastens fruit ripening
Holly twig
Accelerates
abscission of
leaves or fruits
damaged by
various stress
agents
Fig. 27.14
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27.7 Other Plant Hormones
Abscisic Acid (ABA)
May cause synthesis
of ethylene
Plays a role in the
dormancy of seeds
Fig. 27.15
May also function in
transpiration
Opening/closing of
stomata
ABA causes
efflux of K+ out
of guard cells
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27.8 Photoperiodism and Dormancy
Photoperiodism is the physiological response of
organisms to changes in the length of day and night
Angiosperm flower production
Long-day plants
Initiate flowers when nights become shorter than a
certain length
Short-day plants
Initiate flowers when nights become longer than a
certain length
Day-neutral plants
Produce flowers without regard to day length
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Fig. 27.16 How photoperiodism works in plants
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Plants contain a pigment called phytochrome
It exists in two forms converted by darkness
Pr (inactive)
Pfr (active)
To this day,
the existence
of a flowering
hormone
remains
strictly
hypothetical
Fig. 27.17
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Dormancy
Plants respond to their external environment largely
by changes in growth rate
When conditions are not favorable, they become
dormant
They stop growing altogether
In temperate regions, dormancy is generally
associated with winter
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27.9 Tropisms
Tropisms are directional and irreversible growth
responses of plants to external stimuli
Phototropism
Growth toward sources of light
Gravitropism
Growth in response to gravity
Stems grow upward and roots downward
Thigmotropism
Growth in response to touch
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Phototropism
Fig. 27.18 Tropism
guides plant growth
Gravitropism
Thigmotropism
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