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GEORGE B. JOHNSON
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PowerPoint® Lectures prepared by Johnny El-Rady
27 Plant Reproductionand Growth
Essentials of
The Living World
First Edition
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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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RunnersLong, slender stems that grow along the soil surface
RhizomesUnderground horizontal stems that create a network underground
Vegetative reproduction occurs when new individuals are cloned from parts of the parent
Fig. 27.1
“Maternity plant”
SuckersSprouts produced by roots give rise to new plants
Adventitious plantletsNew plants arise from notches along the leaves
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Sexual reproduction in plants is characterized by an alteration of generations
Diploid sporophyte haploid gametophyte
Male gametophytes are pollen grainsDevelop from microspores
Female gametophyte is the embryo sacDevelops 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)
Dioecious plantsContain flowers that produce only ovules or only pollen
Monoecious plantsContain male and female parts in separate flowers, but in the same plant
Often flowers contain both parts, but there are exceptions
Fig. 27.17a
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Pollen grains develop from microspores formed in pollen sacs located in the antherEggs develop in ovules, each of which contains a megaspore mother cell
Fig. 20.2Fig. 27.2
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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
Pollination
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In angiosperms, pollen is carried from flower to flower by insects and other animals
These pollinators are drawn to the flower’s nectar
In certain angiosperms and all gymnosperms, pollen is wind-blown and reaches the stigma passively
Wind-pollinated plants grow in dense strands
Fig. 27.3
Yellow flowers attract bees
Long proboscis gets deep nectar supply
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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
Fertilization
One fertilizes the egg cell to form the zygote (2n)
The other fuses with two polar nuclei to form the endosperm (3n)
Double fertilization
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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
Polarnuclei
MicropyleSperm
Pollentube
Egg(n)
(n)
Triploid endospermmother cell
Zygote(2n)
First celldivision
Endosperm
Suspensor
Basalcell
(3n)
Globularproembryo Cotyledon
Groundmeristem
Procambium
Cotyledons
Protoderm
Root apex (radicle)
Endosperm
Shootapicalmeristem
Hypocotyl
Root apicalmeristem
Cotyledons
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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
Mangroves
Coconuts
By wind By water By attaching to animals
Fig. 27.5e
Dandelion
Fig. 27.5f
Burgrass
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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
Dicot: Soybean
Cotyledons emerge from the underground
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
Fig. 27.10
He called it auxin (from the Gr. word, “to increase”)
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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
Fig. 27.11
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Synthetic auxins are routinely used to control weeds
When treated, the weeds literally “grow to death”
2,4-dichlorophenoxyaceticacidKnown as 2,4-D
Affects only broad-leaved dicots
2,4,5-trichlorophenoxyaceticacidKnown 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
Also hasten seed germination
A large class of over 100 hormones
Play major role in stem elongation
Promote elongation between the node regions
Fig. 27.12
Defective in gibberellin production
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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
Fig. 27.14
Accelerates abscission of leaves or fruits damaged by various stress agents
Holly twig
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27.7 Other Plant Hormones
Abscisic Acid (ABA)
Fig. 27.15
May cause synthesis of ethylene
Plays a role in the dormancy of seeds
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 productionLong-day plants
Initiate flowers when nights become shorter than a certain length
Short-day plantsInitiate flowers when nights become longer than a certain length
Day-neutral plantsProduce 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)
Fig. 27.17
To this day, the existence of a flowering hormone remains strictly hypothetical
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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
Dormancy
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27.9 Tropisms
Tropisms are directional and irreversible growth responses of plants to external stimuli
PhototropismGrowth toward sources of light
GravitropismGrowth in response to gravity
Stems grow upward and roots downward
ThigmotropismGrowth in response to touch
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Fig. 27.18 Tropism guides plant growth
Phototropism
Gravitropism
Thigmotropism