The Living World
Fifth Edition
George B. Johnson
Jonathan B. Losos
Chapter 24
Plant Reproduction and Growth
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
24.1 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 lower investment of energy than
sexual reproduction
24.1 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
Figure 24.1 Vegetative reproduction
24.1 Angiosperm Reproduction
• Sexual reproduction in plants involves an
alternation of generations
 diploid sporophyte generation gives rise to a haploid
gametophyte generation
• the male gametophytes are pollen grains that come from
microspores
• the female gametophyte is the embryo sac, which develops
from a megaspore
• these gametophytes are produced in separate, specialized
structures of the angiosperm flower
– but both usually occur together in the same flower
– they are produced seasonally
24.1 Angiosperm Reproduction
• 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
 plants that contain imperfect flowers that produce only
ovules or only pollen are known as dioecious
 plants that contain imperfect flowers of both male and
female on the same plant are called monoecious
24.1 Angiosperm Reproduction
• When an anther is cut in half, one would
see pollen sacs, containing microspores
 each microspore 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 tube cell nucleus forms the pollen tube
• the generative cell will later divide to form two
sperm cells
24.1 Angiosperm Reproduction
• Pollination is the process by which pollen
is transferred from the anther to the stigma
 if pollen from a flower’s anther pollinates the
same flower’s stigma, then self-fertilization
has occurred
 some plants cannot self-pollinate because of
self-incompatibility
• the pollen and the stigma recognize each other as
being genetically related and block fertilization
24.1 Angiosperm Reproduction
• 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) or deceived into doing it
 coevolution has occurred between plants and
pollinators
– plants may be colored or shaped in ways that attract pollinators
– for pollination by animals to be effective, a particular insect or
animal must visit plant individuals of the same species
Figure 24.3 Insect pollination
24.1 Angiosperm Reproduction
• 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
 the flowers of the wind-pollinated
angiosperms are small, green, and odorless
with inconspicuous or absent petals
24.1 Angiosperm Reproduction
• Eggs develop in the ovules of the
angiosperm flower, which form the base of
the carpel
 each ovule contains a megaspore mother cell
• each megaspore mother cell undergoes meiosis to
produce four haploid megaspores
• 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
24.1 Angiosperm Reproduction
• 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 goes on to
form endosperm
• this process of using two sperm cells in fertilization is called
double fertilization
Figure 24.2 Formation of pollen
and egg
24.2 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 the cells, the embryo
• early in the development of the embryo, the embryo stops
developing and becomes dormant as a result of drying
• this arrestment of development is usually at a point soon
after apical meristems and the seed leaves (called
cotyledons) have developed
24.2 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
Figure 24.4 Development in an
angiosperm embryo
24.3 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
 fruits with fleshy covering are normally dispersed by
bird and other vertebrates
• the animals carry seeds from place to place before excreting
them as solid waste
 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
Figure 24.5 Types of fruits and
common modes of dispersion
24.4 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
 the coleoptile emerges from underground in
in monocots
Figure 24.6 Development of
angiosperms
24.5 Plant Hormones
• Differentiation, the formation of specialized
tissues, is largely reversible in plants
 some differentiated plant tissue are capable of
expressing their hidden genetic information
when provided with suitable environmental
signals
 plant hormones control the expression of
some plant genes
• all hormones in plants are produced in tissues that
are not specialized for that purpose and carry out
many other functions
24.5 Plant Hormones
• F.C. Steward
successfully
regenerated plants
from isolated bits of
phloem tissue
Figure 24.7 How Steward regenerated
a plant from differentiated tissue
Figure 24.8 Stages of plant
differentiation
24.5 Plant Hormones
• At least five major kinds of hormones are
found in plants





auxin
gibberellins
cytokinins
ethylene
abscisic acid
24.6 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
Figure 24.9 The Darwins’
experiment with phototropism
24.6 Auxin
• Frits Went worked out how auxin controls
plant growth
 auxin causes the tissues on the side of the
seedling into which it flowed 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
Figure 24.10 How Went demonstrated
the effects of auxin on plant growth
Figure 24.11 Auxin causes cells to elongate
24.6 Auxin
• Synthetic auxins are used to control weeds
 the work by causing the plant to grow to death and
reducing ATP production
 2,4-D weedkiller affects broadleaf dicots
 a related herbicide, 2,4,5-T, (also known as Agent
Orange) kills woody seedlings and weeds
• this product is easily contaminated with dioxin during the
manufacturing process
• dioxin is an endocrine disruptor, a chemical that interferes
with human development
24.7 Other Plant Hormones
• Gibberellins are synthesized in the apical
portions of roots and shoots and affect
stem elongation
• Cytokinins stimulate cell division in plants
and determine the course of differentiation
• Ethylene, when applied to fruit, hastens
ripening
Plant Hormones
Figure 24.12 The effect of a
gibberellin
Figure 24.13 Cytokinins stimulate lateral
bud growth in the absence of auxin
24.7 Other Plant Hormones
• Abscisic acid appears to stimulate
ethylene synthesis, which promotes
senescence and abscission
 it may also function in transpiration by
stimulating the transport of potassium ions out
of the guard cell
Figure 24.14 The effects of ethylene
Figure 24.15 Effects of abscisic acid
24.8 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
Figure 24.16 How photoperiodism
works in plants
24.8 Photoperiodism and
Dormancy
• Plants contain a pigment called
phytocrome that influences flowering
 this pigment exists in two interconvertible
forms Pr (inactive) and Pfr (active)
 in short-day plants, the presence of Pfr
suppresses flowering
• the amount of Pfr steadily declines in darkness
• when the period of darkness is long enough, the
suppression ceases and the flowering response is
triggered
24.8 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
24.9 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
Figure 24.17 Tropism guides plant growth
Inquiry & Analysis
• What is the effect of auxin
on the rate of pea root
elongation over the range
of concentrations
studied?
• Is the inhibition of root
elongation by high
concentrations of auxin
accompanied by
corresponding increases
in the concentration of
ethylene?
Graph of Effects of Auxin on Root
Elongation