Download 18 PLANT REPRODUCTION AND GROWTH

24 PLANT REPRODUCTION AND GROWTH
CHAPTER OUTLINE
LEARNING OBJECTIVES
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Be able to characterize the flowering plant life cycle.
Diagram the basic structure of a flower and indicate the location of its four whorls.
Describe the steps leading to pollination and fertilization.
Define “double fertilization.”
Characterize the nature and function of a seed.
Discuss the purpose and design of fruit.
Describe the process of germination.
Explain how the growth of the plant body is adjusted by hormones.
Define phototropism and tell what hormone is responsible for this response.
Discuss the functions of cytokinins, gibberellins, ethylene, and abscisic acid.
Explain how plant growth and development are related to photoperiod.
List the three tropisms and what causes them.
Flowering Plant Reproduction (p. 434)
24.1
Angiosperm Reproduction (p. 434; Figs. 24.1, 24.2, 24.3)
A. Asexual Reproduction
1. The forms of vegetative reproduction are runners, rhizomes, suckers, adventitious
plantelets.
B. Sexual Reproduction
1. Angiosperm life cycles involve alternation of generations, with the sporophyte as the
dominant generation and the gametophyte developing within the confines of the
sporophyte.
2. Both microgametophytes (male gametophytes) and megagametophytes (female
gametophytes) are housed within the same structure, the flower.
3. Flower production is seasonal and not a permanent feature of the mature sporophyte.
4. Pollen grains are the male gametophytes while embryo sacs are the female gametophytes.
5. Structure of the Flower
a. Flowers are arranged in whorls.
b. The outermost whorl, the calyx, is made up of the protective sepals.
c. The second whorl is the petals, which collectively are called the corolla, and
functions to attract pollinators.
d. The third whorl is the male androecium, made up of stamens and anthers; pollen
grains develop within the anthers.
e. The innermost whorl is the female gynoecium made up of carpels; each carpel is
composed of a stigma that collects pollen, a style, and an ovary that houses the
ovules in which embryo sacs develop.
6. Pollen Formation
a. Pollen grains develop from microspores that form in the pollen sacs.
b. Within the pollen sacs, the microspore mother cells undergo meiosis to form
microspores; microspores undergo mitosis to form pollen grains.
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7.
24.2
24.3
24.4
Pollination
a. Pollination is the process by which pollen arrives at the stigma.
b. Pollinators include various insects or birds that are attracted to the flowers by scents
and colors, and are often rewarded for their efforts by sugary nectar.
c. Coevolution of flowers and pollinators can be seen in a wide variety of plants and
animals.
d. Wind-pollinated flowers are not fragrant or colorful, and typically produce very large
amounts of pollen to ensure that at least some of it arrives at the female stigma.
8. Egg Formation
a. Eggs develop in the ovules of the angiosperm flower; within each ovule is a
megaspore mother cell.
b. Each megaspore mother cell undergoes meiosis to produce four haploid megaspores,
only one of which survives.
c. The lone megaspore undergoes mitotic divisions and produces 8 haploid nuclei; one
will function as the egg nuclei and two are polar nuclei.
9. Fertilization
a. In angiosperms, fertilization occurs in a unique process called double fertilization.
b. Once pollen grains reach the stigma, a pollen tube grows down through the style until
it reaches the ovule.
c. One of the cells of the pollen within the pollen tube becomes two sperm cells; one of
the sperm cells unites with the egg cell to produce the zygote, and the other sperm
cell fuses with two polar nuclei that were produced during meiosis of the egg cell.
d. The two polar nuclei and the sperm cell fuse and divide rapidly, becoming nutritious
endosperm.
Seeds (p. 437; Fig. 24.4)
A. The entire series of events that occur between fertilization and maturity is called development.
B. In many plants, embryo development is arrested soon after apical meristems and the first
leaves (cotyledons) are differentiated.
C. The integuments surrounding the embryo develop into an impervious seed coat that protects
the embryo throughout its period of dormancy.
D. Resumption of metabolic activity within the seed leads to germination.
Fruit (p. 438; Fig. 24.5)
A. In some plants, the ovary surrounding the ovule develops into fruit.
B. Fruits appear to have evolved to help in seed dispersal, since animals eat the fruit and defecate
or drop the seeds at some distance away from the parent plant.
Germination (p. 439; Fig. 24.6)
A. When the time is right for the embryo to resume its growth, the seed coat absorbs water, and
the embryo becomes metabolically active once again.
B. Oxygen is needed during germination for most seeds, although some seem able to germinate
in anoxic conditions.
Regulating Plant Growth (p. 440)
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Plant Hormones (p. 440; Figs. 24.7, 24.8; Table 24.1)
A. After germination, how rapidly a plant grows depends on its meristematic tissues.
B. As plants grow, cells differentiate.
C. But one critical difference remains between plant cells and animal cells: once animal cells
differentiate, they cannot be anything other than what they are in their differentiated form.
D. Plant cells, on the other hand, can be induced to form whole new plants.
E. Plant cells retain all the genetic information needed for each type of plant cell in the plant.
F. Plant growth is regulated by chemical messengers, known as hormones.
G. At least five major hormones have been found; others also exist but have yet to be chemically
isolated and described.
Auxin (p. 442; Figs. 24.9, 24.10, 24.11)
A. The hormone auxin is a primary growth hormone that is more abundant in shaded portions of
stems.
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24.7
B. Phototropism, or seeming to grow toward the light, is a commonly observed phenomenon in
many plants.
1. The shaded side of the stem elongates faster, causing the plant to lean toward the light.
C. Synthetic auxins are used as herbicides (such as 2, 4-D) because the plants can be induced to
literally growth themselves to death.
Other Plant Hormones (p. 444; Figs. 24.12, 24.13, 24.14, 24.15)
A. Gibberellins
1. Gibberellins are a third type of hormone that is made in the apical areas of stems and
roots.
2. Gibberellins appear to cause elongation of internode areas, and also affect flowering and
seed germination.
B. Cytokinins
1. Cytokinins also induce cell division.
2. Cytokinins are produced in the roots and activate the proteins required for mitosis.
C. Ethylene
1. Ethylene is a chemical produced during certain times of fruit ripening.
2. Ethylene also plays an important ecological role and increases in production when the
plant is stressed by a variety of factors.
D. Abscisic Acid
1. Abscisic acid stimulates leaf drop and suppresses growth during periods when the plant
must become dormant.
2. This hormone also probably induces the formation of winter buds.
Plant Responses to Environmental Stimuli (p. 446)
24.8
24.9
Photoperiodism and Dormancy (p. 446; Fig. 24.16)
A. Plants respond to environmental stimuli in a variety of ways.
B. Photoperiodism
1. Plants also respond to patterns of light and dark as the seasons progress, a phenomenon
referred to as photoperiodism.
2. Day length cues plants to flower.
3. Some plants flower when days are shorter, others as the days become longer.
4. Still other plants are day-neutral and produce flowers when conditions are favorable,
regardless of day length.
C. The Chemical Basis of Photoperiodism
1. One class of chemicals, the phytochromes, are pigment molecules that influence
flowering in plants.
D. Dormancy
1. When environmental conditions are unfavorable to growth, the capacity to become
dormant is critical to the survival of certain plants.
2. Annual plants overwinter as seeds.
Tropisms (p. 447; Fig. 24.17)
A. Tropisms are growth responses of plants to external stimuli and control patterns of growth and
appearance.
B. Gravitropism
1. Stems grow downward and shoots upward as a response to gravity, or gravitropism.
C. Thigmotropism
1. Thigmotropism occurs when plants respond to being touched
KEY TERMS
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Asexual reproduction (p. 434)
Pollen grains (p. 434)
Embryo (p. 434)
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pollination (p. 435) Pollination is the process by which pollen is placed on the stigma. It may arrive
there by wind or by animals, unless it is self-pollinated.
ovules (p. 436) Within the ovules, megasporangia produce haploid megaspores which develop into
embryo sacs containing the eggs.
double fertilization (p. 436) Double fertilization involves two sperms cells, one of which fertilizes the
egg cell, and the other of which fuses with the two haploid polar nuclei to become triploid endosperm.
germination (p. 437) This is the resumption of metabolic activities in the seed, often in response to
water.
phototropism (p. 442) A phenomenon in which plants bend toward the light.
auxin (p. 442) A plant hormone that regulates cell growth in plants.
gibberellins (p. 444) Gibberellins are important for stem elongation.
cytokinins (p. 444) These hormones stimulate cell division and determine the course of differentiation.
ethylene (p. 444) Ethylene is produced during fruit ripening.
abscisic acid (p. 445) This hormone stimulates leaves to age rapidly and fall, and plays a role in
causing dormancy.
photoperiodism (p. 446) Responses of plants are timed to changes in day length.
phytochromes (p. 447) The pigment, phytochrome, is involved in many plant growth responses,
especially those involving day length.
gravitropism (p. 447) This response to gravity causes stems to grow upward and roots downward.
thigmotropism (p. 447) Thigmotropism is the response of plants to touch.
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
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Prepare several examples of tropisms ahead of your lecture time. An easy way to show gravitropism is
to grow corn seedlings in the dark. The etiolated seedlings are white and cannot be growing in response
to light. A house plant positioned in the same direction in reference to a lateral light source for several
weeks shows phototropism, as its leaves and stems are oriented toward light. A mimosa, venus flytrap,
or similar plant is a good example of thigmotropism.
If your class is small enough, bring in different types of flowers for students to pull apart and locate
structures as you discuss them. Usually dissecting tools are not necessary, but paper towels under the
flowers aid in fast cleanup.
CRITICAL THINKING QUESTIONS
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Why would ethylene production increase rapidly when a plant is exposed to ozone, toxic chemicals,
temperature extremes, attacks by predators, or other stresses?
In what directions do roots and stems grow when there is no gravity, such as in a spaceship outside the
pull of earth?
In Chapter 14 of this text, genetic engineering in agriculture was discussed. Might it be feasible to
genetically engineer a plant to respond to touch (become thigmotropic) as a means to deter certain
flying insect predators?
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