1. Plant reproduction and growth
Chapter 34

2. 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.

3. 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.

4. Vegetative reproduction
Runner
Rhizome

5. 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.

6. Angiosperm Reproduction
Parent
sporophyte tissue
(main plant body
and outer whorls
of flowers)
Female
gametophyte
(embryo sac)
is inside base
of flower
Male
gametophytes
(pollen grains)
are on these
filaments

7. 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.

8. The structure of a flower
Anther
Stamen
Stigma
Style
Carpel
Ovary
(b)
(a)
(c)
b: © David Sieren/Visuals Unlimited; c: © Barbara Gerlach/Visuals Unlimited

9. 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.

10. 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.

11. Formation of pollen and egg
Pollen sacs
Pollen grains (n) 1
Anther 2 3
Meiosis
Mitosis
Microspore
mother cell (2n)
Microspores (n)
Tube cell
nucleus
Generative
cell
Stamen
Carpel
Ovary
Megaspore
mother cell (2n)
Megaspores (n)
Surviving
megaspore
Antipodals
Ovule 2 4 1 3
Polar
nuclei
Meiosis
Mitosis
Synergids
Egg
cell
Degenerated
megaspores
8-nucleate embryo sac (n)

12. Gametes Combine Within the Flower
Pollination is the process by which pollen is transferred from the anther to the stigma. 2 3
pollen tube
Tube cell
Sperm cells
Tube cell nucleus
Pollen tube
Growth of
Pollination
Pollen grain
Stigma
Style
Ovary
Carpel
Embryo
sac 1
Tube
cell
Ovule
Generative 4 5
Antipodals
Egg cell
Synergids
Polar nuclei
Release of sperm cells
Double fertilization
Zygote
(2n)
Endosperm
(3n)

13. 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.

14. 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.

15. 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.

16. 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.

17. Development in an angiosperm embryo1 2 3 4
Polar
nuclei
Triploid endosperm
mother cell
Endosperm (3n)
Suspensor
Egg
(n)
Basal cell
Pollen
tube
Zygote
(2n)
Micropyle
First cell
division
Sperm (n)
Hypocotyl
Shoot apical
meristem
Procambium
Cotyledons
Cotyledon
Ground
meristem
Embryo
Protoderm
Endosperm 7 6 5
Cotyledons
Root apex (radicle) 8
Root apical
meristem

18. 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.

19. 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.

20. Types of fruits and common modes of dispersion

21. 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.

22. Development of angiosperms
DICOT
MONOCOT
First leaf
First
leaf
Cotyledon
Withered
cotyledons
Hypocotyl
Coleoptile
Adventitious
root
Cotyledon
Radicle
Seed
coat
Primary
roots
Secondary
roots
Primary
root

23. 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.

24. 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.

25. Plant Hormones auxin gibberellins cytokinins ethylene abscisic acid
At least five major kinds of hormones are found in plants
auxin
gibberellins
cytokinins
ethylene
abscisic acid

26. 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.

27. THE DARWINS’ EXPERIMENT WITH PHOTOTROPISM1 2
Light
Light
Charles and Francis Darwin found that a young grass
seedling normally bent towards the light.
If the tip of the seedling was covered with a light proof
cap, the seedling did not bend toward the light. 3 4
Light
Light
When the tip of the seedling was covered with a
transparent cap, the bending did occur.
When the Darwins placed a lightproof collar below the
tip, the seedling bent toward the light.

28. Auxin Frits Went worked out how auxin controls plant growth.
Shaded side
of seedling
Light
Lighted side
Frits Went worked out how auxin controls plant growth.
Auxin causes the tissues on the side of the seedling into which it flows 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.

29. 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.

30. 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.

31. 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.