1. Plant Reproduction and Growth
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.

2. 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

3. 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

4. Figure 24.1 Vegetative reproduction

5. 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

6. 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

7. 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

8. 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

9. 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

10. Figure 24.3 Insect pollination

11. 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

12. 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

13. 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

14. Figure 24.2 Formation of pollen and egg

15. 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

16. 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

17. Figure 24.4 Development in an angiosperm embryo

18. 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

19. Figure 24.5 Types of fruits and common modes of dispersion

20. 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

21. Figure 24.6 Development of angiosperms

22. 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

23. 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

24. Figure 24.8 Stages of plant differentiation

25. 24.5 Plant Hormones
At least five major kinds of hormones are found in plants
auxin
gibberellins
cytokinins
ethylene
abscisic acid

26. 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

27. Figure 24.9 The Darwins’ experiment with phototropism

28. 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

29. Figure 24.10 How Went demonstrated the effects of auxin on plant growth

30. Figure 24.11 Auxin causes cells to elongate

31. 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

32. 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

33. Plant Hormones Figure 24.12 The effect of a gibberellin
Figure Cytokinins stimulate lateral bud growth in the absence of auxin

34. 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

35. Figure 24.14 The effects of ethylene

36. Figure 24.15 Effects of abscisic acid

37. 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

38. Figure 24.16 How photoperiodism works in plants

39. 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

40. 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

41. 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

42. Figure 24.17 Tropism guides plant growth

43. 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