1. End Show Slide 1 of 42 Copyright Pearson Prentice Hall Biology

2. End Show Slide 2 of 42 Copyright Pearson Prentice Hall 25–1 Hormones and Plant Growth

3. End Show 25–1 Hormones and Plant Growth Slide 3 of 42 Copyright Pearson Prentice Hall Patterns of Plant Growth Biologists have discovered that plant cells send signals to one another that indicate when to divide and when not to divide, and when to develop into a new kind of cell.

4. End Show 25–1 Hormones and Plant Growth Slide 4 of 42 Copyright Pearson Prentice Hall Patterns of Plant Growth One difference between growth in plants and animals is that most animals stop growing once they reach adulthood. In contrast, plants continue to grow new needles, add new wood, and produce cones or new flowers. The secrets of plant growth are found in meristems, regions of tissue that can produce cells that later develop into specialized tissues.

5. End Show 25–1 Hormones and Plant Growth Slide 5 of 42 Copyright Pearson Prentice Hall Patterns of Plant Growth Plants grow in response to environmental factors such as light, moisture, temperature, and gravity. Specific chemicals direct, control, and regulate plant growth.

6. End Show 25–1 Hormones and Plant Growth Slide 6 of 42 Copyright Pearson Prentice Hall Plant Hormones What are plant hormones?

7. End Show 25–1 Hormones and Plant Growth Slide 7 of 42 Copyright Pearson Prentice Hall Plant Hormones A hormone is a substance that is produced in one part of an organism and affects another part of the same individual.

8. End Show 25–1 Hormones and Plant Growth Slide 8 of 42 Copyright Pearson Prentice Hall Plant Hormones Plant hormones are chemical substances that control a plant's patterns of growth and development and its responses to environmental conditions.

9. End Show 25–1 Hormones and Plant Growth Slide 9 of 42 Copyright Pearson Prentice Hall Plant Hormones The portion of an organism affected by a particular hormone is known as its target cell or target tissue. Target cells Hormone producing cells Movement of hormone

10. End Show 25–1 Hormones and Plant Growth Slide 10 of 42 Copyright Pearson Prentice Hall Plant Hormones To respond to a hormone, the target cell must contain a receptor to which the hormone binds. If the receptor is present, the hormone can influence the target cell by: changing its metabolism affecting its growth rate activating the transcription of certain genes

11. End Show 25–1 Hormones and Plant Growth Slide 11 of 42 Copyright Pearson Prentice Hall Plant Hormones Cells that do not contain receptors are generally unaffected by hormones. Different kinds of cells may have different receptors for the same hormone. As a result, a single hormone may affect two different tissues in different ways. For example, a particular hormone may stimulate growth in stem tissues but inhibit growth in root tissues.

12. End Show 25–1 Hormones and Plant Growth Slide 12 of 42 Copyright Pearson Prentice Hall Plant Hormones How do auxins, cytokinins, gibberellins, and ethylene affect plant growth?

13. End Show 25–1 Hormones and Plant Growth Slide 13 of 42 Copyright Pearson Prentice Hall Auxins Charles Darwin and his son Francis carried out the experiment that led to the discovery of the first plant hormone. They described an experiment in which oat seedlings demonstrated a response known as phototropism—the tendency of a plant to grow toward a source of light.

14. End Show 25–1 Hormones and Plant Growth Slide 14 of 42 Copyright Pearson Prentice Hall Auxins In the experiment, they placed an opaque cap over the tip of one of the oat seedlings. This plant did not bend toward the light, even though the rest of the plant was uncovered.

15. End Show 25–1 Hormones and Plant Growth Slide 15 of 42 Copyright Pearson Prentice Hall Auxins However, if an opaque shield was placed a few centimeters below the tip, the plant would bend toward the light as if the shield were not there.

16. End Show 25–1 Hormones and Plant Growth Slide 16 of 42 Copyright Pearson Prentice Hall Auxins The Darwins suspected that the tip of each seedling produced substances that regulated cell growth. Forty years later, these substances were identified and named auxins.

17. End Show 25–1 Hormones and Plant Growth Slide 17 of 42 Copyright Pearson Prentice Hall Auxins Auxins are produced in the apical meristem and are transported downward into the rest of the plant. They stimulate cell elongation.

18. End Show 25–1 Hormones and Plant Growth Slide 18 of 42 Copyright Pearson Prentice Hall Auxins When light hits one side of the stem, the shaded part develops a higher concentration of auxins. This change in concentration stimulates cells on the dark side to elongate.

19. End Show 25–1 Hormones and Plant Growth Slide 19 of 42 Copyright Pearson Prentice Hall Auxins As a result, the stem bends away from the shaded side and toward the light. Recent experiments have shown that auxins migrate toward the shaded side of the stem.

20. End Show 25–1 Hormones and Plant Growth Slide 20 of 42 Copyright Pearson Prentice Hall Auxins Auxins and Gravitropism Auxins are also responsible for gravitropism—the response of a plant to the force of gravity.

21. End Show 25–1 Hormones and Plant Growth Slide 21 of 42 Copyright Pearson Prentice Hall Auxins Auxins build up on the lower sides of roots and stems. In stems, auxins stimulate cell elongation, helping turn the trunk upright.

22. End Show 25–1 Hormones and Plant Growth Slide 22 of 42 Copyright Pearson Prentice Hall Auxins In roots, their effects are exactly the opposite. There, auxins inhibit cell growth and elongation, causing the roots to grow downward.

23. End Show 25–1 Hormones and Plant Growth Slide 23 of 42 Copyright Pearson Prentice Hall Auxins Auxins also influence how roots grow around objects in the soil. If a growing root is forced sideways by an obstacle, auxins accumulate on the lower side of the root.

24. End Show 25–1 Hormones and Plant Growth Slide 24 of 42 Copyright Pearson Prentice Hall Auxins High concentrations of auxins inhibit the elongation of root cells. Uninhibited cells on the top elongate more than auxin-inhibited cells on the bottom and the root grows downward.

25. End Show 25–1 Hormones and Plant Growth Slide 25 of 42 Copyright Pearson Prentice Hall Auxins Auxins and Branching Auxins also regulate cell division in meristems.

26. End Show 25–1 Hormones and Plant Growth Slide 26 of 42 Copyright Pearson Prentice Hall Auxins As a stem grows in length, it produces lateral buds. A lateral bud is a meristematic area on the side of a stem that gives rise to side branches.

27. End Show 25–1 Hormones and Plant Growth Slide 27 of 42 Copyright Pearson Prentice Hall Auxins Most lateral buds do not start growing right away. The reason for this delay is that growth at the lateral buds is inhibited by auxins.

28. End Show 25–1 Hormones and Plant Growth Slide 28 of 42 Copyright Pearson Prentice Hall Auxins Because auxins move out from the apical meristem, the closer a bud is to the stem's tip, the more it is inhibited. This phenomenon is called apical dominance.

29. End Show 25–1 Hormones and Plant Growth Slide 29 of 42 Copyright Pearson Prentice Hall Auxins When the apical meristem is removed, the concentration of auxin is reduced and the side branches begin to grow more rapidly. Apical meristem removed

30. End Show 25–1 Hormones and Plant Growth Slide 30 of 42 Copyright Pearson Prentice Hall Auxins Auxinlike Weed Killers Chemists have produced compounds that mimic the effects of auxins. Since high concentrations of auxins inhibit growth, many of these are used as herbicides— compounds toxic to plants.

31. End Show 25–1 Hormones and Plant Growth Slide 31 of 42 Copyright Pearson Prentice Hall Cytokinins Cytokinins are plant hormones produced in growing roots and developing fruits and seeds. Cytokinins delay the aging of leaves and play important roles in early stages of plant growth.

32. End Show 25–1 Hormones and Plant Growth Slide 32 of 42 Copyright Pearson Prentice Hall Cytokinins In plants, cytokinins stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout.

33. End Show 25–1 Hormones and Plant Growth Slide 33 of 42 Copyright Pearson Prentice Hall Cytokinins Cytokinins and auxins often produce opposite effects. Auxins stimulate cell elongation. Cytokinins inhibit cell elongation and cause cells to grow thicker. Auxins inhibit the growth of lateral buds. Cytokinins stimulate lateral bud growth.

34. End Show 25–1 Hormones and Plant Growth Slide 34 of 42 Copyright Pearson Prentice Hall Cytokinins Recent experiments show that the rate of cell growth in most plants is determined by the ratio of the concentration of auxins to cytokinins. In growing plants, therefore, the relative concentrations of auxins, cytokinins and other hormones determine how the plant grows.

35. End Show 25–1 Hormones and Plant Growth Slide 35 of 42 Copyright Pearson Prentice Hall Gibberellins A gibberellin is a growth-promoting substance in plants. Gibberellins produce dramatic increases in size, particularly in stems and fruit. Gibberellins are also produced by seed tissue and are responsible for the rapid early growth of many plants.

36. End Show 25–1 Hormones and Plant Growth Slide 36 of 42 Copyright Pearson Prentice Hall Ethylene In response to auxins, fruit tissues release small amounts of the hormone ethylene. Ethylene is a plant hormone that causes fruits to ripen. Commercial producers of fruit sometimes use this hormone to control the ripening process.

37. End Show - or - Continue to: Click to Launch: Slide 37 of 42 Copyright Pearson Prentice Hall 25–1

38. End Show Slide 38 of 42 Copyright Pearson Prentice Hall 25–1 The tendency of a plant to grow toward a source of light is a.gravitropism. b.phototropism. c.meristematic growth. d.apical dominance.

39. End Show Slide 39 of 42 Copyright Pearson Prentice Hall 25–1 A plant part in which hormones are produced is a.the apical meristem. b.a target cell. c.a hormone receptor. d.xylem.

40. End Show Slide 40 of 42 Copyright Pearson Prentice Hall 25–1 If you snip off the tip of a stem, the a.plant grows tall and narrow. b.plant dies. c.side branches begin to grow more quickly. d.stem stops growing.

41. End Show Slide 41 of 42 Copyright Pearson Prentice Hall 25–1 Ethylene is a plant hormone that causes a.plant cells to grow longer. b.flowers to develop. c.fruit to ripen. d.roots to grow downward.

42. End Show Slide 42 of 42 Copyright Pearson Prentice Hall 25–1 The herbicides produced by chemists have a structure that is similar to a.auxins. b.gibberellins. c.cytokinins. d.ethylene.

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