1. CHAPTER 29
The Working Plant

6. Biology and Society: What’s in—or Not in—That Organic Tomato?
If you buy tomatoes labeled “organic” at a grocery store, what does that label mean?
In order to use the term organic on a food label,
Food producers must grow and produce their products according to strict guidelines.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

7. In the United States,
Over 2 million acres are dedicated to organic farming.

8. Figure 29.1

9. The benefits of organic farming are clear:
Fewer synthetic chemicals in the environment
Less risk of exposing farm workers and wildlife to potential toxins
However, an organic label is no guarantee of extra health benefits.

10. How Plants Acquire and Transport Nutrients
Nutrients a plant needs to build sugar via photosynthesis come from the air, water, and soil.

11. Figure 29.2

12. A plant uses the sugars, along with minerals, to construct all the other organic materials it needs.

13. Figure 29.3

14. Plant Nutrition An essential element
Is one the plant must obtain from its environment to complete its life cycle.

15. Macronutrients Micronutrients
Are those that a plant requires in relatively large amounts.
Micronutrients
Are those that a plant requires in extremely small amounts.

16. Macronutrients
Plants use macronutrients to produce organic compounds, such as proteins, nucleic acids, ATP, and chlorophyll.

17. There are eight micronutrients required by all plants.
A plant recycles the atoms of micronutrients over and over,
So it needs them only in minute quantities.

18. Nutrient Deficiencies
The quality of soil
Affects the growth of plants, and in turn, our own nutrition.

19. Figure 29.4

20. Nutrient deficiencies in plants can be diagnosed through the symptoms the deficiency produces.

21. Figure 29.5

22. From the Soil into the Roots
The roots of a plant
Absorb water and all other essential nutrients.

23. Root hairs
Provide a huge surface area available for absorption.

24. Figure 29.6

25. Water and solutes
Move from the soil through the epidermis and cortex and then into the xylem.

26. Many plants form mycorrhizae, symbiotic associations with fungi.
Increase the absorptive surface area of the roots.

27. Figure 29.7

28. The Roles of Bacteria in Nitrogen Nutrition
Most plants rely on bacteria to supply them with usable nitrogen in the form of ammonium.

29. Soil Bacteria and Nitrogen
Three types of soil bacteria play an essential role in supplying plants with nitrogen.

30. Figure 29.8

31. One type of bacteria carries out nitrogen fixation, which is
The conversion of atmospheric N2 into ammonium.
A process vital to plants.

32. Root Nodule Bacteria and Nitrogen
Plants called legumes have their own built-in source of ammonium.
Nitrogen-fixing bacteria live in swellings called root nodules.

33. Root nodules
Consist of plant root cells containing vesicles filled with nitrogen-fixing bacteria.

34. Figure 29.9

35. As a plant grows upward toward sunlight,
The Transport of Water
As a plant grows upward toward sunlight,
It needs an increasing supply of resources from the soil.
Plants transport xylem sap from the roots to the tips of the leaves.

36. Transpiration, the loss of water vapor from a plant,
The Ascent of Xylem Sap
Transpiration, the loss of water vapor from a plant,
Pulls xylem sap up the plant against gravity.
Occurs through the stomata of leaves.
Transpiration
Transport in Roots

37. Figure part 1

38. Figure part 2

39. Figure part 3

40. Transpiration relies on two properties of water:
Cohesion and adhesion
The ascent of xylem sap
Is called the transpiration-cohesion-tension mechanism.

41. The Regulation of Transpiration by Stomata
Works both for and against plants.
Can cause plants to lose large amounts of water.

42. The leaf stomata
Help plants adjust their transpiration rates to changing environmental conditions.
Plasmolysis
Turgid Elodea

43. Figure 29.11

44. The Transport of Sugars
The main function of phloem
Is to transport sugars a plant makes by photosynthesis.

45. Phloem
Consists of living food-conducting cells arranged end-to-end into tubes.
Transports phloem sap, a sugary solution.
Translocation of Phloem Sap in Spring
Translocation of Phloem Sap in Summer

46. Figure 29.12

47. A sugar source A sugar sink
Is a location in a plant where sugar is being produced.
A sugar sink
Is where the sugar will be stored or consumed in a plant.

48. Phloem sap moves from a sugar source to a sugar sink by the pressure-flow mechanism.

49. Figure 29.13

50. Plant Hormones
Plants use hormones to regulate their internal activities.

51. Plant biologists have identified five major types of plant hormones.

52. Table 29.1

53. A houseplant on a windowsill
Auxin
A houseplant on a windowsill
Grows toward light.
Phototropism
Is the growth of a shoot toward light.
Phototropism

54. Figure 29.14

55. Cell Elongation
Cellular mechanisms
Underlie phototropism.

56. Figure 29.15

57. Cells on the darker side of the stem
Are larger than those on the brighter side, causing the shoot to bend toward the light.
What causes plant cells on the dark side of a shoot to grow faster than those on the bright side?

58. The Process of Science: Do Chemical Signals Affect Plant Growth?
Charles Darwin and his son Francis
Performed some of the earliest experiments on phototropism.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

59. They noticed that grass seedlings
Could bend toward light only if the tips of their shoots were present.

60. Figure 29.16

61. They showed that
Removing the tip of a grass shoot prevents growth toward light.
Preventing the cap of a shoot from receiving light also made the shoot remain straight.

62. The chemical responsible for phototropism is a hormone called auxin.
The Action of Auxin
The chemical responsible for phototropism is a hormone called auxin.
An uneven distribution of auxin on each side of a shoot
Makes cells on the dark side elongate, causing the shoot to bend.

63. Ethylene
Ethylene is a hormone that triggers a variety of aging responses in plants, including fruit ripening and dropping of leaves.

64. Fruit Ripening Fruit ripening
Is triggered by a burst of ethylene production in the fruit.
Ethylene is a gas causing the signal to ripen to spread from fruit to fruit.

65. Some fruits ripen faster if they are stored in a plastic bag so that ethylene accumulates.

66. Figure 29.17

67. The loss of leaves in autumn is affected by ethylene. Leaf drop
Is triggered by environmental stimuli, which cause a change in the balance of ethylene and auxin.

68. Cytokinins Cytokinins
Are growth regulators that promote cell division.
Are produced in actively growing tissues.
Counter the inhibitory effects of auxin, resulting in complex growth patterns in plants.

69. Gibberellins Gibberellins Are growth-regulating plant hormones.
Stimulate cell elongation and division in stems.

70. In combination with auxin, gibberellins can influence fruit development, creating seedless fruits.

71. Figure 29.18

72. Abscisic Acid Abscisic acid (ABA) Generally slows down growth.
Can inhibit seed germination, allowing seeds to go dormant.

73. Some desert plants remain dormant until a downpour of rain washes out the ABA, allowing the seeds to germinate when water is available.

74. Figure 29.19

75. Response to Stimuli
The most important stimulus in the life of a plant is light.

76. Tropisms
Tropisms
Are directed growth responses that cause parts of a plant to grow toward or away from a stimulus.

77. Thigmotropism
Is a response to touch, as when a pea plant tendril coils around a string or wire it touches for support.
Gravitropism

78. Figure 29.20a

79. In gravitropism Plants grow in response to gravity.
Roots grow down and shoots grow upward regardless of the orientation of the seed.
Mimosa Leaf

80. Figure 29.20b

81. Light helps regulate a plant’s life cycle. A photoperiod
Photoperiods
Light helps regulate a plant’s life cycle.
A photoperiod
Is an important environmental stimulus plants most often use to detect the time of year.
Is defined as the relative lengths of day and night.

82. Plants whose flowering is triggered by photoperiod fall into two groups:
Long-night plants
Short-night plants

83. Figure 29.21

84. We distinguish short-night from long-night plants not by absolute night length
But by whether the plants flower in response to a night that is shorter or longer than a critical period.

85. Evolution Connection: The Interdependence of Organisms
Plants rely on organisms from two other kingdoms to help acquire nutrients: bacteria and fungi.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

86. Aphids
Have evolved specialized feeding mechanisms to obtain food from plants.
In turn provide food for other animals.

87. Figure 29.22