BIOLOGY. CONCEPTS & CONNECTIONS Fourth Edition. Neil A. Campbell Jane B. Reece Lawrence G. Mitchell Martha R. Taylor

Transcription

1 BIOLOGY CONCEPTS & CONNECTIONS Fourth Edition Neil A. Campbell Jane B. Reece Lawrence G. Mitchell Martha R. Taylor CHAPTER 31 Plant Structure, Reproduction, and Development Modules From PowerPoint Lectures for Biology: Concepts & Connections

2 A Gentle Giant This giant sequoia, the General Sherman, is the largest plant on Earth It is 84 m (275 ft) tall Its trunk is 10m in diameter

3 The General Sherman has been growing for about 2,500 years

4 Growth rings mark each year in a tree's life Rings vary in thickness depending on weather conditions during the growing season

5 Humans depend on plant products Lumber Fabric Paper Food Industrial chemicals

6 Plants are vital to Earth's well-being They provide food for land animals They offer shelter and breeding areas for animals, fungi, and microorganisms Their roots prevent soil erosion Photosynthesis in plant leaves helps reduce carbon dioxide and adds oxygen to the air

7 PLANT STRUCTURE AND FUNCTION 31.2 The two main groups of angiosperms are the monocots and the dicots Angiosperms, or flowering plants, are the most familiar and diverse plants There are two main types of angiosperms Monocots include orchids, bamboos, palms, lilies, grains, and other grasses Dicots include shrubs, ornamental plants, most trees, and many food crops

8 Monocots and dicots differ in seed leaf number and in the structure of roots, stems, leaves, and flowers MONOCOTS SEED LEAVES LEAF VEINS STEMS FLOWERS ROOTS One cotyledon Main veins usually parallel Vascular bundles in complex arrangement Floral parts usually in multiples of three Fibrous root system DICOTS Two cotyledons Main veins usually branched Vascular bundles arranged in ring Floral parts usually in multiples of four or five Taproot usually present Figure 31.2

9 31.3 The plant body consists of roots and shoots Root system Provides anchorage Absorbs and transports minerals and water Stores food Root hairs increase the surface area for absorption

10 Shoot system Consists of stems, leaves, and flowers in angiosperms Stems are located above the ground and support the leaves and flowers Leaves are the main sites of photosynthesis in most plants

11 Terminal bud Leaf Blade Petiole Axillary bud Flower SHOOT SYSTEM Stem Node Internode ROOT SYSTEM Taproot Root hairs Figure 31.3

12 The terminal bud is located at the tip of a stem It is the growth point of the stem Axillary buds can give rise to branches In apical dominance, the terminal bud produces hormones that inhibit the growth of axillary buds This results in a taller plant that has greater exposure to light

13 31.4 Many plants have modified roots and shoots Roots and stems are adapted for a variety of functions Storing food Asexual reproduction Protection Plant breeders have improved the yields of root crops by selecting varieties, such as the sugar beet plant, with very large taproots Figure 31.4A

14 Modified stems include STRAWBERRY PLANT runners, for asexual reproduction Runner rhizomes, for plant growth and food storage POTATO PLANT Rhizome tubers, for food storage in the form of starch IRIS PLANT Root Rhizome Taproot Tuber Figure 31.4B

15 Modified leaves include tendrils and spines Tendrils help plants to climb Spines may protect the plant from plant-eating animals Figure 31.4C

16 31.5 Plant cells and tissues are diverse in structure and function Figure 31.5A

17 There are five major types of plant cells Parenchyma Collenchyma Sclerenchyma Water-conducting cells (xylem) Food-conducting cells (phloem)

18 Parenchyma cells function in food storage, photosynthesis, and aerobic respiration. They have a primary cell wall that is thin and flexible and lack a secondary cell wall. Primary wall (thin) Pit Figure 31.5B

19 Collenchyma cells provide support in parts of the plant that are still growing. They have an unevenly thick primary cell wall and lack a secondary cell wall. The strings of celery are made of collenchyma cells. Primary wall (thick) Figure 31.5C

20 Sclerenchyma cells provide a rigid scaffold that supports the plant. They have very thick primary and secondary cell walls that are fortified with lignin. Their function is purely for support. Fiber cells Pits Secondary wall Fiber cells Primary wall FIBER Figure 31.5D

21 Water-conducting cells, generally known as xylem, convey water from the roots to the stems and leaves Chains of tracheids or vessel elements form a system of tubes for water transport Openings in end wall Pits Vessel element Tracheids Pits Figure 31.5E

22 Food-conducting cells, generally known as phloem, function in the transport of sugars, other compounds, and some mineral ions Sieve-tube members are arranged end-to-end, forming tubes Their end walls are perforated with plasmodesmata, forming sieve plates At least one companion cell flanks each sievetube member

23 Sieve plate Companion cell Primary wall Cytoplasm Figure 31.5F

24 Complex tissues are composed of more than one type of plant cell Vascular tissues are complex tissues that conduct water and food Xylem contains water-conducting cells that convey water and dissolved minerals Phloem contains sieve-tube members that transport sugars

25 31.6 Three tissue systems make up the plant body Roots, stems, and leaves are made of three tissue systems Leaf Stem The epidermis The vascular tissue system The ground tissue system Epidermis Root Ground tissue system Vascular tissue system Figure 31.6A

26 The epidermis covers and protects the plant The cuticle is a waxy coating secreted by epidermal cells that helps the plant retain water The vascular tissue contains xylem and phloem It provides support and transports water and nutrients

27 The ground tissue system functions mainly in storage and photosynthesis It consists of parenchyma cells and supportive collenchyma and sclerenchyma cells

28 The ground tissue system of the root forms the cortex The cortex consists mostly of parenchyma tissue The selective barrier forming the innermost layer of the cortex is the endodermis

29 VASCULAR TISSUE SYSTEM Xylem Phloem Epidermis GROUND TISSUE SYSTEM Cortex Endodermis Figure 31.6B

30 Monocot Root cross section

31 These microscopic cross sections of a dicot and a monocot indicate several differences in their tissue systems Figure 31.6C

32 The three tissue systems in dicot leaves The epidermis consist of pores called stomata (singular, stoma) flanked by regulatory guard cells Figure 31.6D

33 The ground tissue system of a leaf is called mesophyll and is the site of photosynthesis Figure 31.6D

34 The vascular tissue consists of a network of veins composed of xylem and phloem Figure 31.6D

35 PLANT GROWTH 31.7 Primary growth lengthens roots and shoots Most plants exhibit indeterminate growth They continue to grow as long as they live In contrast, animals are characterized by determinate growth They cease growing after reaching a certain size

36 Terminal bud Axillary buds Arrows = direction of growth Root tips Figure 31.7A

37 Annuals complete their life cycle in a single year or growing season Examples: wheat, corn, rice, and most wildflowers Biennials complete their life cycle in two years, with flowering occurring in the second year Examples: beets and carrots Perennials live and reproduce for many years Examples: trees, shrubs, and some grasses

38 Growth in all plants originates in tissues called meristems Meristems are areas of unspecialized, dividing cells Apical meristems are located at the tips of roots and in the terminal buds and axillary buds of shoots They initiate primary growth, lengthwise growth by the production of new cells Roots and stems lengthen further as cells elongate and differentiate

39 CELL DIVISION ELONGATION DIFFERENTIATION Vascular cylinder Cortex Epidermis Root hair Cellulose fibers Apical meristem region Root cap Figure 31.7B

40 31.8 Secondary growth increases the girth of woody plants An increase in a plant's girth results from secondary growth Secondary growth involves cell division in two cylindrical meristems Vascular cambium Cork cambium

41 Figure 31.8A

42 Vascular cambium thickens a stem by adding layers of secondary xylem, or wood, next to its inner surface It also produces the secondary phloem, which is a tissue of the bark Cork cambium produces protective cork cells located in the bark

43 Everything external to the vascular cambium is considered bark Secondary phloem Cork cambium Protective cork cells Heartwood in the center of the trunk consists of older, clogged layers of secondary xylem Sapwood consists of younger, secondary xylem that still conducts water

44 A woody log is the result of several years of secondary growth Sapwood Rings Wood rays Heartwood Sapwood Vascular cambium Heartwood Bark Secondary phloem Cork cambium Cork Figure 31.8B

45 PLANT REPRODUCTION 31.9 Overview: The sexual life cycle of a flowering plant The angiosperm flower is a reproductive shoot consisting of Carpel Ovary Stigma Anther sepals petals Stamen stamen carpels Petal Ovule Sepal Figure 31.9A

46 Sepals are usually green and resemble leaves in appearance Sepals enclose and protect the flower bud before the flower opens Petals are often bright and colorful They attract insects (pollinators)

47 Stamens are the male reproductive organs of plants Pollen grains develop in anthers, at the tips of stamens Each pollen grain contains three haploid cells (a tube cell and two sperm cells) Carpels are the female reproductive organs of plants The ovary at the base of the carpel houses the ovule

48 Figure 31.10

49 The life cycle of an angiosperm involves several stages Ovary, containing ovule Embryo Fruit, containing seed Seed Mature plant with flowers, where fertilization occurs Seedling Germinating seed Figure 31.9B

50 31.11 The ovule develops into a seed After fertilization, the ovule becomes a seed The fertilized egg (first sperm cell) within the seed divides to become an embryo The other fertilized cell (second sperm cell) develops into the endosperm, which stores food for the embryo A resistant seed coat protects the embryo and endosperm

51 Seed dormancy is an important evolutionary adaptation in which growth and development are suspended temporarily It allows time for a plant to disperse its seeds It increases the chance that a new generation of plants will begin growing only when environmental conditions favor survival

52 Comparison between dicot and monocot seeds Seed coat Embryonic shoot Embryonic leaves Embryonic root Cotyledons COMMON BEAN (DICOT) Fruit tissue Cotyledon Seed coat Endosperm Embryonic leaf Sheath Embryonic shoot Embryonic root CORN (MONOCOT) Figure 31.11B

53 31.12 The ovary develops into a fruit The ovary develops into a fruit which helps protect and disperse the seeds Figure 31.12A

54 There is a correspondence between flower and fruit in a pea plant The wall of the ovary becomes the pod The ovules develop into the seeds Upper part of carpel Ovule Seed Ovary wall Sepal Pod (opened) Figure 31.12B

55 The small, threadlike structure at the end of the pod is what remains of the upper part of the flower's carpel The sepals of the flower stay attached to the base of the green pod Upper part of carpel Ovule Seed Ovary wall Sepal Pod (opened) Figure 31.12B

56 Simple fruits develop from a flower with a single carpel and ovary Apples, pea pods, cherries Aggregate fruits develop from a flower with many carpels Raspberries Multiple fruits develop from a group of flowers clustered tightly together Pineapples Figure 31.12C

57 31.13 Seed germination continues the life cycle A seed starts to germinate when it takes up water, expands, and bursts its seed coat Metabolic changes cause the embryo to resume growth and absorb nutrients from the endosperm An embryonic root emerges, and a shoot pushes upward and expands its leaves

58 Pea germination (a dicot) Embryonic shoot Foliage leaves Cotyledons Embryonic root Corn germination (a monocot) Protective sheath enclosing shoot Foliage leaves Embryonic root Cotyledon Figure 31.13A, B

59 31.14 Asexual reproduction produces plant clones Many plants can reproduce asexually via bulbs, sprouts, or runners Asexual reproduction often involves fragmentation Fragmentation is the separation of parts from the parent plant and regeneration of those parts into whole plants Figure 31.14A

60 Sprouts from the roots of a coast redwood tree may eventually take the place of its parent in the forest Figure 31.14B

61 These creosote bushes came from generations of vegetative reproduction by roots Figure 31.14C

62 Most grasses can propagate asexually by sprouting shoots and roots from runners Figure 31.14D

63 31.15 Connection: Vegetative reproduction is a mainstay of modern agriculture Propagating plants from cuttings or bits of tissue can increase agricultural productivity But it can also reduce genetic diversity

64 Test-tube cloning is the growth of a plantlet from a few meristem cells cultured on a chemical medium A single plant can be cloned into thousands of copies that will continue to grow when planted in soil Orchids and certain pine trees used in mass plantings are propagated this way Figure 31.15A

65 "GM" (genetically modified) plants are created when foreign genes are incorporated into a single parenchyma cell The cell is then cultured until it develops into a new plantlet The commercial adoption of GM crops has been rapid However, many people are concerned about the potential environmental risks associated with their use

66 Monocultures are large areas of land planted with a single crop Gene-cloning techniques and monocultures have led to crop plants with little genetic diversity This increases the likelihood that a small number of diseases could devastate large crop areas

67 THE UPTAKE AND TRANSPORT OF PLANT NUTRIENTS 32.1 Plants acquire their nutrients from soil and air As a plant grows, its roots absorb water, minerals (inorganic ions), and some oxygen from the soil Its leaves take carbon dioxide from the air Figure 32.1A

68 Photosynthesis makes use of the uptake of water, carbon dioxide, and minerals to produce sugars These sugars are composed of carbon, oxygen, and hydrogen The nitrogen and magnesium absorbed from the soil are components of chlorophyll Phosphorus, also absorbed from the soil, is a major component of nucleic acids, phospholipids, and ATP

69 The ability to move water from roots to leaves and to deliver sugars to specific areas of the body are remarkable feats of evolutionary engineering Figure 32.1B

70 32.2 The plasma membranes of root cells control solute uptake Root hairs greatly increase a root's absorptive surface Figure 32.2A

71 In order for upward transport, water and solutes must enter the xylem Water and solutes move through the root's epidermis and cortex by two main routes Through cells (intracellular route) Between cells (extracellular route) Water and solutes typically follow a combination of routes and passages through numerous plasma membranes and cell walls en route to the xylem

72 Root hair Epidermis Cortex Phloem Xylem Casparian strip Endodermis EXTRACELLULAR ROUTE, via cell walls; stopped by Casparian strip Root hair Casparian strip Xylem INTRACELLULAR ROUTE, via cell interiors; through plasmodesmata Epidermis Cortex Endodermis Figure 32.2B

73 The Casparian strip stops water and solutes from entering the xylem via cell walls Thus water and ions that travel the extracellular route can enter the xylem only by crossing a plasma membrane into an endodermal cell

74 32.3 Transpiration pulls water up xylem vessels Xylem sap is the solution of inorganic nutrients conveyed in xylem tissue from a plant's roots to its shoots Root pressure can push xylem sap up only a few meters Solute transport raises water pressure in the xylem Plants pull xylem sap upward from the soil through the transpiration-cohesion-tension mechanism

75 Transpiration is the loss of water from the leaves It exerts a pull on the xylem sap Cohesion causes water molecules to stick together It relays the pull of transpiration along a string of water molecules all the way to the roots The adhesion of water molecules to xylem cell walls helps counter the effect of gravity

76 Figure 32.3

77 32.4 Guard cells control transpiration Guard cells surrounding stomata in the leaves control transpiration The opening and closing of stomata is an adaptation to help plants regulate their water content and adjust to changing environmental conditions H 2 O Guard cells H 2 O H 2 O H 2 O H 2 O H 2 O K + H 2 O H 2 O Vacuole H 2 O H 2 O Stoma opening Stoma closing Figure 32.4

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79 32.5 Phloem transports sugars While xylem sap flows upwards from the roots, phloem sap moves throughout the plant in various directions The main function of phloem is to transport the sugars made by photosynthesis

80 Phloem contains food-conducting cells arranged end-to-end as tubes Sievetube member Sieve plate Figure 32.5A

81 Phloem transports food molecules made by photosynthesis by a pressure-flow mechanism Sugar is loaded into a phloem tube at the sugar source, raising the solute concentration inside the tube Figure 32.5B

82 Water is drawn into the tube by osmosis, raising the pressure in the tube Sugar and water leave the tube at the sugar sink Figure 32.5B

83 The increase in pressure at the sugar source and decrease at the sugar sink causes phloem sap to flow from source to sink Figure 32.5B

84 Plant biologists have used aphids to study phloem sap These studies have supported the pressureflow model Honeydew droplet Stylet of aphid Figure 32.5C

85 PLANT NUTRIENTS AND THE SOIL 32.6 Plant health depends on a complete diet of essential inorganic nutrients A plant must obtain nutrients from its surroundings Macronutrients, such as carbon, oxygen, nitrogen, and phosphorus, are needed in large amounts They are used to build organic molecules Figure 32.6B

86 32.8 Soil contains rock particles, humus, organisms, water, and crucial solutes Soil characteristics determine whether a plant will be able to obtain the nutrients it needs to grow Fertile soil contains a mixture of small rock and clay particles They hold water and ions and allow oxygen to diffuse into plant roots

87 Humus is decaying organic material It provides nutrients, holds water and air, and supports the growth of organisms that enhance soil fertility

88 Soil horizons are distinct layers of soil Horizon A, or topsoil, contains rock particles (sand and clay), humus, and living organisms Horizon B contains fine clay particles and nutrients that have drained down from Horizon A Horizon C is composed mainly of partially broken-down rock Figure 32.8A

89 A plant's root hairs are in direct contact with the water that surrounds the tiny particles of topsoil The root hairs take up dissolved oxygen, ions, and water from the film of soil water that surrounds them Soil particle surrounded by film of water Root hair Water Air space Figure 32.8B

90 Anions, such as nitrate (NO 3- ), are readily available to plants because they are not bound to soil particles But they tend to drain out of the soil quickly This reduces soil fertility

91 32.9 Connection: Soil conservation is essential to human life Good soil management includes water-conserving irrigation erosion control the prudent use of herbicides and fertilizers Figure 32.9A, B

92 32.10 Connection: Organic farmers avoid the use of commercial chemicals Organic farmers rely on the principles of ecology rather than the use of synthetic chemicals or pesticides that can damage the environment Organic farmers try to restore as much to the soil as is drawn from it Figure 32.10

93 32.11 Fungi help most plants absorb nutrients from the soil Relationships with other organisms help plants obtain nutrients Many plants form mycorrhizae A network of fungal threads increases a plant's absorption of nutrients and water The fungus receives some nutrients from the plant Figure 32.11

94 32.12 The plant kingdom includes parasites and carnivores Some plants have evolved parasitic ways of obtaining food from other plants Dodder obtains organic molecules from other plant species using specialized roots that tap into the host s vascular tissue Figure 32.12A

95 Mistletoe supplements its diet by siphoning sap from the vascular tissue of its host plants Figure 32.12B

96 Carnivorous plants obtain some of their nutrients from animal tissues The sundew and Venus flytrap use insects as a source of nitrogen This nutritional adaptation enables them to thrive in highly acidic soil Figure 32.12C, D

97 32.13 Most plants depend on bacteria to supply nitrogen Plants cannot use atmospheric nitrogen, gaseous N 2, although it is very plentiful Instead, nearly all plants depend to some extent on nitrogen supplies in the soil

98 Bacteria in the soil convert N 2 from the air and nitrogen compounds from decomposing organic matter into forms that plants can take up and use Nitrate ions (N0 3- ) and ammonium ions (NH 4+ )

99 This process of converting atmospheric nitrogen to ammonium is called nitrogen fixation ATMOSPHERE N 2 N 2 Nitrogen-fixing bacteria Amino acids NH 4 + Soil Organic material Ammonifying bacteria NH 4 + (ammonium) Nitrifying bacteria NO 3 (nitrate) Root Figure 32.13

100 32.14 Legumes and certain other plants house nitrogen-fixing bacteria Legumes and certain other plants have nodules in their roots that contain nitrogen-fixing bacteria Shoot Nodules Roots Figure 32.14A

101 Most of the nitrogen-fixing bacteria in legume nodules belong to the genus Rhizobium The relationship between the plant and the nitrogen-fixing bacteria is mutually beneficial Bacteria within vesicle Figure 32.14B

102 PLANT NUTRIENTS AND AGRICULTURE Connection: A major goal of agricultural research is to improve the protein content of crops Plants are the main nutritional source for most people in the world Therefore, improving the protein content of crops is an important research goal Figure 32.15A

103 One of the most promising lines of agricultural research is directed toward improving the output of the Rhizobium bacteria that inhabit the root nodules of legumes Rhizobium DNA Genes for nitrogen fixation TURN OFF GENES Nitrogen compounds in root nodules Nitrogen-fixing enzymes N 2 Figure 32.15B

104 PLANT HORMONES 33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone Hormones coordinate the activities of plant cells and tissues The study of plant hormones began with observations of plants bending toward light This phenomenon is called phototropism Figure 33.1A

105 Experiments carried out by Darwin and others showed that the tip of a grass seedling detects light and transmits a signal down to the growing region of the shoot Light Control Tip removed Tip covered by opaque cap Tip covered by transparent cap Base covered by opaque shield Tip separated by gelatin block Tip separated by mica Figure 33.1C DARWIN AND DARWIN (1880) BOYSEN-JENSEN (1913)

106 It was discovered in the 1920s that a hormone was responsible for the signaling Darwin observed This hormone was dubbed auxin Auxin plays an important role in phototropism

107 33.2 Five major types of hormones regulate plant growth and development Hormones regulate plant growth and development by affecting cell division cell elongation cell differentiation Only small amounts of hormones are necessary to trigger the signal-transduction pathways that regulate plant growth and development

108 Table 33.2

109 Plants produce auxin (IAA) in the apical meristems at the tips of shoots At different concentrations, auxin stimulates or inhibits the elongation of shoots and roots STEMS ROOTS 0.9 g/l Figure 33.3B

110 Phototropism results from faster cell growth on the shaded side of the shoot than on the illuminated side Shaded side of shoot Light Illuminated side of shoot Figure 33.1B

111 The effect of auxin on pea plants Figure 33.3A

112 Auxin stimulates cell division and the development of vascular tissues in vascular cambium This promotes growth in stem diameter Auxins are also used as a rooting powder to develop roots quickly in plant cuttings. Synthetic auxins can be sprayed on tomato plants to induce fruit production without pollination producing seedless tomatoes

113 33.4 Cytokinins stimulate cell division Cytokinins are hormones that promote cell division and delay senescence (aging) by inhibiting protein breakdown. They are produced in actively growing roots, embryos, and fruits The antagonistic interaction of auxin and cytokinin may be one way a plant coordinates the growth of its root and shoot systems

114 Cytokinins from roots may balance the effects of auxin from apical meristems, causing lower buds to develop into branches The basil plant on the right has had its terminal bud removed The inhibitory effect of auxin on axillary buds was thus eliminated Cytokinins from the roots activated the axillary buds, making the plant grow more branches Terminal bud No terminal bud Figure 33.4

115 33.5 Gibberellins affect stem elongation and have numerous other effects Gibberellins stimulate cell elongation and cell division in stems and leaves. This causes rapid growth in some stems known as bolting. Figure 33.5A

116 Gibberellins, in combination with auxin, can influence fruit development Gibberellins can make grapes grow larger and farther apart in a cluster The grapes at right were treated with gibberellin, while those at left were not Figure 33.5B

117 Fig

118 Gibberellin-auxin sprays can make apples, currants, and eggplants develop without fertilization Gibberellins released from embryos function in some of the early events of seed germination

119 33.6 Abscisic acid inhibits many plant processes Abscisic acid (ABA) inhibits the germination of seeds The ratio of ABA to gibberellins often determines whether a seed will remain dormant or will germinate

120 Seeds of many plants remain dormant until their ABA is inactivated or washed away These flowers grew from seeds that germinated after a rainstorm in the Mojave Desert Figure 33.6

121 ABA also acts as a stress hormone It causes stomata to close when a plant is dehydrated Thus the rate of transpiration is decreased and further water loss prevented

122 33.7 Ethylene triggers fruit ripening and other aging processes Ethylene is a gaseous hormone that triggers fruit ripening Ethylene is given off as cells age These bananas were exposed to different amounts of ethylene over the same time period Figure 33.7A

123 Fruit growers use ethylene to control ripening Apple farmers take measures to retard the ripening action of natural ethylene Tomato farmers pick unripe fruit and then pipe ethylene into storage bins to promote ripening

124 The shorter days of autumn trigger a changing ratio of auxin to ethylene Leaf stalk Stem (twig) This is the likely cause of the changes seen in deciduous trees color changes, drying, and the loss of leaves Protective layer Abscission layer Figure 33.7B Stem Leaf stalk

125 33.8 Connection: Plant hormones have many agricultural uses Plant hormones have a variety of agricultural uses Farmers use auxin to delay or promote fruit drop Auxin and gibberellins are used to produce seedless fruits A synthetic auxin (2,4-D) is used to kill weeds

126 There are many questions and concerns about the safety of using such chemicals in food production Figure 33.8

127 GROWTH RESPONSES AND BIOLOGICAL RHYTHMS IN PLANTS 33.9 Tropisms orient plant growth toward or away from environmental stimuli Plants sense and respond to environmental changes in a variety of ways Tropisms are growth responses that change the shape of a plant or make it grow toward or away from a stimulus

128 Phototropism is the bending toward light It may result from auxin moving from the illuminated side to the shaded side of a stem Figure 33.1A

129 Gravitropism is a response to gravity It may be caused by the settling of special organelles on the low sides of shoots and roots This may trigger a change in the distribution of hormones Figure 33.9A

130 Gravitropism is an important adaptation It ensures that the shoot will grow upward toward light and the roots will grow down into the soil, no matter how the seed lands in the soil

131 Thigmotropism is a response to touch It is responsible for the coiling of tendrils and vines around objects It enables plants to use other objects for support while growing toward sunlight Figure 33.9B

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133 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (TE Haploid Art) n n n n Spore Spores Gametophyte (n) Mitosis Egg Sperm Meiosis 2n Spore mother cell Gamete fusion 2n Zygote Diploid Sporangia Sporophyte (2n) 2n Embryo

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136 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (TE Art) Antheridium Sperm FERTILIZATION Egg Archegonium n 2n Zygote Developing sporophyte in archegonium Mature sporophyte Male Female Sporangium Gametophytes Bud Mitosis Rhizoid Germinating spores MEIOSIS Spores Parent gametophyte

137 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (TE Art) Archegonium Mitosis Antheridium Rhizoids Spore Gametophyte n Sperm MEIOSIS FERTILIZATION Mature 2n sporangium Mature frond Egg Embryo Sorus (cluster of sporangia) Adult sporophyte Rhizome Leaf of young sporophyte Gametophyte

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140 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (TE Art) Megaspore Pollen tube Mature seed cone (2nd year) Mitosis Pollination Mitosis Scale Pollen Megaspore mother cell n Microspores Microspore mother cell FERTILIZATION (15 months after pollination) Mitosis Embryo Longisection of seed, showing 2n embryo Pine seed Pollen-bearing cone Mitosis Scale Seedling Ovulate (seed-bearing) cone Sporophyte

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