1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 30 Plant Diversity II: The Evolution of Seed Plants

2. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: Transforming the World Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems A seed consists of an embryo and nutrients surrounded by a protective coat

3. Fig. 30-1

4. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 30.1: Seeds and pollen grains are key adaptations for life on land In addition to seeds, the following are common to all seed plants – Reduced gametophytes – Heterospory – Ovules – Pollen

5. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Advantages of Reduced Gametophytes The gametophytes of seed plants develop within the walls of spores that are retained within tissues of the parent sporophyte

6. Fig. 30-2 Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition Reduced, independent (photosynthetic and free-living) Gametophyte Sporophyte (2n) Gametophyte (n) Sporophyte Example Gametophyte (n) Dominant Reduced, dependent on gametophyte for nutrition Mosses and other nonvascular plants Ferns and other seedless vascular plants Seed plants (gymnosperms and angiosperms) PLANT GROUP GymnospermAngiosperm Microscopic female gametophytes (n) inside ovulate cone Microscopic male gametophytes (n) inside pollen cone Sporophyte (2n) Microscopic female gametophytes (n) inside these parts of flowers Microscopic male gametophytes (n) inside these parts of flowers

7. Fig. 30-2a Gametophyte Sporophyte (2n) Gametophyte (n) Sporophyte Example Dominant Reduced, dependent on gametophyte for nutrition Mosses and other nonvascular plants

8. Fig. 30-2b Reduced, independent (photosynthetic and free-living) Sporophyte (2n) Gametophyte (n) Dominant Ferns and other seedless vascular plants Example Gametophyte Sporophyte

9. Fig. 30-2c Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition Dominant Seed plants (gymnosperms and angiosperms) Gymnosperm Angiosperm Microscopic female gametophytes (n) inside ovulate cone Microscopic male gametophytes (n) inside pollen cone Sporophyte (2n) Microscopic female gametophytes (n) inside these parts of flowers Microscopic male gametophytes (n) inside these parts of flowers Example Gametophyte Sporophyte

10. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Heterospory: The Rule Among Seed Plants The ancestors of seed plants were likely homosporous, while seed plants are heterosporous Megasporangia produce megaspores that give rise to female gametophytes Microsporangia produce microspores that give rise to male gametophytes

11. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Ovules and Production of Eggs An ovule consists of a megasporangium, megaspore, and one or more protective integuments Gymnosperm megaspores have one integument Angiosperm megaspores usually have two integuments

12. Fig. 30-3-1 Megasporangium (2n) Megaspore (n) (a) Unfertilized ovule Integument Spore wall Immature female cone

13. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Pollen and Production of Sperm Microspores develop into pollen grains, which contain the male gametophytes Pollination is the transfer of pollen to the part of a seed plant containing the ovules Pollen eliminates the need for a film of water and can be dispersed great distances by air or animals If a pollen grain germinates, it gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule

14. Fig. 30-3-2 Male gametophyte (within a germinated pollen grain) (n) Female gametophyte (n) (b) Fertilized ovule Micropyle Pollen grain (n) Spore wall Discharged sperm nucleus (n) Egg nucleus (n)

15. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Evolutionary Advantage of Seeds A seed develops from the whole ovule A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat Seeds provide some evolutionary advantages over spores: – They may remain dormant for days to years, until conditions are favorable for germination – They may be transported long distances by wind or animals

16. Fig. 30-3-3 Seed coat (derived from integument) (c) Gymnosperm seed Embryo (2n) (new sporophyte) Food supply (female gametophyte tissue) (n)

17. Fig. 30-3-4 Seed coat (derived from integument) (c) Gymnosperm seed Embryo (2n) (new sporophyte) Food supply (female gametophyte tissue) (n) (b) Fertilized ovule(a) Unfertilized ovule Integument Immature female cone Spore wall Megasporangium (2n) Male gametophyte (within a germinated pollen grain) (n) Megaspore (n) MicropylePollen grain (n) Egg nucleus (n) Discharged sperm nucleus (n) Female gametophyte (n)

18. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones The gymnosperms have “naked” seeds not enclosed by ovaries and consist of four phyla: – Cycadophyta (cycads) – Gingkophyta (one living species: Ginkgo biloba) – Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia) – Coniferophyta (conifers, such as pine, fir, and redwood)

19. Fig. 30-UN1 Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms

20. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Gymnosperm Evolution Fossil evidence reveals that by the late Devonian period some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants

21. Fig. 30-4 Archaeopteris, a progymnosperm

22. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Living seed plants can be divided into two clades: gymnosperms and angiosperms Gymnosperms appear early in the fossil record and dominated the Mesozoic terrestrial ecosystems Gymnosperms were better suited than nonvascular plants to drier conditions Today, cone-bearing gymnosperms called conifers dominate in the northern latitudes

23. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Phylum Cycadophyta Individuals have large cones and palmlike leaves These thrived during the Mesozoic, but relatively few species exist today

24. Fig. 30-5a Cycas revoluta

25. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Phylum Ginkgophyta This phylum consists of a single living species, Ginkgo biloba It has a high tolerance to air pollution and is a popular ornamental tree

26. Fig. 30-5b Ginkgo biloba pollen-producing tree

27. Fig. 30-5c Ginkgo biloba leaves and fleshy seeds

28. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Phylum Gnetophyta This phylum comprises three genera Species vary in appearance, and some are tropical whereas others live in deserts

29. Fig. 30-5d Gnetum

30. Fig. 30-5e Ephedra

31. Fig. 30-5f Welwitschia

32. Fig. 30-5g Welwitschia Ovulate cones

33. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Phylum Coniferophyta This phylum is by far the largest of the gymnosperm phyla Most conifers are evergreens and can carry out photosynthesis year round

34. Fig. 30-5h Douglas fir

35. Fig. 30-5i European larch

36. Fig. 30-5j Bristlecone pine

37. Fig. 30-5k Sequoia

38. Fig. 30-5l Wollemi pine

39. Fig. 30-5m Common juniper

40. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Life Cycle of a Pine: A Closer Look Three key features of the gymnosperm life cycle are: – Dominance of the sporophyte generation – Development of seeds from fertilized ovules – The transfer of sperm to ovules by pollen The life cycle of a pine provides an example Animation: Pine Life Cycle Animation: Pine Life Cycle

41. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The pine tree is the sporophyte and produces sporangia in male and female cones Small cones produce microspores called pollen grains, each of which contains a male gametophyte The familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes It takes nearly three years from cone production to mature seed

42. Fig. 30-6-1 Microsporangium (2n) Microsporocytes (2n) Pollen grains (n) Pollen cone Microsporangia MEIOSIS Mature sporophyte (2n) Haploid (n) Diploid (2n) Key Ovulate cone

43. Fig. 30-6-2 Microsporangium (2n) Microsporocytes (2n) Pollen grains (n) Pollen cone Microsporangia MEIOSIS Mature sporophyte (2n) Haploid (n) Diploid (2n) Key MEIOSIS Surviving megaspore (n) Pollen grain Megasporangium (2n) Megasporocyte (2n) Ovule Integument Ovulate cone

44. Fig. 30-6-3 Microsporangium (2n) Microsporocytes (2n) Pollen grains (n) Pollen cone Microsporangia MEIOSIS Mature sporophyte (2n) Haploid (n) Diploid (2n) Key MEIOSIS Surviving megaspore (n) Pollen grain Megasporocyte (2n) Ovule Integument Ovulate cone FERTILIZATION Pollen tube Female gametophyte Sperm nucleus (n) Egg nucleus (n) Archegonium Megasporangium (2n)

45. Fig. 30-6-4 Microsporangium (2n) Microsporocytes (2n) Pollen grains (n) Pollen cone Microsporangia MEIOSIS Mature sporophyte (2n) Haploid (n) Diploid (2n) Key MEIOSIS Surviving megaspore (n) Pollen grain Megasporocyte (2n) Ovule Integument Ovulate cone FERTILIZATION Pollen tube Female gametophyte Sperm nucleus (n) Egg nucleus (n) Archegonium Seedling Seeds Seed coat (2n) Food reserves (n) Embryo (2n) Megasporangium (2n)

46. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits Angiosperms are seed plants with reproductive structures called flowers and fruits They are the most widespread and diverse of all plants

47. Fig. 30-UN2 Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms

48. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Characteristics of Angiosperms All angiosperms are classified in a single phylum, Anthophyta The name comes from the Greek anthos, flower

49. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Flowers The flower is an angiosperm structure specialized for sexual reproduction Many species are pollinated by insects or animals, while some species are wind- pollinated

50. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings A flower is a specialized shoot with up to four types of modified leaves: – Sepals, which enclose the flower – Petals, which are brightly colored and attract pollinators – Stamens, which produce pollen on their terminal anthers – Carpels, which produce ovules

51. Fig. 30-7 Carpel Ovule Sepal Petal Stigma Style Ovary Stamen Anther Filament

52. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings A carpel consists of an ovary at the base and a style leading up to a stigma, where pollen is received Video: Flower Blooming (time lapse) Video: Flower Blooming (time lapse)

53. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fruits A fruit typically consists of a mature ovary but can also include other flower parts Fruits protect seeds and aid in their dispersal Mature fruits can be either fleshy or dry Animation: Fruit Development Animation: Fruit Development

54. Fig. 30-8 Hazelnut Ruby grapefruit Tomato Nectarine Milkweed

55. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Various fruit adaptations help disperse seeds Seeds can be carried by wind, water, or animals to new locations

56. Fig. 30-9 Barbs Seeds within berries Wings

57. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Angiosperm Life Cycle The flower of the sporophyte is composed of both male and female structures Male gametophytes are contained within pollen grains produced by the microsporangia of anthers The female gametophyte, or embryo sac, develops within an ovule contained within an ovary at the base of a stigma Most flowers have mechanisms to ensure cross-pollination between flowers from different plants of the same species

58. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings A pollen grain that has landed on a stigma germinates and the pollen tube of the male gametophyte grows down to the ovary The ovule is entered by a pore called the micropyle Double fertilization occurs when the pollen tube discharges two sperm into the female gametophyte within an ovule

59. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings One sperm fertilizes the egg, while the other combines with two nuclei in the central cell of the female gametophyte and initiates development of food-storing endosperm The endosperm nourishes the developing embryo Within a seed, the embryo consists of a root and two seed leaves called cotyledons

60. Fig. 30-10-1 MEIOSIS Key Microsporangium Microsporocytes (2n) Generative cell Anther Tube cell Pollen grains Microspore (n) Male gametophyte (in pollen grain) (n) Mature flower on sporophyte plant (2n) Haploid (n) Diploid (2n)

61. Fig. 30-10-2 MEIOSIS Key Microsporangium Microsporocytes (2n) Generative cell Anther Tube cell Pollen grains Microspore (n) Male gametophyte (in pollen grain) (n) Mature flower on sporophyte plant (2n) Haploid (n) Diploid (2n) MEIOSIS Ovule (2n) Ovary Megasporangium (2n) Megaspore (n) Female gametophyte (embryo sac) Antipodal cells Central cell Synergids Egg (n)

62. Fig. 30-10-3 MEIOSIS Key Microsporangium Microsporocytes (2n) Generative cell Anther Tube cell Pollen grains Microspore (n) Male gametophyte (in pollen grain) (n) Mature flower on sporophyte plant (2n) Haploid (n) Diploid (2n) MEIOSIS Ovule (2n) Ovary Megasporangium (2n) Megaspore (n) Female gametophyte (embryo sac) Antipodal cells Central cell Synergids Egg (n) Pollen tube Stigma Sperm (n) Discharged sperm nuclei (n) FERTILIZATION Egg nucleus (n) Style Sperm

63. Fig. 30-10-4 MEIOSIS Key Microsporangium Microsporocytes (2n) Generative cell Anther Tube cell Pollen grains Microspore (n) Male gametophyte (in pollen grain) (n) Mature flower on sporophyte plant (2n) Haploid (n) Diploid (2n) MEIOSIS Ovule (2n) Ovary Megasporangium (2n) Megaspore (n) Female gametophyte (embryo sac) Antipodal cells Central cell Synergids Egg (n) Pollen tube Stigma Sperm (n) Discharged sperm nuclei (n) FERTILIZATION Germinating seed Embryo (2n) Endosperm (3n) Seed coat (2n) Seed Nucleus of developing endosperm (3n) Zygote (2n) Egg nucleus (n) Style Sperm

64. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Video: Flowering Plant Life Cycle (time lapse) Video: Flowering Plant Life Cycle (time lapse) Animation: Seed Development Animation: Seed Development Animation: Plant Fertilization Animation: Plant Fertilization

65. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Angiosperm Evolution Clarifying the origin and diversification of angiosperms poses fascinating challenges to evolutionary biologists Angiosperms originated at least 140 million years ago During the late Mesozoic, the major branches of the clade diverged from their common ancestor

66. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fossil Angiosperms Primitive fossils of 125-million-year-old angiosperms display derived and primitive traits Archaefructus sinensis, for example, has anthers and seeds but lacks petals and sepals

67. Fig. 30-11 Carpel Stamen Archaefructus sinensis, a 125-million-year-old fossil (a) (b)Artist’s reconstruction of Archaefructus sinensis 5 cm

68. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Angiosperm Phylogeny The ancestors of angiosperms and gymnosperms diverged about 305 million years ago Angiosperms may be closely related to Bennettitales, extinct seed plants with flowerlike structures Amborella and water lilies are likely descended from two of the most ancient angiosperm lineages

69. Fig. 30-12 Microsporangia (contain microspores) Ovules A possible ancestor of the angiosperms? (a) (b) Angiosperm phylogeny Most recent common ancestor of all living angiosperms Millions of years ago 300 250 200 150 100 50 0 Living gymnosperms Bennettitales Amborella Star anise and relatives Water lilies Monocots Magnoliids Eudicots

70. Fig. 30-12a Microsporangia (contain microspores) Ovules A possible ancestor of the angiosperms? (a)

71. Fig. 30-12b (b) Angiosperm phylogeny Most recent common ancestor of all living angiosperms Millions of years ago 300 250 200 150 100 50 0 Living gymnosperms Bennettitales Amborella Star anise and relatives Water lilies Monocots Magnoliids Eudicots

72. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Developmental Patterns in Angiosperms Egg formation in the angiosperm Amborella resembles that of the gymnosperms Researchers are currently studying expression of flower development genes in gymnosperm and angiosperm species

73. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Angiosperm Diversity The two main groups of angiosperms are monocots (one cotyledon) and eudicots (“true” dicots) The clade eudicot includes some groups formerly assigned to the paraphyletic dicot (two cotyledons) group

74. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Basal angiosperms are less derived and include the flowering plants belonging to the oldest lineages Magnoliids share some traits with basal angiosperms but are more closely related to monocots and eudicots

75. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Basal Angiosperms Three small lineages constitute the basal angiosperms These include Amborella trichopoda, water lilies, and star anise

76. Fig. 30-13a Amborella trichopoda

77. Fig. 30-13b Water lily

78. Fig. 30-13c Star anise

79. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Magnoliids Magnoliids include magnolias, laurels, and black pepper plants Magnoliids are more closely related to monocots and eudicots than basal angiosperms

80. Fig. 30-13d Southern magnolia

81. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Monocots More than one-quarter of angiosperm species are monocots

82. Fig. 30-13e Orchid

83. Fig. 30-13e1 Pygmy date palm (Phoenix roebelenii)

84. Fig. 30-13f

85. Fig. 30-13g Anther Barley Stigma Ovary Filament

86. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Eudicots More than two-thirds of angiosperm species are eudicots

87. Fig. 30-13h California poppy

88. Fig. 30-13i Pyrenean oak

89. Fig. 30-13j Dog rose

90. Fig. 30-13k Snow pea

91. Fig. 30-13l Zucchini flowers

92. Fig. 30-13m Monocot Characteristics Eudicot Characteristics Vascular tissue usually arranged in ring Veins usually parallel Veins usually netlike Vascular tissue scattered Leaf venation One cotyledon Embryos Two cotyledons Stems Roots Pollen Root system usually fibrous (no main root) Pollen grain with three openings Taproot (main root) usually present Pollen grain with one opening Floral organs usually in multiples of three Flowers Floral organs usually in multiples of four or five

93. Fig. 30-13n Monocot Characteristics Eudicot Characteristics Vascular tissue usually arranged in ring Veins usually parallel Vascular tissue scattered Leaf venation One cotyledon Embryos Two cotyledons Stems Veins usually netlike

94. Fig. 30-13o Roots Pollen Root system usually fibrous (no main root) Pollen grain with three openings Pollen grain with one opening Floral organs usually in multiples of three Flowers Floral organs usually in multiples of four or five Monocot Characteristics Eudicot Characteristics Taproot (main root) usually present

95. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Evolutionary Links Between Angiosperms and Animals Pollination of flowers and transport of seeds by animals are two important relationships in terrestrial ecosystems Clades with bilaterally symmetrical flowers have more species than those with radially symmetrical flowers This is likely because bilateral symmetry affects the movement of pollinators and reduces gene flow in diverging populations Video: Bat Pollinating Agave Plant Video: Bat Pollinating Agave Plant Video: Bee Pollinating Video: Bee Pollinating

96. Fig. 30-14 Common ancestor Radial symmetry (N = 4) Bilateral symmetry (N = 15) Compare numbers of species Time since divergence from common ancestor “Radial” clade “Bilateral” clade 3,000 2,000 1,000 0 EXPERIMENT RESULTS Mean difference in number of species

97. Fig. 30-14a Common ancestor Compare numbers of species Time since divergence from common ancestor “Radial” clade “Bilateral” clade EXPERIMENT

98. Fig. 30-14b Radial symmetry (N = 4) Bilateral symmetry (N = 15) 3,000 2,000 1,000 0 RESULTS Mean difference in number of species

99. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 30.4: Human welfare depends greatly on seed plants No group of plants is more important to human survival than seed plants Plants are key sources of food, fuel, wood products, and medicine Our reliance on seed plants makes preservation of plant diversity critical

100. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Products from Seed Plants Most of our food comes from angiosperms Six crops (wheat, rice, maize, potatoes, cassava, and sweet potatoes) yield 80% of the calories consumed by humans Modern crops are products of relatively recent genetic change resulting from artificial selection Many seed plants provide wood Secondary compounds of seed plants are used in medicines

101. Table 30-1a

102. Table 30-1b Cinchona bark, source of quinine

103. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Threats to Plant Diversity Destruction of habitat is causing extinction of many plant species Loss of plant habitat is often accompanied by loss of the animal species that plants support At the current rate of habitat loss, 50% of Earth’s species will become extinct within the next 100–200 years

104. Fig. 30-UN3 Reduced gametophytes Microscopic male and female gametophytes (n) are nourished and protected by the sporophyte (2n) Five Derived Traits of Seed Plants Male gametophyte Female gametophyte HeterosporyMicrospore (gives rise to a male gametophyte) Megaspore (gives rise to a female gametophyte) Ovules Ovule (gymnosperm) Pollen Pollen grains make water unnecessary for fertilization Integument (2n) Megaspore (2n) Megasporangium (2n) Seeds Seeds: survive better than unprotected spores, can be transported long distances Integument Food supply Embryo

105. Fig. 30-UN4 Charophyte green algae Mosses Ferns Gymnosperms Angiosperms

106. Fig. 30-UN5

107. Fig. 30-UN6

108. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1.Explain why pollen grains were an important adaptation for successful reproduction on land 2.List and distinguish among the four phyla of gymnosperms 3.Describe the life history of a pine; indicate which structures are part of the gametophyte generation and which are part of the sporophyte generation

109. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings You should now be able to: 4.Identify and describe the function of the following floral structures: sepals, petals, stamens, carpels, filament, anther, stigma, style, ovary, and ovule 5.Explain how fruits may be adapted to disperse seeds 6.Diagram the generalized life cycle of an angiosperm; indicate which structures are part of the gametophyte generation and which are part of the sporophyte generation

110. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 7.Explain the significance of Archaefructus and Amborella 8.Describe the current threat to plant diversity caused by human population growth