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Chapter 30

Chapter 30. Plant Diversity II: The Evolution of Seed Plants. Figure 30.1. Overview: Feeding the World Seeds changed the course of plant evolution Enabling their bearers to become the dominant producers in most terrestrial ecosystems.

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Chapter 30

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  1. Chapter 30 Plant Diversity II: The Evolution of Seed Plants

  2. Figure 30.1 • Overview: Feeding the World • Seeds changed the course of plant evolution • Enabling their bearers to become the dominant producers in most terrestrial ecosystems

  3. Concept 30.1: The reduced gametophytes of seed plants are protected in ovules and pollen grains • In addition to seeds, the following are common to all seed plants • Reduced gametophytes • Heterospory • Ovules • Pollen

  4. Advantages of Reduced Gametophytes • The gametophytes of seed plants • Develop within the walls of spores retained within tissues of the parent sporophyte

  5. (a) Sporophyte dependent on gametophyte (mosses and other bryophytes). (b) Large sporophyte and small, independent gametophyte (ferns and other seedless vascular plants). (c) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms). • Gametophyte/sporophyte relationships Sporophyte (2n) Sporophyte (2n) Gametophyte (n) Gametophyte (n) Microscopic female gametophytes (n) in ovulate cones (dependent) Microscopic female gametophytes (n) inside these parts of flowers (dependent) Microscopic male gametophytes (n) inside these parts of flowers (dependent) Microscopic male gametophytes (n) in pollen cones (dependent) Sporophyte (2n), the flowering plant (independent) Sporophyte (2n) (independent) Figure 30.2a–c

  6. Heterospory: The Rule Among Seed Plants • Seed plants evolved from plants that had megasporangia • Which produce megaspores that give rise to female gametophytes • Seed plants evolved from plants that had microsporangia • Which produce microspores that give rise to male gametophytes

  7. Integument Spore wall Megasporangium (2n) Megaspore (n) (a) Unfertilized ovule. In this sectional view through the ovule of a pine (a gymnosperm), a fleshy megasporangium is surrounded by a protective layer of tissue called an integument. (Angiosperms have two integuments.) Figure 30.3a Ovules and Production of Eggs • An ovule consists of • A megasporangium, megaspore, and protective integuments

  8. Pollen and Production of Sperm • Microspores develop into pollen grains • Which contain the male gametophytes of plants • Pollination • Is the transfer of pollen to the part of a seed plant containing the ovules

  9. If a pollen grain germinates • It gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule Female gametophyte (n) Egg nucleus (n) Spore wall Male gametophyte (within germinating pollen grain) (n) Discharged sperm nucleus (n) Pollen grain (n) Micropyle (b) Fertilized ovule. A megaspore develops into a multicellular female gametophyte. The micropyle,the only opening through the integument, allowsentry of a pollen grain. The pollen grain contains amale gametophyte, which develops a pollen tubethat discharges sperm. Figure 30.3b

  10. Pollen, which can be dispersed by air or animals • Eliminated the water requirement for fertilization

  11. Seed coat (derived from Integument) Food supply (female gametophyte tissue) (n) Embryo (2n) (new sporophyte) (c) Gymnosperm seed. Fertilization initiatesthe transformation of the ovule into a seed,which consists of a sporophyte embryo, a food supply, and a protective seed coat derived from the integument. The Evolutionary Advantage of Seeds • A seed • Develops from the whole ovule • Is a sporophyte embryo, along with its food supply, packaged in a protective coat Figure 30.3c

  12. Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones • Among the gymnosperms are many well-known conifers • Or cone-bearing trees, including pine, fir, and redwood

  13. The gymnosperms include four plant phyla • Cycadophyta • Gingkophyta • Gnetophyta • Coniferophyta

  14. PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA Cycas revoluta PHYLUM GNETOPHYTA Gnetum Welwitschia Ovulate cones Ephedra • Exploring Gymnosperm Diversity Figure 30.4

  15. Douglas fir Common juniper Wollemia pine Pacific yew Sequoia Bristlecone pine • Exploring Gymnosperm Diversity PHYLUM CYCADOPHYTA Figure 30.4

  16. Gymnosperm Evolution • Fossil evidence reveals that by the late Devonian • Some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants Figure 30.5

  17. Gymnosperms appear early in the fossil record • And dominated the Mesozoic terrestrial ecosystems • Living seed plants • Can be divided into two groups: gymnosperms and angiosperms

  18. A Closer Look at the Life Cycle of a Pine • Key features of the gymnosperm life cycle include • Dominance of the sporophyte generation, the pine tree • The development of seeds from fertilized ovules • The role of pollen in transferring sperm to ovules

  19. 2 An ovulate cone scale has two ovules, each containing a mega- sporangium. Only one ovule is shown. 1 8 4 5 6 7 3 Key In most conifer species, each tree has both ovulate and pollen cones. Haploid (n) Ovule Diploid (2n) A pollen grain enters through the micropyle and germinates, forming a pollen tube that slowly digests through the megasporangium. Megasporocyte (2n) Ovulate cone Integument Longitudinal section of ovulate cone Micropyle Pollen cone Microsporocytes (2n) Megasporangium Mature sporophyte (2n) Germinating pollen grain Pollen grains (n) (containing male gametophytes) MEIOSIS MEIOSIS While the pollen tube develops, the megasporocyte (megaspore mother cell) undergoes meiosis, producing four haploid cells. One survives as a megaspore. Longitudinal section of pollen cone Surviving megaspore (n) Sporophyll Microsporangium A pollen cone contains many microsporangia held in sporophylls. Each microsporangium contains microsporocytes (microspore mother cells). These undergo meiosis, giving rise to haploid microspores that develop into pollen grains. Seedling Germinating pollen grain Archegonium Egg (n) Integument Female gametophyte Seeds on surface of ovulate scale Germinating pollen grain (n) Food reserves (gametophyte tissue) (n) The female gametophyte develops within the megaspore and contains two or three archegonia, each with an egg. Seed coat (derived from parent sporophyte) (2n) Fertilization usually occurs more than a year after pollination. All eggs may be fertilized, but usually only one zygote develops into an embryo. The ovule becomes a seed, consisting of an embryo, food supply, and seed coat. Discharged sperm nucleus (n) Pollen tube By the time the eggs are mature, two sperm cells have developed in the pollen tube, which extends to the female gametophyte. Fertilization occurs when sperm and egg nuclei unite. Embryo (new sporophyte) (2n) FERTILIZATION Egg nucleus (n) Figure 30.6 • The life cycle of a pine

  20. Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits • Angiosperms • Are commonly known as flowering plants • Are seed plants that produce the reproductive structures called flowers and fruits • Are the most widespread and diverse of all plants

  21. Characteristics of Angiosperms • The key adaptations in the evolution of angiosperms • Are flowers and fruits

  22. Flowers • The flower • Is an angiosperm structure specialized for sexual reproduction

  23. Carpel Stigma Anther Style Stamen Ovary Filament Petal Sepal Receptacle Ovule • A flower is a specialized shoot with modified leaves • Sepals, which enclose the flower • Petals, which are brightly colored and attract pollinators • Stamens, which produce pollen • Carpels, which produce ovules Figure 30.7

  24. (b) Ruby grapefruit, a fleshy fruitwith a hard outer layer andsoft inner layer of pericarp (a) Tomato, a fleshy fruit withsoft outer and inner layersof pericarp (c) Nectarine, a fleshyfruit with a soft outerlayer and hard innerlayer (pit) of pericarp (d) Milkweed, a dry fruit thatsplits open at maturity (e) Walnut, a dry fruit that remains closed at maturity Figure 30.8a–e Fruits • Fruits • Typically consist of a mature ovary

  25. (a) Wings enable maple fruits to be easily carried by the wind. (b) Seeds within berries and other edible fruits are often dispersed in animal feces. (c) The barbs of cockleburs facilitate seed dispersal by allowing the fruits to “hitchhike” on animals. • Can be carried by wind, water, or animals to new locations, enhancing seed dispersal Figure 30.9a–c

  26. The Angiosperm Life Cycle • In the angiosperm life cycle • Double fertilization occurs when a pollen tube discharges two sperm into the female gametophyte within an ovule • One sperm fertilizes the egg, while the other combines with two nuclei in the center cell of the female gametophyte and initiates development of food-storing endosperm • The endosperm • Nourishes the developing embryo

  27. Key 1 Anthers contain microsporangia. Each microsporangium contains micro- sporocytes (microspore mother cells) that divide by meiosis, producing microspores. 2 Microspores form pollen grains (containing male gametophytes). The generative cell will divide to form two sperm. The tube cell will produce the pollen tube. Haploid (n) Diploid (2n) Microsporangium Anther Microsporocytes (2n) Mature flower on sporophyte plant (2n) MEIOSIS Microspore (n) Generative cell 7 When a seed germinates, the embryo develops into a mature sporophyte. Ovule with megasporangium (2n) Tube cell Male gametophyte (in pollen grain) Ovary Pollen grains MEIOSIS Germinating Seed Stigma In the megasporangium of each ovule, the megasporocyte divides by meiosis and produces four megaspores. The surviving megaspore in each ovule forms a female gametophyte (embryo sac). 3 Pollen tube Megasporangium (n) Embryo (2n) Sperm Endosperm (food Supply) (3n) 6 The zygote develops into an embryo that is packaged along with food into a seed. (The fruit tissues surround- ing the seed are not shown). Surviving megaspore (n) Seed Pollen tube Seed coat (2n) Style Antipodal cells Polar nuclei Synergids Egg (n) Female gametophyte (embryo sac) Pollen tube Zygote (2n) Nucleus of developing endosperm (3n) Egg Nucleus (n) Sperm (n) After pollina- tion, eventually two sperm nuclei are discharged in each ovule. 4 FERTILIZATION Double fertilization occurs. One sperm fertilizes the egg, forming a zygote. The other sperm combines with the two polar nuclei to form the nucleus of the endosperm, which is triploid in this example. 5 Discharged sperm nuclei (n) • The life cycle of an angiosperm Figure 30.10

  28. Angiosperm Evolution • Clarifying the origin and diversification of angiosperms • Poses fascinating challenges to evolutionary biologists • Angiosperms originated at least 140 million years ago • And during the late Mesozoic, the major branches of the clade diverged from their common ancestor

  29. Carpel Stamen 5 cm (a) (b) Archaefructus sinensis, a 125-million-year- old fossil. Artist’s reconstruction of Archaefructus sinensis Fossil Angiosperms • Primitive fossils of 125-million-year-old angiosperms • Display both derived and primitive traits Figure 30.11a, b

  30. An “Evo-Devo” Hypothesis of Flower Origins • In hypothesizing how pollen-producing and ovule-producing structures were combined into a single flower • Scientist Michael Frohlich proposed that the ancestor of angiosperms had separate pollen-producing and ovule-producing structures

  31. Angiosperm Diversity • The two main groups of angiosperms • Are monocots and eudicots • 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

  32. BASAL ANGIOSPERMS Amborella trichopoda Star anise (Illicium floridanum) Water lily (Nymphaea “Rene Gerard”) HYPOTHETICAL TREE OF FLOWERING PLANTS Monocots Eudicots Star anise and relatives Water lilies Amborella Magnoliids MAGNOLIIDS Southern magnolia (Magnolia grandiflora) • Exploring Angiosperm Diversity Figure 30.12

  33. EUDICOTS MONOCOTS Monocot Characteristics Eudicot Characteristics California poppy (Eschscholzia californica) Orchid (Lemboglossum fossii) Embryos One cotyledon Two cotyledons Leaf venation Pyrenean oak (Quercus pyrenaica) Veins usually netlike Veins usually parallel Pygmy date palm (Phoenix roebelenii) Stems Vascular tissue usually arranged in ring Lily (Lilium “Enchant- ment”) Vascular tissue scattered Roots Dog rose (Rosa canina), a wild rose Root system Usually fibrous (no main root) Taproot (main root) usually present Barley (Hordeum vulgare), a grass Pea (Lathyrus nervosus, Lord Anson’sblue pea), a legume Pollen Pollen grain with three openings Pollen grain with one opening Flowers Zucchini (Cucurbita Pepo), female (left) andmale flowers Anther Floral organs usually in multiples of three Floral organs usually in multiples of four or five Stigma Filament Ovary • Exploring Angiosperm Diversity Figure 30.12

  34. (c) A flower pollinated by nocturnal animals. Some angiosperms, such as this cactus, depend mainly on nocturnal pollinators, including bats. Common adaptations of such plants include large, light-colored, highly fragrant flowers that nighttime pollinators can locate. (a) A flower pollinated by honeybees. This honeybee is harvesting pollen and nectar (a sugary solution secreted by flower glands) from a Scottish broom flower. The flower has a tripping mechanism that arches the stamens over the beeand dusts it with pollen, some ofwhich will rub off onto the stigmaof the next flower the bee visits. (b) A flower pollinated by hummingbirds.The long, thin beak and tongue of this rufous hummingbird enable the animal to probe flowers that secrete nectar deep within floral tubes. Before the hummer leaves, anthers will dust its beak and head feathers with pollen. Many flowers that are pollinated by birds are red or pink, colors to which bird eyes are especially sensitive. Evolutionary Links Between Angiosperms and Animals • Pollination of flowers by animals and transport of seeds by animals • Are two important relationships in terrestrial ecosystems Figure 30.13a–c

  35. Concept 30.4: Human welfare depends greatly on seed plants • No group is more important to human survival than seed plants

  36. Products from Seed Plants • Humans depend on seed plants for • Food • Wood • Many medicines Table 30.1

  37. Threats to Plant Diversity • Destruction of habitat • Is causing extinction of many plant species and the animal species they support

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