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Chapter 30 Plant Diversity II: The Evolution of Seed Plants PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Overview: Feeding the World • Seeds changed the course of plant evolution – Enabling their bearers to become the dominant producers in most terrestrial ecosystems Figure 30.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Advantages of Reduced Gametophytes • The gametophytes of seed plants – Develop within the walls of spores retained within tissues of the parent sporophyte Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Gametophyte/sporophyte relationships Sporophyte (2n) Sporophyte (2n) Gametophyte (n) (a) Sporophyte dependent on gametophyte (mosses and other bryophytes). Gametophyte (n) (b) Large sporophyte and small, independent gametophyte (ferns and other seedless vascular plants). Microscopic female gametophytes (n) in ovulate cones (dependent) Microscopic male gametophytes (n) inside these parts of flowers (dependent) Microscopic male gametophytes (n) in pollen cones (dependent) Figure 30.2a–c Sporophyte (2n) (independent) (c) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms). Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Microscopic female gametophytes (n) inside these parts of flowers (dependent) Sporophyte (2n), the flowering plant (independent) 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ovules and Production of Eggs • An ovule consists of – A megasporangium, megaspore, and protective integuments Integument Spore wall Megasporangium (2n) Megaspore (n) Figure 30.3a (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.) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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) Micropyle Figure 30.3b Pollen grain (n) (b) Fertilized ovule. A megaspore develops into a multicellular female gametophyte. The micropyle, the only opening through the integument, allows entry of a pollen grain. The pollen grain contains a male gametophyte, which develops a pollen tube that discharges sperm. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Pollen, which can be dispersed by air or animals – Eliminated the water requirement for fertilization Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Seed coat (derived from Integument) Food supply (female gametophyte tissue) (n) Embryo (2n) (new sporophyte) Figure 30.3c (c) Gymnosperm seed. Fertilization initiates the 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones • Among the gymnosperms are many wellknown conifers – Or cone-bearing trees, including pine, fir, and redwood Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The gymnosperms include four plant phyla – Cycadophyta – Gingkophyta – Gnetophyta – Coniferophyta Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Exploring Gymnosperm Diversity PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA Cycas revoluta PHYLUM GNETOPHYTA Gnetum Welwitschia Ovulate cones Ephedra Figure 30.4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Exploring Gymnosperm Diversity PHYLUM CYCADOPHYTA Douglas fir Common juniper Wollemia pine Pacific yew Bristlecone pine Figure 30.4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sequoia 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The life cycle of a pine 2 An ovulate cone scale has two ovules, each containing a megasporangium. Only one ovule is shown. Key 1 In most conifer species, each tree has both ovulate and pollen cones. Haploid (n) Diploid (2n) Ovule Megasporocyte (2n) Ovulate cone Pollen cone Integument Longitudinal section of ovulate cone Microsporocytes (2n) Mature sporophyte (2n) MEIOSIS Longitudinal section of Sporophyll pollen cone Microsporangium Seedling Micropyle Megasporangium Germinating pollen grain Pollen grains (n) MEIOSIS (containing male gametophytes) Surviving megaspore (n) 3 A pollen cone contains many microsporangia held in sporophylls. Each microsporangium Germinating contains microsporocytes (microspore mother pollen grain cells). These undergo meiosis, giving rise to Archegonium haploid microspores that develop into Egg (n) Integument pollen grains. Female Seeds on surface gametophyte 4 A pollen grain enters through the micropyle and germinates, forming a pollen tube that slowly digests through the megasporangium. 5 While the pollen tube develops, the megasporocyte (megaspore mother cell) undergoes meiosis, producing four haploid cells. One survives as a megaspore. of ovulate scale Germinating pollen grain (n) 8 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. Figure 30.6 Embryo (new sporophyte) (2n) Food reserves (gametophyte tissue) (n) Seed coat (derived from parent sporophyte) (2n) Discharged sperm nucleus (n) Pollen tube FERTILIZATION Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Egg nucleus (n) 6 The female gametophyte develops within the megaspore and contains two or three archegonia, each with an egg. 7 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. • 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Characteristics of Angiosperms • The key adaptations in the evolution of angiosperms – Are flowers and fruits Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Flowers • The flower – Is an angiosperm structure specialized for sexual reproduction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • 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 Carpel Stigma Stamen Anther Style Ovary Filament Petal Sepal Receptacle Figure 30.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ovule Fruits • Fruits – Typically consist of a mature ovary (a) Tomato, a fleshy fruit with soft outer and inner layers of pericarp (b) Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp (c) Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp Figure 30.8a–e (d) Milkweed, a dry fruit that splits open at maturity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (e) Walnut, a dry fruit that remains closed at maturity • Can be carried by wind, water, or animals to new locations, enhancing seed dispersal (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. Figure 30.9a–c Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (c) The barbs of cockleburs facilitate seed dispersal by allowing the fruits to “hitchhike” on animals. 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The life cycle of an angiosperm Key Haploid (n) Diploid (2n) 1 Anthers contain microsporangia. Each microsporangium contains microsporocytes (microspore mother cells) that divide by meiosis, producing microspores. Microsporangium Anther Microsporocytes (2n) Mature flower on sporophyte plant (2n) 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. MEIOSIS Microspore (n) Ovule with megasporangium (2n) 7 When a seed germinates, the embryo develops into a mature sporophyte. Generative cell Tube cell Male gametophyte (in pollen grain) Ovary 3 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 Seed (embryo sac). Embryo (2n) Endosperm (food Supply) (3n) 6 The zygote develops into an embryo that is packaged along with food into a seed. (The fruit tissues surrounding the seed are not shown). Pollen grains MEIOSIS Germinating Seed Stigma Megasporangium (n) Pollen tube Sperm Surviving megaspore (n) Seed coat (2n) Pollen tube Style Female gametophyte (embryo sac) Antipodal cells Polar nuclei Synergids Egg (n) Pollen tube Zygote (2n) Nucleus of developing endosperm (3n) Egg Nucleus (n) Sperm (n) 4 After pollination, eventually two sperm nuclei are discharged in each ovule. FERTILIZATION Figure 30.10 5 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Discharged sperm nuclei (n) 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fossil Angiosperms • Primitive fossils of 125-million-year-old angiosperms – Display both derived and primitive traits Carpel Stamen 5 cm (a) Archaefructus sinensis, a 125-million-yearold fossil. (b) Artist’s reconstruction of Archaefructus sinensis Figure 30.11a, b Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 pollenproducing and ovule-producing structures Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Exploring Angiosperm Diversity BASAL ANGIOSPERMS Amborella trichopoda Star anise (Illicium floridanum) Water lily (Nymphaea “Rene Gerard”) MAGNOLIIDS Southern magnolia (Magnolia grandiflora) Figure 30.12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Eudicots Monocots Magnoliids Star anise and relatives Water lilies Amborella HYPOTHETICAL TREE OF FLOWERING PLANTS • Exploring Angiosperm Diversity EUDICOTS MONOCOTS Monocot Characteristics Orchid (Lemboglossum fossii) Eudicot Characteristics California poppy (Eschscholzia californica) Embryos One cotyledon Two cotyledons Leaf venation Veins usually netlike Veins usually parallel Pygmy date palm (Phoenix roebelenii) Pyrenean oak (Quercus pyrenaica) Stems Lily (Lilium “Enchantment”) Vascular tissue usually arranged in ring Vascular tissue scattered Roots Barley (Hordeum vulgare), a grass Root system Usually fibrous (no main root) Dog rose (Rosa canina), a wild rose Taproot (main root) usually present Pollen Pollen grain with one opening Pea (Lathyrus nervosus, Lord Anson’s blue pea), a legume Pollen grain with three openings Flowers Anther Stigma Filament Figure 30.12 Ovary Floral organs usually in multiples of three Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Floral organs usually in multiples of four or five Zucchini (Cucurbita Pepo), female (left) and male flowers Evolutionary Links Between Angiosperms and Animals • Pollination of flowers by animals and transport of seeds by animals – Are two important relationships in terrestrial ecosystems (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 bee and dusts it with pollen, some of which will rub off onto the stigma of 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. Figure 30.13a–c Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (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. • Concept 30.4: Human welfare depends greatly on seed plants • No group is more important to human survival than seed plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Products from Seed Plants • Humans depend on seed plants for – Food – Wood – Many medicines Table 30.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Threats to Plant Diversity • Destruction of habitat – Is causing extinction of many plant species and the animal species they support Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings