Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 30 Plant Diversity II: The Evolution of Seed Plants
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview Seeds changed the course of plant evolution – arose about 360 million years ago – enabling their bearers to become the dominant producers in most terrestrial ecosystems Figure 30.1
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Enclosed or Not Gymnosperms – cycads – ginkgoes – conifers – seeds do not develop in an enclosed structure Angiosperms seeds develop inside a carpel – protective structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Common Characteristics Common to all seed plants – Reduced gametophytes protected in pollen grains and ovules – Heterospory producing two different sporangia which produce 2 different gametophytes Ovules Pollen
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gametophyte/sporophyte relationships Figure 30.2a–c Sporophyte dependent on gametophyte (mosses and other bryophytes). (a) Large sporophyte and small, independent gametophyte (ferns and other seedless vascular plants). (b) Microscopic female gametophytes (n) in ovulate cones (dependent) Sporophyte (2n), the flowering plant (independent) Microscopic male gametophytes (n) inside these parts of flowers (dependent) Microscopic male gametophytes (n) in pollen cones (dependent) Sporophyte (2n) (independent) Microscopic female gametophytes (n) inside these parts of flowers (dependent) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms). (c) Gametophyte (n) Gametophyte (n) Sporophyte (2n) Sporophyte (2n)
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Heterospory: The Rule Among Seed Plants
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ovules and Production of Eggs An ovule consists of – A megasporangium, megaspore, and protective integuments 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.) Integument Spore wall Megasporangium (2n) Megaspore (n)
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 Figure 30.3b (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. Spore wall Male gametophyte (within germinating pollen grain) (n) Female gametophyte (n) Egg nucleus (n) Discharged sperm nucleus (n) Pollen grain (n) Micropyle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Evolution of Pollen When pollen evolved, seed plants lost their dependence on water 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 Figure 30.3c 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. (c) Seed coat (derived from Integument) Food supply (female gametophyte tissue) (n) Embryo (2n) (new sporophyte)
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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 Gymnosperms bear “naked” seeds, typically on cones Among the gymnosperms are many well- known conifers – Or cone-bearing trees, including pine, fir, and redwood include four plant phyla – Cycadophyta – Gingkophyta – Gnetophyta – Coniferophyta
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Exploring Gymnosperm Diversity Figure 30.4 Gnetum Ephedra Ovulate cones Welwitschia PHYLUM GNETOPHYTA PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA Cycas revoluta
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Exploring Gymnosperm Diversity Figure 30.4 Douglas fir Pacific yew Common juniper Wollemia pine Bristlecone pine Sequoia PHYLUM CYCADOPHYTA
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 Figure 30.6 Ovule Megasporocyte (2n) Integument Longitudinal section of ovulate cone Ovulate cone Pollen cone Mature sporophyte (2n) Longitudinal section of pollen cone Microsporocytes (2n) Pollen grains (n) (containing male gametophytes) MEIOSIS Micropyle Germinating pollen grain Megasporangium MEIOSIS Sporophyll Microsporangium Surviving megaspore (n) Germinating pollen grain Archegonium IntegumentEgg (n) Female gametophyte Germinating pollen grain (n) Discharged sperm nucleus (n) Pollen tube Egg nucleus (n) FERTILIZATION Seed coat (derived from parent sporophyte) (2n) Food reserves (gametophyte tissue) (n) Embryo (new sporophyte) (2n) Seeds on surface of ovulate scale Seedling Key Diploid (2n) Haploid (n) The life cycle of a pine 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. 3 In most conifer species, each tree has both ovulate and pollen cones. 1 A pollen grain enters through the micropyle and germinates, forming a pollen tube that slowly digests through the megasporangium. 4 While the pollen tube develops, the megasporocyte (megaspore mother cell) undergoes meiosis, producing four haploid cells. One survives as a megaspore. 5 The female gametophyte develops within the megaspore and contains two or three archegonia, each with an egg. 6 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. 7 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. 8 An ovulate cone scale has two ovules, each containing a mega- sporangium. Only one ovule is shown. 2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Angiosperms 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 The Evolution of the Flower Angiosperms are the most diverse land plants living today. About 250,000 species have been described, and more are discovered each year. The success of angiosperms in terms of geographical distribution, number of individuals, and number of species revolves around a reproductive organ: the flower
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Flowers 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 Anther Filament Stigma Style Ovary Carpel Petal Receptacle Ovule Sepal Stamen
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Fruits – Typically consist of a mature ovary Figure 30.8a–e (b) Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp (a) Tomato, a fleshy fruit with soft outer and inner layers of pericarp (c) Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp (e) Walnut, a dry fruit that remains closed at maturity (d) Milkweed, a dry fruit that splits open at maturity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Can be carried by wind, water, or animals to new locations, enhancing seed dispersal Figure 30.9a–c Wings enable maple fruits to be easily carried by the wind. (a) Seeds within berries and other edible fruits are often dispersed in animal feces. (b) The barbs of cockleburs facilitate seed dispersal by allowing the fruits to “hitchhike” on animals. (c)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Figure Key Mature flower on sporophyte plant (2n) Ovule with megasporangium (2n) Female gametophyte (embryo sac) Nucleus of developing endosperm (3n) Discharged sperm nuclei (n) Pollen tube Male gametophyte (in pollen grain) Pollen tube Sperm Surviving megaspore (n) Microspore (n) Generative cell Tube cell Stigma Ovary MEIOSIS Megasporangium (n) Pollen grains Egg Nucleus (n) Zygote (2n) Antipodal cells Polar nuclei Synergids Egg (n) Embryo (2n) Endosperm (food Supply) (3n) Seed coat (2n) Seed FERTILIZATION Haploid (n) Diploid (2n) Anther Sperm (n) Pollen tube Style Microsporangium Microsporocytes (2n) Germinating Seed Anthers contain microsporangia. Each microsporangium contains micro- sporocytes (microspore mother cells) that divide by meiosis, producing microspores. 1 Microspores form pollen grains (containing male gametophytes). The generative cell will divide to form two sperm. The tube cell will produce the pollen tube. 2 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 After pollina- tion, eventually two sperm nuclei are discharged in each ovule. 4 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 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). 6 When a seed germinates, the embryo develops into a mature sporophyte. 7
Copyright © 2005 Pearson Education, Inc. publishing as 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 – 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 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
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 Figure 30.11a, b Carpel Stamen Archaefructus sinensis, a 125-million-year- old fossil. (a) Artist’s reconstruction of Archaefructus sinensis (b) 5 cm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Adaptations Allowing Land Reproduction
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Angiosperm Radiation Angiosperms represent one of the great adaptive radiations in the history of life. An adaptive radiation occurs when a single lineage produces a large number of descendant species that are adapted to a wide variety of habitats.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Their diversification is associated with three key adaptations: (1) vessels, (2) flowers, and (3) fruits.
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 Monocots are monophyletic; dicots are paraphyletic. Because dicots are not a natural grouping, most biologists call them eudicots
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Exploring Angiosperm Diversity Figure Amborella trichopoda Water lily (Nymphaea “Rene Gerard”) Star anise (Illicium floridanum) BASAL ANGIOSPERMS HYPOTHETICAL TREE OF FLOWERING PLANTS MAGNOLIIDS Amborella Water lilies Star anise and relatives Magnoliids Monocots Eudicots Southern magnolia (Magnolia grandiflora)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Exploring Angiosperm Diversity Figure Orchid (Lemboglossum fossii) Monocot Characteristics Embryos Leaf venation Stems Roots Pollen Flowers Pollen grain with one opening Root system Usually fibrous (no main root) Vascular tissue scattered Veins usually parallel One cotyledonTwo cotyledons Veins usually netlike Vascular tissue usually arranged in ring Taproot (main root) usually present Pollen grain with three openings Zucchini (Cucurbita Pepo), female (left) and male flowers Pea (Lathyrus nervosus, Lord Anson’s blue pea), a legume Dog rose (Rosa canina), a wild rose Pygmy date palm (Phoenix roebelenii) Lily (Lilium “Enchant- ment”) Barley (Hordeum vulgare), a grass Anther Stigma California poppy (Eschscholzia californica) Pyrenean oak (Quercus pyrenaica) Floral organs usually in multiples of three Floral organs usually in multiples of four or five Filament Ovary Eudicot Characteristics MONOCOTS EUDICOTS
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 (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. (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. (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.
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Seed Plants The seed plants are a monophyletic group that consists of the gymnosperms and the angiosperms. Seed plants are defined by the production of seeds and pollen grains.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gnetophyta (Gnetophytes) There are three living genera; the drug ephedrine has been isolated from this group
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Cycadophyta (Cycads) They harbor large numbers of symbiotic, nitrogen-fixing cyanobacteria, which are important sources of nutrients (Figure 29.38).
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cycads
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ginkgophyta (Ginkgoes) One species is alive today. It is deciduous, and individual trees are either male or female
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Coniferophyta (Conifers) This group is named for its reproductive structure, the cone. Conifers dominate all high- latitude and high-altitude forests
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Anthophyta (Angiosperms) The defining adaptation of angiosperms is the flower. Flowering plants supply the food that supports virtually every other species (
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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