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Ch. 30: Plant Diversity II: Seed Plants 1 © 2014 Pearson Education, Inc. 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 Seeds can disperse over long distances by wind or other means In addition to seeds, the following are common to all seed plants: Reduced gametophytes Ovules © 2014 Pearson Education, Inc. Heterospory Pollen Figure 30.2 PLANT GROUP COMPARISONS Mosses and other nonvascular plants Gametophyte Dominant Sporophyte Ferns and other seedless vascular plants Reduced, Independent (photosynthetic and free-living) Reduced, dependent on gametophyte for Dominant nutrition Seed plants (gymnosperms and angiosperms) Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition Dominant Gymnosperm Sporophyte (2n) Sporophyte (2n) Microscopic female gametophytes (n) inside ovulate cone Gametophyte (n) Example Gametophyte (n) © 2014 Pearson Education, Inc. Microscopic male gametophytes (n) inside pollen cone Sporophyte (2n) Angiosperm Microscopic female gametophytes (n) inside these parts of flowers Microscopic male gametophytes (n) inside these parts of flowers Sporophyte (2n) Heterospory: The Rule Among Seed Plants Seed plants are heterosporous The dependent gametophytes of seed plants are microscopically small and develop from spores that are retained within tissues of the parent sporophyte Within the parent sporophyte are the Megasporangia which produce megaspores that give rise to female gametophytes Microsporangia produce microspores that give rise to male gametophytes © 2014 Pearson Education, Inc. 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 sperm into the female gametophyte within the ovule A seed develops from the whole ovule A seed is a sporophyte embryo, along with its food supply (in the endosperm), packaged in a protective seed coat , the integuments © 2014 Pearson Education, Inc. An ovule consists of a megasporangium, megaspore, and one or more protective integuments Immature ovulate cone Integument (2n) Megaspore (n) Female gametophyte (n) Spore wall Egg nucleus (n) Megasporangium (2n) Micropyle Pollen grain (n) (a) Unfertilized ovule Seed coat Spore wall Discharged sperm nucleus (n) Male Pollen tube gametophyte (n) (b) Fertilized ovule Food supply (n) Embryo (2n) (c) Gymnosperm seed Gymnosperm megasporangia have one integument Angiosperm megasporangia usually have two integuments © 2014 Pearson Education, Inc. Concept 30.2: Gymnosperms bear “naked” seeds, typically on cones • Gymnosperms were better suited than nonvascular plants to drier conditions. • Cone-bearing gymnosperms called conifers dominate in the northern latitudes • Gymnosperms means “naked seeds” The seeds are exposed on sporophylls that (typically) form cones. © 2014 Pearson Education, Inc. The Life Cycle of a Pine Three key features of the gymnosperm life cycle are Miniaturization of their gametophytes Development of seeds from fertilized ovules The transfer of sperm to ovules by pollen The life cycle of a pine provides an example © 2014 Pearson Education, Inc. • The pine tree is the sporophyte and produces sporangia in male and female cones. Most have both types of cones • Small cones contain hundreds of microsporangia held on small sporophylls. Each cone produces microspores that develop into pollen grains containing the male gametophyte. • The familiar larger cones contain ovules that undergo meiosis, which produce haploid megaspores that develop into female gametophytes (retained in the sporangia cone) • It takes nearly three years from cone production to mature seed • During pollination, windblown pollen falls on the ovulate cone and grows into the ovule through the micropyle-fertilization follows 9 © 2014 Pearson Education, Inc. Figure 30.4 Ovule Megasporocyte (2n) Ovulate cone Integument Pollen cone Microsporocytes Pollen (2n) grains (n) Mature sporophyte (2n) Megasporangium (2n) MEIOSIS Germinating MEIOSIS pollen grain Microsporangia Microsporangium (2n) Seedling Surviving megaspore (n) Archegonium Seeds Female gametophyte Food reserves Seed coat (2n) Sperm nucleus (n) Pollen tube Embryo (new sporophyte) (2n) © 2014 Pearson Education, Inc. Key FERTILIZATION Egg nucleus (n) Haploid (n) Diploid (2n) Generations and Seed Production: The gymnosperm embryo, the new sporophyte, is surrounded by the female gametophyte which nourishes the embryo. Therefore, The ovule develops into a seed, which consists of an embryo (new sporophyte), its food supply (derived from gametophyte tissue), and a seed coat derived from the integuments of the parent tree (parent sporophyte)-- 3 generations in 1! It takes three years from the appearance of young cones on a tree to the formation of mature seeds. The scales of ovulate cone separate and the seeds are typically dispersed by the wind. 11 © 2014 Pearson Education, Inc. The gymnosperms 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) © 2014 Pearson Education, Inc. Phylum Cycadophyta Individuals have large cones and palmlike leaves Unlike most seed plants, cycads have flagellated sperm These thrived during the Mesozoic, but most of the few surviving species are endangered Cycas revoluta © 2014 Pearson Education, Inc. Figure 30.7b Phylum Ginkgophyta This phylum consists of a single living species, Ginkgo biloba Like the cycads, this group also has flagellated sperm It has a high tolerance to air pollution and is a popular ornamental tree Ginkgo biloba © 2014 Pearson Education, Inc. right picture from nature2health.blogspot.com Phylum Gnetophyta This phylum comprises three genera: Gnetum, Ephedra, and Welwitschia Species vary in appearance, and some are tropical whereas others live in deserts Ovulate cones Welwitschia Gnetum © 2014 Pearson Education, Inc. Ephedra Phylum Coniferophyta • This phylum is by far the largest of the gymnosperm phyla • Most conifers are evergreens and can carry out photosynthesis year round • Named for the cone, which is a cluster of scalelike sporophylls • Needle shaped leaves are adapted to dry conditions as well as more easily shed snow • Waxy cuticles and stomates located in pits help limit water loss • Examples include fir, spruce, hemlock, yew, sequoia, cedar, pine, cypress, tamarack 16 © 2014 Pearson Education, Inc. Douglas fir Sequoia © 2014 Pearson Education, Inc. European larch Figure 30.7e Phylum Coniferophyta Common juniper Wollemi pine © 2014 Pearson Education, Inc. Bristlecone pine Concept 30.3: The reproductive adaptations of angiosperms include flowers and fruits Angiosperms are the most widespread and diverse of all plants. Angiosperms now dominate more terrestrial ecosystems though conifers still dominate in some regions including the northern latitudes. All angiosperms are classified in a single phylum, Anthophyta, from the Greek anthos for flower Angiosperms are seed plants with two key adaptations: Reproductive structures called Flowers and Fruits, the mature ovaries where seeds are © 2014 Pearson Education, Inc. 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 A flower is a specialized shoot with up to four types of modified leaves called floral organs: Sepals, which enclose the flower Petals, which are brightly colored and attract pollinators Stamens, which produce pollen-- A stamen consists of a stalk called a filament, with a sac called an anther where the pollen is produced Carpels, which produce ovules--A carpel consists of an ovary at the base and a style leading up to a stigma, where pollen is received © 2014 Pearson Education, Inc. Figure 30.8 Stigma Stamen Anther Carpel Style Filament Ovary Petal Sepal Ovule Receptacle © 2014 Pearson Education, Inc. Flowers that have all four organs are called complete flowers Those lacking one or more organs are called incomplete flowers Symmetry Flowers may have radial symmetry or bilateral symmetry For flowers with radial symmetry, any imaginary line through the central axis divides the flower into two equal parts In bilateral symmetry, a flower can only be divided into two equal parts by a single imaginary line © 2014 Pearson Education, Inc. Figure 30.9 Location of Stamens and Carpels Symmetry Common holly flowers with stamens Sepal Radial symmetry (daffodil) Fused petals Stamens Carpel Bilateral symmetry (orchid) © 2014 Pearson Education, Inc. Common holly flowers with carpels Nonfunctional stamen Location of Stamens and Carpels Most species have flowers with both functional stamens and carpels, but in some species they occur on separate flowers= monoecious Flowers with stamens may be on the same plant as those with carpels, or they may occur on different plants © 2014 Pearson Education, Inc. Fruits • A fruit is formed when the ovary wall thickens and matures • 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 • Various fruit adaptations help disperse seeds • Examples include burrs, milkweed fuzz, maple helicopters, nuts, pits, and seeds in all different types of fruits • Seeds can be carried by wind, water, or animals to new locations 25 © 2014 Pearson Education, Inc. Figure 30.10 Tomato: a berry Apple: a pome Ruby grapefruit: a hesperidium Nectarine: a drupe Pumpkin: a pepo Hazelnut: a nut © 2014 Pearson Education, Inc. Milkweed: a follicle Figure 30.11 ▼ Seeds provide some evolutionary advantages over spores ▼ They may remain dormant for days to years, until conditions are favorable for germination © 2014 Pearson Education, Inc. Wings ▼ ▼ Seeds have a supply of stored food They may be transported long distances by wind or animals Mechanisms that disperse seeds by explosive action Barbs Seeds within berries and other edible fruits 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 crosspollination between flowers from different plants of the same species 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 © 2014 Pearson Education, Inc. Double fertilization occurs when the 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 central cell of the female gametophyte and initiates development of food-storing endosperm The triploid endosperm nourishes the developing embryo Within a seed, the embryo consists of a root and two seed leaves called cotyledons © 2014 Pearson Education, Inc. Figure 30.12 Carpel Anther Mature flower on sporophyte plant (2n) Germinating seed Nucleus of developing endosperm (3n) Microsporangium Microsporocytes (2n) MEIOSIS Microspore (n) Generative cell Tube cell Ovule with megasporangium Tube nucleus (2n) Male gametophyte (in pollen grain) (n) Ovary Pollen MEIOSIS grains Stigma Pollen tube Megasporangium Surviving Sperm (2n) megaspore Embryo (2n) Integuments (n) Tube Endosperm (3n) nucleus Seed Micropyle Seed coat (2n) Antipodal cells Polar nuclei Style Female in central cell gametophyte (embryo sac) Synergids Egg (n) Zygote (2n) Egg nucleus (n) FERTILIZATION Key Discharged sperm nuclei (n) © 2014 Pearson Education, Inc. Haploid (n) Diploid (2n) Angiosperm Diversity Angiosperms comprise more than 250,000 living species Previously, angiosperms were divided into two main groups Monocots (one cotyledon) Dicots (two dicots) DNA studies suggest that monocots from a clade while dicots are paraphyletic. The clade eudicot (“true” dicots) includes most dicots The rest of the former dicots form several small lineages and include water lilies, magnolias, and laurels © 2014 Pearson Education, Inc. Figure 30.17c Monocots Orchid (Lemboglossum rossii) Pygmy date palm (Phoenix roebelenii) Monocots More than one-quarter of angiosperm species are monocots The largest groups are the orchids, grasses, and palms Barley (Hordeum vulgare), a grass © 2014 Pearson Education, Inc. Figure 30.17d Eudicots Snow pea (Pisum Dog rose (Rosa canina), a wild rose sativum), a legume Pyrenean oak (Quercus pyrenaica) © 2014 Pearson Education, Inc. Eudicots More than two-thirds of angiosperm species are eudicots Eudicots include the large legume family and the economically important rose family Figure 30.16 Embryos Leaf venation One cotyledon Veins usually parallel Stems Roots Pollen Flowers Monocot Characteristics Root system Vascular tissue usually fibrous Pollen grain scattered with one (no main root) opening Floral organs usually in multiples of three Eudicot Characteristics Vascular tissue Two Veins usually usually arranged cotyledons netlike in ring © 2014 Pearson Education, Inc. Taproot Pollen grain (main root) with three usually present openings Floral organs usually in multiples of four or five Evolutionary Links with Animals Animals influence the evolution of plants and vice versa For example, animal herbivory selects for plant defenses For example, interactions between pollinators and flowering plants select for mutually beneficial adaptations Bilateral symmetry affects the movement of pollinators and reduces gene flow in diverging populations Plants with bilateral symmetry may have increased rates of speciation © 2014 Pearson Education, Inc. Figure 30.15 Stamens © 2014 Pearson Education, Inc. Concept 30.4: Human welfare depends on seed plants Seed plants are key sources of food, fuel, wood products, and medicine Our reliance on seed plants makes preservation of plant diversity critical 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 © 2014 Pearson Education, Inc. Table 30.1 © 2014 Pearson Education, Inc. Threats to Plant Diversity • Destruction of habitat is causing extinction of many plant species • In the tropics 55,000 km2 are cleared each year • At this rate, the remaining tropical forests will be eliminated in 200 years • Loss of forests reduces the absorption of atmospheric CO2 that occurs during photosynthesis • 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 • The tropical rain forests may contain undiscovered medicinal compounds 39 © 2014 Pearson Education, Inc. Figure 30.18 A satellite image from 2000 shows clear-cut areas in Brazil surrounded by dense tropical forest. © 2014 Pearson Education, Inc. By 2009, much more of this same5 km tropical forest had been cut down. Figure 30.UN05 Five Derived Traits of Seed Plants Reduced gametophytes Heterospory Microscopic male and female gametophytes (n) are nourished and protected by the sporophyte (2n) Male gametophyte Female gametophyte Microspore (gives rise to a male gametophyte) Megaspore (gives rise to a female gametophyte) Ovules Integument (2n) Ovule (gymnosperm) Megaspore (n) Megasporangium (2n) Pollen Pollen grains make water unnecessary for fertilization Seeds Seeds: survive better than unprotected spores, can be transported long distances © 2014 Pearson Education, Inc. Seed coat Food supply Embryo