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Chapter 29 Plant Diversity I How Plants Colonized Land 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: The Greening of Earth • Looking at a lush landscape – It is difficult to imagine the land without any plants or other organisms Figure 29.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • For more than the first 3 billion years of Earth’s history – The terrestrial surface was lifeless • Since colonizing land – Plants have diversified into roughly 290,000 living species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 29.1: Land plants evolved from green algae • Researchers have identified green algae called charophyceans as the closest relatives of land plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Morphological and Biochemical Evidence • Many characteristics of land plants – Also appear in a variety of algal clades Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • There are four key traits that land plants share only with charophyceans – Rose-shaped complexes for cellulose synthesis Figure 29.2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 30 nm – Peroxisome enzymes – Structure of flagellated sperm – Formation of a phragmoplast Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Evidence • Comparisons of both nuclear and chloroplast genes – Point to charophyceans as the closest living relatives of land plants (a) Chara, a pond organism 10 mm 40 µm Figure 29.3a, b (b) Coleochaete orbicularis, a diskshaped charophycean (LM) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Adaptations Enabling the Move to Land • In charophyceans – A layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out • The accumulation of traits that facilitated survival on land – May have opened the way to its colonization by plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 29.2: Land plants possess a set of derived terrestrial adaptations • Many adaptations – Emerged after land plants diverged from their charophycean relatives Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Defining the Plant Kingdom • Systematists – Are currently debating the boundaries of the plant kingdom Viridiplantae Streptophyta Plantae Red algae Figure 29.4 Chlorophytes Charophyceans Embryophytes Ancestral alga Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Some biologists think that the plant kingdom – Should be expanded to include some or all green algae • Until this debate is resolved – This textbook retains the embryophyte definition of kingdom Plantae Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Derived Traits of Plants • Five key traits appear in nearly all land plants but are absent in the charophyceans – Apical meristems – Alternation of generations – Walled spores produced in sporangia – Multicellular gametangia – Multicellular dependent embryos Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Apical meristems and alternation of generations APICAL MERISTEMS Apical meristem of shoot Developing leaves Apical meristems of plant shoots and roots. The light micrographs are longitudinal sections at the tips of a shoot and root. Apical meristem of root Shoot Root 100 µm 100 µm Haploid multicellular organism (gametophyte) Mitosis Mitosis n n n ALTERNATION OF GENERATIONS Spores n n Gametes MEIOSIS FERTILIZATION 2n Figure 29.5 2n Zygote Mitosis Diploid multicellular organism (sporophyte) Alternation of generations: a generalized scheme Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Walled spores; multicellular gametangia; and multicellular, dependent embryos WALLED SPORES PRODUCED IN SPORANGIA Spores Sporangium Sporophyte and sporangium of Sphagnum (a moss) Longitudinal section of Sphagnum sporangium (LM) Sporophyte Gametophyte MULTICELLULAR GAMETANGIA Female gametophyte Archegonium with egg Antheridium with sperm Archegonia and antheridia of Marchantia (a liverwort) Male gametophyte MULTICELLULAR, DEPENDENT EMBRYOS Embryo and placental transfer cell of Marchantia Figure 29.5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Embryo Maternal tissue 2 µm 10 µm Wall ingrowths Placental transfer cell • Additional derived units – Such as a cuticle and secondary compounds, evolved in many plant species Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Origin and Diversification of Plants • Fossil evidence – Indicates that plants were on land at least 475 million years ago Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Fossilized spores and tissues – Have been extracted from 475-million-year-old rocks (a) Fossilized spores. Unlike the spores of most living plants, which are single grains, these spores found in Oman are in groups of four (left; one hidden) and two (right). (b) Fossilized Figure 29.6 a, b sporophyte tissue. The spores were embedded in tissue that appears to be from plants. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Whatever the age of the first land plants – Those ancestral species gave rise to a vast diversity of modern plants Table 29.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Land plants can be informally grouped – Based on the presence or absence of vascular tissue Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • An overview of land plant evolution Land plants Vascular plants Figure 29.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Angiosperms Origin of seed plants (about 360 mya) Origin of vascular plants (about 420 mya) Origin of land plants (about 475 mya) Ancestral green alga Seed plants Gymnosperms Pterophyte (ferns, horsetails, whisk fern) Seedless vascular plants Lycophytes (club mosses, spike mosses, quillworts) Mosses Hornworts Liverworts Charophyceans Bryophytes (nonvascular plants) • Concept 29.3: The life cycles of mosses and other bryophytes are dominated by the gametophyte stage • Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants – Liverworts, phylum Hepatophyta – Hornworts, phylum Anthocerophyta – Mosses, phylum Bryophyta Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Debate continues over the sequence of bryophyte evolution • Mosses are most closely related to vascular plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bryophyte Gametophytes • In all three bryophyte phyla – Gametophytes are larger and longer-living than sporophytes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The life cycle of a moss Raindrop 1 Spores develop into threadlike protonemata. Key Male gametophyte Haploid (n) Diploid (2n) Sperm “Bud” 2 The haploid protonemata produce “buds” that grow into gametophytes. Protonemata 4 A sperm swims through a film of moisture to an archegonium and fertilizes the egg. Antheridia 3 Most mosses have separate male and female gametophytes, with antheridia and archegonia, respectively. “Bud” Egg Spores Gametophore Female Archegonia spores develop in the sporangium gametophyte of the sporophyte. When the Rhizoid sporangium lid pops off, the peristome “teeth” regulate 6 The sporophyte grows a gradual release of the spores. long stalk, or seta, that emerges Seta from the archegonium. 8 Meiosis occurs and haploid Peristome Sporangium MEIOSIS Mature Mature sporophytes sporophytes Capsule (sporangium) FERTILIZATION (within archegonium) Calyptra Zygote Embryo Foot Archegonium Young sporophyte Capsule with peristome (LM) Figure 29.8 Female gametophytes 5 The diploid zygote develops into a sporophyte embryo within Attached by its foot, the 7 the archegonium. sporophyte remains nutritionally dependent on the gametophyte. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Bryophyte gametophytes – Produce flagellated sperm in antheridia – Produce ova in archegonia – Generally form ground-hugging carpets and are at most only a few cells thick • Some mosses – Have conducting tissues in the center of their “stems” and may grow vertically Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bryophyte Sporophytes • Bryophyte sporophytes – Grow out of archegonia – Are the smallest and simplest of all extant plant groups – Consist of a foot, a seta, and a sporangium • Hornwort and moss sporophytes – Have stomata Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Bryophyte diversity Gametophore of female gametophyte LIVERWORTS (PHYLUM HEPATOPHYTA) Plagiochila deltoidea, a “leafy” liverwort Foot Seta Marchantia sporophyte (LM) HORNWORTS (PHYLUM ANTHOCEROPHYTA) An Anthoceros hornwort species Sporophyte Sporangium 500 µm Marchantia polymorpha, a “thalloid” liverwort MOSSES (PHYLUM BRYOPHYTA) Polytrichum commune, hairy-cap moss Sporophyte Gametophyte Gametophyte Figure 29.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ecological and Economic Importance of Mosses • Sphagnum, or “peat moss” – Forms extensive deposits of partially decayed organic material known as peat – Plays an important role in the Earth’s carbon cycle (a) Peat being harvested from a peat bog (b) Closeup of Sphagnum. Note the “leafy” gametophytes and their offspring, the sporophytes. Gametophyte (c) Sphagnum “leaf” (LM). The combination of living photosynthetic cells and dead water-storing cells gives the moss its spongy quality. Figure 29.10 a–d (d) “Tolland Man,” a bog mummy dating from 405–100 B.C. The acidic, oxygen-poor conditions produced by Sphagnum canpreserve human or other animal bodies for thousands of years. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sporangium at tip of sporophyte Living photo- Dead watersynthetic storing cells 100 µm cells • Concept 29.4: Ferns and other seedless vascular plants formed the first forests • Bryophytes and bryophyte-like plants – Were the prevalent vegetation during the first 100 million years of plant evolution • Vascular plants – Began to evolve during the Carboniferous period Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Origins and Traits of Vascular Plants • Fossils of the forerunners of vascular plants – Date back about 420 million years Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • These early tiny plants – Had independent, branching sporophytes – Lacked other derived traits of vascular plants Figure 29.11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Life Cycles with Dominant Sporophytes • In contrast with bryophytes – Sporophytes of seedless vascular plants are the larger generation, as in the familiar leafy fern – The gametophytes are tiny plants that grow on or below the soil surface Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The life cycle of a fern 1 Sporangia release spores. Most fern species produce a single type of spore that gives rise to a bisexual gametophyte. Key 2 The fern spore develops into a small, photosynthetic gametophyte. 3 Although this illustration shows an egg and sperm from the same gametophyte, a variety of mechanisms promote cross-fertilization between gametophytes. Haploid (n) Diploid (2n) Antheridium Spore MEIOSIS Young gametophyte Sporangium Archegonium Mature sporophyte New sporophyte Sperm Egg Zygote Sporangium FERTILIZATION Sorus 6 On the underside of the sporophyte‘s reproductive leaves are spots called sori. Each sorus is a cluster of sporangia. Gametophyte Fiddlehead Figure 29.12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 A zygote develops into a new sporophyte, and the young plant grows out from an archegonium of its parent, the gametophyte. 4 Fern sperm use flagella to swim from the antheridia to eggs in the archegonia. Transport in Xylem and Phloem • Vascular plants have two types of vascular tissue – Xylem and phloem Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Xylem – Conducts most of the water and minerals – Includes dead cells called tracheids • Phloem – Distributes sugars, amino acids, and other organic products – Consists of living cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolution of Roots • Roots – Are organs that anchor vascular plants – Enable vascular plants to absorb water and nutrients from the soil – May have evolved from subterranean stems Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolution of Leaves • Leaves – Are organs that increase the surface area of vascular plants, thereby capturing more solar energy for photosynthesis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Leaves are categorized by two types – Microphylls, leaves with a single vein – Megaphylls, leaves with a highly branched vascular system Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • According to one model of evolution – Microphylls evolved first, as outgrowths of stems Vascular tissue Figure 29.13a, b (a) Microphylls, such as those of lycophytes, may have originated as small stem outgrowths supported by single, unbranched strands of vascular tissue. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) Megaphylls, which have branched vascular systems, may have evolved by the fusion of branched stems. Sporophylls and Spore Variations • Sporophylls – Are modified leaves with sporangia • Most seedless vascular plants – Are homosporous, producing one type of spore that develops into a bisexual gametophyte Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • All seed plants and some seedless vascular plants – Are heterosporous, having two types of spores that give rise to male and female gametophytes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Classification of Seedless Vascular Plants • Seedless vascular plants form two phyla – Lycophyta, including club mosses, spike mosses, and quillworts – Pterophyta, including ferns, horsetails, and whisk ferns and their relatives Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The general groups of seedless vascular plants LYCOPHYTES (PHYLUM LYCOPHYTA) Strobili (clusters of sporophylls) Isoetes gunnii, a quillwort Selaginella apoda, a spike moss Diphasiastrum tristachyum, a club moss PTEROPHYTES (PHYLUM PTEROPHYTA) Psilotum nudum, a whisk fern Equisetum arvense, field horsetail Athyrium filix-femina, lady fern Vegetative stem Strobilus on fertile stem Figure 29.14 WHISK FERNS AND RELATIVES HORSETAILS Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings FERNS Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts • Modern species of lycophytes – Are relics from a far more eminent past – Are small herbaceous plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives • Ferns – Are the most diverse seedless vascular plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Significance of Seedless Vascular Plants • The ancestors of modern lycophytes, horsetails, and ferns – Grew to great heights during the Carboniferous, forming the first forests Figure 29.15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The growth of these early forests – May have helped produce the major global cooling that characterized the end of the Carboniferous period – Decayed and eventually became coal Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings