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Chapter 29

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Chapter 29

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  1. Chapter 29 Plant Diversity IHow Plants Colonized Land

  2. Figure 29.1 • Overview: The Greening of Earth • Looking at a lush landscape • It is difficult to imagine the land without any plants or other organisms

  3. 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

  4. Concept 29.1: Land plants evolved from green algae • Researchers have identified green algae called charophyceans as the closest relatives of land plants

  5. Morphological and Biochemical Evidence • Many characteristics of land plants • Also appear in a variety of algal clades

  6. 30 nm Figure 29.2 • There are four key traits that land plants share only with charophyceans • Rose-shaped complexes for cellulose synthesis

  7. Peroxisome enzymes • Structure of flagellated sperm • Formation of a phragmoplast

  8. Chara, a pond organism (a) 10 mm 40 µm Coleochaete orbicularis, a disk- shaped charophycean (LM) (b) Figure 29.3a, b Genetic Evidence • Comparisons of both nuclear and chloroplast genes • Point to charophyceans as the closest living relatives of land plants

  9. 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

  10. Concept 29.2: Land plants possess a set of derived terrestrial adaptations • Many adaptations • Emerged after land plants diverged from their charophycean relatives

  11. Viridiplantae Streptophyta Plantae Red algae Chlorophytes Charophyceans Embryophytes Ancestral alga Figure 29.4 Defining the Plant Kingdom • Systematists • Are currently debating the boundaries of the plant kingdom

  12. 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

  13. 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

  14. Developing leaves Apical meristem of shoot Apical meristems of plant shoots and roots. The light micrographs are longitudinal sections at the tips of a shoot and root. APICAL MERISTEMS Apical meristem of root Shoot Root 100 µm 100 µm Haploid multicellular organism (gametophyte) Mitosis Mitosis ALTERNATION OF GENERATIONS n n n n n Spores Gametes MEIOSIS FERTILIZATION 2n 2n Zygote Mitosis Diploid multicellular organism (sporophyte) Figure 29.5 Alternation of generations: a generalized scheme • Apical meristems and alternation of generations Figure 29.5

  15. Walled spores; multicellular gametangia; and multicellular, dependent embryos Spores WALLED SPORES PRODUCED IN SPORANGIA 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 Maternal tissue 2 µm Embryo and placental transfer cell of Marchantia 10 µm Wall ingrowths Figure 29.5 Placental transfer cell

  16. Additional derived units • Such as a cuticle and secondary compounds, evolved in many plant species

  17. The Origin and Diversification of Plants • Fossil evidence • Indicates that plants were on land at least 475 million years ago

  18. (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 sporophyte tissue. The spores were embedded in tissue that appears to be from plants. Figure 29.6 a, b • Fossilized spores and tissues • Have been extracted from 475-million-year-old rocks

  19. Table 29.1 • Whatever the age of the first land plants • Those ancestral species gave rise to a vast diversity of modern plants

  20. Land plants can be informally grouped • Based on the presence or absence of vascular tissue

  21. An overview of land plant evolution Land plants Vascular plants Bryophytes (nonvascular plants) Seedless vascular plants Seed plants Mosses Hornworts Liverworts Angiosperms Gymnosperms Charophyceans Pterophyte (ferns, horsetails, whisk fern) Origin of seed plants (about 360 mya) Lycophytes (club mosses, spike mosses, quillworts) Origin of vascular plants (about 420 mya) Origin of land plants (about 475 mya) Ancestral green alga Figure 29.7

  22. 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

  23. Debate continues over the sequence of bryophyte evolution • Mosses are most closely related to vascular plants

  24. Bryophyte Gametophytes • In all three bryophyte phyla • Gametophytes are larger and longer-living than sporophytes

  25. Raindrop Key Male gametophyte A sperm swims through a film of moisture to an archegonium and fertilizes the egg. Haploid (n) Spores develop into threadlike protonemata. Sperm Diploid (2n) 5 6 3 8 4 2 1 “Bud” Most mosses have separate male and female gametophytes, with antheridia and archegonia, respectively. Antheridia The haploid protonemata produce “buds” that grow into gametophytes. Protonemata “Bud” Egg Spores Gametophore Femalegametophyte Archegonia Meiosis occurs and haploid spores develop in the sporangium of the sporophyte. When the sporangium lid pops off, the peristome “teeth” regulate gradual release of the spores. Peristome Rhizoid The sporophyte grows a long stalk, or seta, that emerges from the archegonium. Sporangium FERTILIZATION Seta MEIOSIS (within archegonium) Capsule(sporangium) Zygote Calyptra Maturesporophytes Mature sporophytes Embryo Foot Archegonium Youngsporophyte The diploid zygote develops into a sporophyte embryo within the archegonium. Femalegametophytes Capsule with peristome (LM) Attached by its foot, the sporophyte remains nutritionally dependent on the gametophyte. 7 Figure 29.8 • The life cycle of a moss

  26. 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

  27. 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

  28. LIVERWORTS (PHYLUM HEPATOPHYTA) Gametophore of female gametophyte Plagiochila deltoidea, a “leafy” liverwort Foot Seta Sporangium Marchantia polymorpha, a “thalloid” liverwort Marchantia sporophyte (LM) 500 µm MOSSES (PHYLUM BRYOPHYTA) HORNWORTS (PHYLUM ANTHOCEROPHYTA) An Anthoceros hornwort species Polytrichum commune, hairy-cap moss Sporophyte Sporophyte Gametophyte Gametophyte Figure 29.9 • Bryophyte diversity

  29. 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 Peat being harvested from a peat bog (a) Sporangium at tip of sporophyte Gametophyte Living photo- synthetic cells Closeup of Sphagnum. Note the “leafy” gametophytes and their offspring, the sporophytes. (b) Dead water- storing cells 100 µm Sphagnum “leaf” (LM). The combination of living photosynthetic cells and dead water-storing cells gives the moss its spongy quality. (c) (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. Figure 29.10 a–d

  30. 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

  31. Origins and Traits of Vascular Plants • Fossils of the forerunners of vascular plants • Date back about 420 million years

  32. Figure 29.11 • These early tiny plants • Had independent, branching sporophytes • Lacked other derived traits of vascular plants

  33. 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

  34. 1 • The life cycle of a fern Sporangia release spores. Most fern species produce a single type of spore that gives rise to a bisexual gametophyte. 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. 2 Key Haploid (n) Diploid (2n) Antheridium Young gametophyte Spore MEIOSIS Sporangium Sperm Archegonium Egg Mature sporophyte New sporophyte Zygote Sporangium FERTILIZATION Sorus On the underside of the sporophyte‘s reproductive leaves are spots called sori. Each sorus is a cluster of sporangia. 6 Fern sperm use flagella to swim from the antheridia to eggs in the archegonia. 4 Gametophyte 5 A zygote develops into a new sporophyte, and the young plant grows out from an archegonium of its parent, the gametophyte. Fiddlehead Figure 29.12

  35. Transport in Xylem and Phloem • Vascular plants have two types of vascular tissue • Xylem and phloem

  36. 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

  37. 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

  38. Evolution of Leaves • Leaves • Are organs that increase the surface area of vascular plants, thereby capturing more solar energy for photosynthesis

  39. Leaves are categorized by two types • Microphylls, leaves with a single vein • Megaphylls, leaves with a highly branched vascular system

  40. Vascular tissue (a) Microphylls, such as those of lycophytes, may have originated as small stem outgrowths supported by single, unbranched strands of vascular tissue. Megaphylls, which have branched vascular systems, may have evolved by the fusion of branched stems. (b) Figure 29.13a, b • According to one model of evolution • Microphylls evolved first, as outgrowths of stems

  41. 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

  42. All seed plants and some seedless vascular plants • Are heterosporous, having two types of spores that give rise to male and female gametophytes

  43. 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

  44. 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 FERNS HORSETAILS WHISK FERNS AND RELATIVES • The general groups of seedless vascular plants

  45. Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts • Modern species of lycophytes • Are relics from a far more eminent past • Are small herbaceous plants

  46. Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives • Ferns • Are the most diverse seedless vascular plants

  47. Figure 29.15 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

  48. 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