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Chapter 29

Chapter 29. Plant Diversity I: How Plants Colonized Land. Overview: The Greening of Earth. 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

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Chapter 29

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  1. Chapter 29 Plant Diversity I:How Plants Colonized Land

  2. Overview: The Greening of Earth • 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 • Plants supply oxygen and are the ultimate source of most food eaten by land animals

  3. Fig. 29-1

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

  5. Morphological and Molecular Evidence • Many characteristics of land plants also appear in a variety of protists, mainly algae • Protists include: • Plants, animals, fungi, & algae • However, land plants share four key traits only with charophytes (green algae): • Rose-shaped complexes for cellulose synthesis • Peroxisome enzymes • Structure of flagellated sperm • Formation of a phragmoplast

  6. Morphological and Molecular Evidence • Phragmoplast: • Microtubules connecting the two daughter nuclei of a dividing cell • A cell plate develops in the middle of the phragmoplast across the midline of the dividing cell • A cell plate then gives rise to new cross wall (new cell wall) that separates the daughter cells

  7. Fig. 29-2 30 nm

  8. Comparisons of both nuclear and chloroplast genes point to: • Charophytes as the closest living relatives of land plants • Note that: • Land plants are not descended from modern/living charophytes, but • Share a common ancestor with modern charophytes

  9. Fig. 29-3 Chara species, a pond organism 5 mm Coleochaete orbicularis, a disk-shaped charophyte that also lives in ponds (LM) 40 µm

  10. Adaptations Enabling the Move to Land • Sporopollenin: • A durable polymer layer in charophytes protecting exposed zygotes from drying out • The movement onto land provided: • Unfiltered sun • More plentiful CO2 • Nutrient-rich soil • Few herbivores or pathogens • Land also presented challenges: • Scarcity of water • Lack of structural support (e.g gravity)

  11. Accumulation of traits facilitating survival on land has triggered land colonization by plants • Systematists currently debating the boundaries of the plant kingdom • Some think the plant kingdom should be expanded to include some or all the green algae • Until this debate is resolved, embryophyte ( plants with embryos) will be retained as the definition of the kingdom Plantae

  12. Derived Traits of Plants • Four key traits appear in nearly all land plants but are absent in the charophytes: • Alternation of generations • Walled spores produced in sporangia • Multicellular gametangia • Apical meristems

  13. Additional derived traits evolved in many plant species such as: • A cuticle (epidermal waxy covering) • Secondary compounds (bitter protective comp) • Symbiotic associations: • Between fungi and the first land plants • May have helped plants without true roots to obtain nutrients

  14. Alternation of Generations • Plants alternate between two multicellular stages, • This is a reproductive cycle called alternation of generations: • The gametophyte is haploid and produces haploidgametes by mitosis • The sporophyte is diploid, resulting from the fusion of the gametes, and produces haploidsporesby meiosis

  15. Fig. 29-5a Gamete from another plant Gametophyte (n) Mitosis Mitosis n n n n Spore Gamete MEIOSIS FERTILIZATION Zygote 2n Mitosis Sporophyte (2n) Alternation of generations

  16. Multicellular, Dependent Embryos • The diploid embryo is retained within the tissue of the female gametophyte • Nutrients are transferred from parent to embryo through placental transfer cells • Land plants are called embryophytes because of the dependency of the embryo on the parent

  17. Fig. 29-5b Embryo 2 µm Maternal tissue Wall ingrowths 10 µm Placental transfer cell (outlined in blue) Embryo (LM) and placental transfer cell (TEM) of Marchantia (a liverwort)

  18. Walled Spores Produced in Sporangia • The sporophyte produces spores in organscalled sporangia • Diploid cells called sporocytesundergo meiosis to generate haploid spores • Spore walls contain sporopollenin, which makes them resistant to harsh environments

  19. Fig. 29-5c Spores Sporangium Longitudinal section of Sphagnum sporangium (LM) Sporophyte Gametophyte Sporophytes and sporangia of Sphagnum (a moss)

  20. MulticellularGametangia • Gametes are produced within organs called gametangia • Female gametangia, called archegonia,produce eggs and are the site of fertilization • Male gametangia, called antheridia, are the site of sperm production and release

  21. Fig. 29-5d Archegonium with egg Female gametophyte Antheridium with sperm Male gametophyte Archegonia and antheridia of Marchantia (a liverwort)

  22. Apical Meristems • Plants sustain continual growth in their apical meristems • Cells from the apical meristems differentiate into various tissues

  23. Fig. 29-5e Apical meristems Developing leaves Apical meristem of shoot Apical meristem of root Shoot Root 100 µm 100 µm

  24. The Origin and Diversification of Plants • Fossil evidence indicates that plants were on land at least 475 million years ago • Fossilized spores and tissues have been extracted from 475-million-year-old rocks

  25. Fig. 29-6 (a) Fossilized spores (b) Fossilized sporophyte tissue

  26. Whatever the age of the first land plants: • Those ancestral species gave rise to a vast diversity of modern plants

  27. Based on the presence or absence of vascular tissue, land plants are informally grouped into: • Nonvascular plants: • Are commonly called bryophytes • Vascular plants: • Most plants have vascular tissue

  28. Seedless Vascular Plants • Seedless vascular plants can be divided into: • Lycophytes (club mosses and their relatives) • Pterophytes (ferns and their relatives) • Seedless vascular plants are: • Paraphyletic (not monophyletic=not a clade=don’t share a common ancestor) , • Are of the same level of biological organization, & called grade (instead of a clade)

  29. Vascular Plants with seeds • A seed is an embryo & nutrients surrounded by a protective coat • Seed plants (a clade of vascular plants): • Are the vast majority of living plant species • Divided into further clades based on where seeds mature, : • Gymnosperms: seed maturation is not in enclosed chambers; ex. conifers • Angiosperms: seeds mature in enclosed chambers called ovaries; ex. all flowering plants

  30. Fig. 29-7 Origin of land plants (about 475 mya) 1 Origin of vascular plants (about 420 mya) 2 Origin of extant seed plants (about 305 mya) 3 Liverworts Nonvascular plants (bryophytes) Land plants Hornworts ANCES- TRAL GREEN ALGA 1 Mosses Lycophytes (club mosses, spike mosses, quillworts) Seedless vascular plants Vascular plants 2 Pterophytes (ferns, horsetails, whisk ferns) Gymnosperms 3 Seed plants Angiosperms 50 500 450 400 0 350 300 Millions of years ago (mya)

  31. Mosses and other nonvascular plants have life cycles dominated by gametophytes • Bryophytes (non-vascular plants): • Represented today by three phyla of small herbaceous (nonwoody) plants: • Liverworts, phylum Hepatophyta • Hornworts, phylum Anthocerophyta • Mosses, phylum Bryophyta • Mosses are most closely related to vascular plants

  32. Fig. 29-UN1 Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms

  33. Bryophyte Gametophytes • Compared to sporophytes in all three bryophyte phyla, gametophytes are: • Larger • Live longer • Sporophytes are typically present only part of the time

  34. Fig. 29-8-3 Raindrop Sperm “Bud” Antheridia Male gametophyte (n) Key Haploid (n) Protonemata (n) Diploid (2n) “Bud” Egg Gametophore Spores Archegonia Female gametophyte (n) Spore dispersal Rhizoid Peristome FERTILIZATION Sporangium (within archegonium) MEIOSIS Seta Zygote (2n) Capsule (sporangium) Mature sporophytes Foot Embryo Archegonium Young sporophyte (2n) 2 mm Female gametophytes Capsule with peristome (SEM)

  35. Fig. 29-8a 2 mm Capsule with peristome (SEM)

  36. A spore germinates into a gametophyte , which is composed of: • A protonema (green, branched one-cell filaments) • Gametophore(s) (gamete-producing structure (s)) • Rhizoids (tubular/filament one-cell structures): • anchor gametophytes to substrate

  37. The height of gametophytes is constrained by lack of vascular tissues • Mature gametophytes produce: • Flagellated sperm in antheridia, and • An egg in each archegonium • Sperm swim through a film of water to reach and fertilize the egg

  38. Bryophyte Sporophytes • Bryophyte sporophytes: • Green & photosynthetic when young, but can’t live independantly • Remain attached to parent gametophyte • The smallest & simplest sporophytes of all extant plant groups

  39. Bryophyte Sporophytes • A sporophyte consists of a: • Foot: (embeded in archegonium; absorbs nutrient from gametophyte) • Seta: (stalk; conduct nutrients to sporangium) • Sporangium, also called a capsule:(produces spores & discharges them through a peristome • Peristome: (a ring of interlocking tooth-like structures for spore discharge) • Some sporophytes have stomata (pores for gas”O2&Co2” exchange & water evaporation)

  40. Fig. 29-9a Gametophore of female gametophyte Thallus Sporophyte Foot Seta Capsule (sporangium) Marchantia polymorpha, a “thalloid” liverwort 500 µm Marchantia sporophyte (LM)

  41. Fig. 29-9b Plagiochila deltoidea, a “leafy” liverwort

  42. Fig. 29-9c An Anthoceros hornwort species Sporophyte Gametophyte

  43. Fig. 29-9d Polytrichum commune, hairy-cap moss Sporophyte (a sturdy plant that takes months to grow) Capsule Seta Gametophyte

  44. Fig. 29-11a (a) Peat being harvested

  45. Fig. 29-11b (b) “Tollund Man,” a bog mummy

  46. Seedless Vascular Plants (Ferns and other) were the first plants to grow tall • Bryophytes and bryophyte-like plants: • The prevalent vegetation during the first 100 million years of plant evolution • Vascular plants: • Began to diversify during the Devonian and Carboniferous periods • Vascular tissue allowed them to grow tall • Seedless vascular plants have: • Flagellated sperm • Usually restricted to moist environments

  47. Fig. 29-UN2 Nonvascular plants (bryophytes) Seedless vascular plants Gymnosperms Angiosperms

  48. Origins and Traits of Vascular Plants • Fossils of the forerunners of vascular plants date back about 420 million years • These early tiny plants had independent, branching sporophytes • Living vascular plants are characterized by: • Life cycles with dominant sporophytes • Vascular tissues called xylem and phloem • Well-developed roots and leaves

  49. Fig. 29-12 Sporophytes of Aglaophyton major

  50. 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 Animation: Fern Life Cycle

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