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Chapter 29, 30 & 31. Plant Diversity I: How Plants Colonized Land Biology in Focus: Ch. 26—The colonization of land by plants and fungi. Key Concepts. Fossils show that plants colonized the land more than 470 million years ago Fungi played an essential role in the colonization of land

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chapter 29 30 31

Chapter 29, 30 & 31

Plant Diversity I: How Plants Colonized Land

Biology in Focus: Ch. 26—The colonization of land by plants and fungi

key concepts
Key Concepts
  • Fossils show that plants colonized the land more than 470 million years ago
  • Fungi played an essential role in the colonization of land
  • Early land plants radiated into a diverse set of lineages
  • Seeds and pollen grains are key adaptations for life on land
  • Land plants and fungi fundamentally changed chemical cycling and biotic interactions
overview the greening of earth
Overview: The Greening of Earth
  • Looking at a lush landscape, it is difficult to imagine the land without any plants or other organisms
  • For more than the first 2 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
concept 1 evidence of algal ancestry
Concept 1: Evidence of Algal Ancestry
  • Green algae called charophytes are the closest relatives of land plants
morphological and molecular evidence
Morphological and Molecular Evidence
  • Many characteristics of land plants also appear in a variety of algal clades, mainly algae
  • However, land plants share four key traits only with charophytes:
    • Rose-shaped complexes for cellulose synthesis
    • Peroxisome enzymes
    • Structure of flagellated sperm
    • Formation of a phragmoplast
slide6
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 charophytes, but share a common ancestor with modern charophytes
slide7

Fig. 29-3

  • Chara species, a pond organism
  • 5 mm
  • Coleochaete orbicularis, a
  • disk-shaped charophyte that
  • also lives in ponds (LM)
  • 40 µm
adaptations enabling the move to land
Adaptations Enabling the Move to Land
  • In charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out
  • The movement onto land by charophyte ancestors provided unfiltered sun, more plentiful CO2, nutrient-rich soil, and few herbivores or pathogens
  • Land presented challenges: a scarcity of water and lack of structural support
slide9
The accumulation of traits that facilitated survival on land may have opened the way to its colonization by plants
  • Systematists are currently debating the boundaries of the plant kingdom
  • Some biologists think the plant kingdom should be expanded to include some or all green algae
  • Until this debate is resolved, we will retain the embryophyte definition of kingdom Plantae
slide10

Fig. 29-4

  • Red algae
  • ANCESTRAL
  • ALGA
  • Chlorophytes
  • Viridiplantae
  • Charophytes
  • Streptophyta
  • Plantae
  • Embryophytes
derived traits of plants
Derived Traits of Plants
  • Four key traits appear in nearly all land plants but are absent in the charophytes:
    • Alternation of generations (with multicellular, dependent embryos)
      • http://education-portal.com/academy/lesson/alternation-of-generations-the-gametophyte-and-sporophyte.html
    • Walled spores produced in sporangia
    • Multicellular gametangia
    • Apical meristems
slide12
Additional derived traits such as a cuticle and secondary compounds evolved in many plant species
  • Symbiotic associations between fungi and the first land plants may have helped plants without true roots to obtain nutrients
alternation of generations and multicellular dependent embryos
Alternation of Generations and Multicellular, Dependent Embryos
  • Plants alternate between two multicellular stages, a reproductive cycle called alternation of generations
  • The gametophyte is haploid and produces haploid gametes by mitosis
  • Fusion of the gametes gives rise to the diploid sporophyte,which produces haploid spores by meiosis
slide14
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
slide15

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
slide16

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)
walled spores produced in sporangia
Walled Spores Produced in Sporangia
  • The sporophyte produces spores in organs called sporangia
  • Spore walls contain sporopollenin, which makes them resistant to harsh environments
slide18

Fig. 29-5c

  • Spores
  • Sporangium
  • Longitudinal section of
  • Sphagnum sporangium (LM)
  • Sporophyte
  • Gametophyte
  • Sporophytes and sporangia of Sphagnum (a moss)
slide19

Fig. 29-5d

  • Archegonium
  • with egg
  • Female gametophyte
  • Antheridium
  • with sperm
  • Male
  • gametophyte
  • Archegonia and antheridia of Marchantia (a liverwort)
apical meristems
Apical Meristems
  • Plants sustain continual growth in their apical meristems
  • Cells from the apical meristems differentiate into various tissues
slide21

Fig. 29-5e

  • Apical meristems
  • Developing
  • leaves
  • Apical
  • meristem
  • of shoot
  • Apical meristem
  • of root
  • Shoot
  • Root
  • 100 µm
  • 100 µm
the origin and diversification of plants
The Origin and Diversification of Plants

(a) Fossilized spores

  • Fossil evidence indicates that plants were on land at least 470 million years ago
  • Fossilized spores and tissues have been extracted from 450-million-year-old rocks

(b) Fossilized

sporophyte tissue

concept 2 essential role of fungi in colonization
Concept 2: Essential role of fungi in colonization
  • How did early land plants obtain nutrients without roots and leaves?
  • Fossil evidence reveals that early land plants formed symbiotic associations with fungi to aid in nutrient absorption from the soil. (mycorrhizae)
overview mighty mushrooms
Overview: Mighty Mushrooms
  • Fungi are diverse and widespread
  • Essential to most terrestrial ecosystems because they break down organic material and recycle vital nutrients
  • Fungi exhibit diverse lifestyles:
    • Decomposers
    • Parasites
    • Mutualists
fungal nutrition
Fungal Nutrition
  • Fungi are heterotrophs that feed by absorption.
  • Fungi use enzymes to break down complex molecules into smaller organic compounds that can be absorbed.
  • Fungi can digest compounds from a wide range of sources, living or dead.
adaptations for feeding by absorption
Adaptations for Feeding by Absorption
  • Most fungi have cell walls made of chitin
  • The most common body structures are multicellular filaments and single cells (yeasts)
  • Morphology enhances absorption. Some species grow as either filaments or yeasts; others grow as both
  • Fungi consist of mycelia, networks of branched hyphae adapted for absorption
specialized hyphae in mycorrhizal fungi
Specialized Hyphae in Mycorrhizal Fungi
  • Some unique fungi have specialized hyphae called haustoriathat allow them to penetrate the tissues of their host
  • Mutually beneficial relationships between such fungi and plant roots are called mycorrhizae(“fungus roots”)
  • Mycorrhizal fungi deliver phosphate ions and other minerals to plants in exchange for organic nutrients such as carbohydrates
slide29

Fig. 31-4b

Plant

cell

wall

Fungal hypha

Plant cell

Plant cell

plasma

membrane

Haustorium

(b) Haustoria

slide30
Two main types of mycorrhizal fungi:
  • Ectomycorrhizal fungi (ektos, out)form sheaths of hyphae over a root and also grow into the extracellular spaces of the root cortex
  • Arbuscularmycorrhizal fungi (arbor, tree) extend hyphae through the cell walls of root cells and into tubes formed by invagination (pushing inward) of the root cell membrane
slide31

Fig. 31-2

Reproductive structure

Hyphae

Spore-producing

structures

20 µm

Mycelium

sexual and asexual reproduction
Sexual and Asexual Reproduction
  • Fungi propagate themselves by producing vast numbers of spores, either sexually or asexually
  • Plasmogamyis the union of two parent mycelia
  • Hours, days, or even centuries may pass before the occurrence of karyogamy, nuclear fusion
  • During karyogamy, the haploid nuclei fuse, producing diploid cells
  • The diploid phase is short-lived and undergoes meiosis, producing haploid spores
slide33

Fig. 31-5-1

Key

Haploid (n)

Heterokaryotic

(unfused nuclei from

different parents)

Diploid (2n)

Spore-producing

structures

Spores

ASEXUAL

REPRODUCTION

Mycelium

GERMINATION

slide34

Fig. 31-5-2

Key

Heterokaryotic

stage

Haploid (n)

Heterokaryotic

(unfused nuclei from

different parents)

PLASMOGAMY

(fusion of cytoplasm)

Diploid (2n)

KARYOGAMY

(fusion of nuclei)

Spore-producing

structures

Zygote

SEXUAL

REPRODUCTION

Spores

ASEXUAL

REPRODUCTION

Mycelium

GERMINATION

slide35

Fig. 31-5-3

Key

Heterokaryotic

stage

Haploid (n)

Heterokaryotic

(unfused nuclei from

different parents)

PLASMOGAMY

(fusion of cytoplasm)

Diploid (2n)

KARYOGAMY

(fusion of nuclei)

Spore-producing

structures

Zygote

SEXUAL

REPRODUCTION

Spores

ASEXUAL

REPRODUCTION

Mycelium

MEIOSIS

GERMINATION

GERMINATION

Spores

asexual reproduction
Asexual Reproduction
  • In addition to sexual reproduction, many fungi can reproduce asexually
  • Molds produce haploid spores by mitosis and form visible mycelia
  • Other fungi that can reproduce asexually are yeasts, which inhabit moist environments
  • Instead of producing spores, yeasts reproduce asexually by simple cell division and the pinching of “bud cells” from a parent cell
slide37

Fig. 31-7

10 µm

Parent

cell

Bud

the origin of fungi
The Origin of Fungi
  • Fungi and animals are more closely related to each other than they are to plants or other eukaryotes
  • DNA evidence suggests that fungi are most closely related to unicellular nucleariids while animals are most closely related to unicellular choanoflagellates
slide39

Fig. 31-8

Animals (and their close

protistan relatives)

UNICELLULAR,

FLAGELLATED

ANCESTOR

Nucleariids

Opisthokonts

Chytrids

Fungi

Other fungi

the move to land
The Move to Land
  • Fungi were among the earliest colonizers of land and probably formed mutualistic relationships with early land plants
  • Among fossil evidence, molecular studies on sym genes have shown evidence of gene conservation for hundreds of millions of years.
  • Molecular analyses have also helped clarify evolutionary relationships among fungal groups, although areas of uncertainty remain
slide41

Fig. 31-11

Hyphae

25 µm

Chytrids (1,000 species)

Zygomycetes (1,000 species)

Fungal hypha

Glomeromycetes (160 species)

Ascomycetes (65,000 species)

Basidiomycetes (30,000 species)