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CHAPTER 30

PLANTS AND THE

CONQUEST

OF LAND

Prepared by

Brenda Leady, University of Toledo

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Eukaryotic, primarily photosynthetic organisms that mostly live on land and display many adaptations to life in terrestrial habitats

Most likely evolved from aquatic algal ancestors

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Ancestry

Monophyletic kingdom Probably originated from a single common

protist ancestor Either Chara or Coleochaete are modern

protists most closely related to ancestry of land plants

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10 plant phyla

Liverworts Hornworts Mosses Lycophytes Pteridophytes Cycads Ginkgos Conifers Gnetophytes Angiosperms

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Bryophytes

Include liverworts, hornworts, and mosses Monophyletic phyla Share common structural, reproductive

and ecological features Models of earliest plants

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Bryophytes display features absent from charophycean algae but present in plants

Likely early adaptations to land Charophycean display a zygotic life cycle with a

one cell diploid zygote Bryophytes and other plants exhibit a sporic life

cycle with alternation of generations

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Adaptations to life on land

Sporic life cycle has 2 multicellular life stagesDiploid sporophyte produces haploid spores by

meiosis Spores grow into gametophytes

Haploid gametophyte produces gametes by mitosis

Gametes are nonflagellate eggs and smaller flagellate sperm fuse into single-celled diploid zygotes

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Gametophytes

Produces haploid gametes Gametangia protects developing gametes from

drying out and microbial attack Antheridia – round or elongate gametangia

producing sperm Archegonia – flask shaped gametangia enclosing

an egg Sperm swim to egg and fuse to form diploid zygote Zygotes grow into sporophytes

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Sporophytes

Zygotes remain sheltered and fed within gametophyte tissue

Young sporophytes are embryos When mature, spores are produced in

protective enclosures known as sporangia Plant spore cell walls contain sporopollenin

to help prevent cellular damage During evolution, plant sporophytes become

larger and more complex

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Distinguishing bryophyte features

Gametophytes dominant generation (as opposed to dominant sporophyte generation in other plants)

Sporophytes are dependent on gametophtye and small and short lived (as opposed to independent, large and long-lived in other plants)

Nonvascular or lacking tissues for structural support and conduction found in other plants (vascular plants)

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Lycophytes and pteridophytes

Lycophytes- more numerous and larger in the past but now about 1000 relatively small species

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Pteridophytes – about 12,000 species of ferns, horsetails and whisk ferns

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Diverged prior to the origin of seedsSeedless vascular plantsBryophytes are seedless and nonvascular

Lycophytes, pteridophytes and seed-producing plants are vascular plants or tracheophytesPossess tracheids for water and mineral

conduction and structural support

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Roots, stems and leaves Produce specialized organs

like other tracheophytes Stems

Contain vascular tissue and produce leaves and reproductive structures

Contain phloem and xylem (contains tracheids and lignin)

Roots Specialized for uptake of water

and minerals from the soil Leaves

Photosynthetic function

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Adaptations That Foster Stable Internal Water Content

Waxy cuticle present on most surfaces of vascular plant sporophytes

Cutin found in cuticle that helps prevent pathogen attack

Wax prevents desiccation Stomata are pores that open and close to

allow gas exchange while minimizing water loss

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Gymnosperms

Cycads, ginkgos, conifers and gnetophytes

Reproduce using spores and seeds (like angiosperms)

Seed plants Seeds protect and provide

energy for young sporophyte

“Naked seeds” meaning seeds are not enclosed by fruit

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Angiosperms Distinguished by the

presence of flowers and endosperm

Flowers are specialized to enhance seed production

Fruits develop from flowers and enclose the seed and foster seed dispersal

Endosperm is a nutritive seed tissue with increased storage efficiency

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Evolutionary history of land plants

A billion years ago, terrestrial surface bare Some cyanobacteria crusts Origin of land plants essential to the

development of substantial soils, evolution of modern plants, and animals colonizing land

Living plant phyla reveal the order plants appeared

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Molecular approaches

Compare gene sequences from diverse plants Arrangements of branches on phylogenetic trees

changes as new data becomes available Selection acts on expressed genes, so introns

change more slowly than encoding regions Can reveal ancient phylogenetic divergences Analysis shows pteridophytes to be

monophyletic Horsetails and whisk ferns had been classified

separately based on structural features

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Use of fossils

Tough plant compounds help to preserve plant structures

Compare fossils to other fossils and living plants

Compared modern lycophytes treated to degrade all but the most resistant plant materials (those likely to fossilize) and found similarities with particular fossils

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3 steps to plants conquering land

1. Aquatic charophycean algae give rise to the first land-adapted plants

2. Seedless plants transform Earth’s atmosphere and climate

3. Ancient cataclysm marks the rise of angiosperms

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Algae give rise to land plants

Plants most likely evolved from an aquatic ancestor similar to modern, complex charophycean algae

PhragmoplastDistinctive feature of plant cytokinesisPromotes the development of intercellular

connections (plasmodesmata)Land plants used these traits to build

increasingly more complex bodies better adapted to terrestrial stresses

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Early plants acquired other features in response to life on land, not water

All land plants possess several features not found in charophyceans

All land plants posses xyloglucan carbohydrates that cross-link cellulose microfibrils

The Number of Genes That Controls Cellulose Production Increased During Plant Evolutionary History

Cellulose-rich cell walls are a hallmark of plants and many green algae

Spun from terminal complexes located in plasma membranes (rosettes)

CesA gene encodes cellulose synthase Compared CesA genes of charophycean

algae, seedless plants, and seed plants CesA gene family has diversified by gene

duplication and divergence Correlated with evolution of greater plant

structural complexity

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Seedless plants transformed ecology Liverworts and mosses produce decay-

resistant body tissues Used modern data to estimate ecological

impact of early nonvascular plants Helped enrich soils Could have begun process of organic

carbon burial that helps to reduce amount of greenhouse gas CO2 in the atmosphere

Influences temperature and precipitation

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Modern bryophytes also store CO2

Under cooler than normal conditions, Sphagnum grows more slowly and thus absorbs less CO2, allowing atmospheric CO2 to rise a bit

Since atmospheric CO2 helps to warm Earth’s climate, increasing CO2 warms the climate a little

When the climate warms sufficiently, Sphagnum grows faster, thereby sponging up more CO2 as peat deposits

Reducing atmospheric CO2 returns the climate to slightly cooler conditions

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Ecological effects of vascular plants

First appear 420-430 mya - Coal Age Carboniferous plants converted huge amounts of

atmospheric CO2 into decay-resistant organic material

Carboniferous proliferation of vascular plants was correlated with a dramatic decrease in atmospheric carbon dioxide, which reached a historic low about 300 mya

Atmospheric oxygen levels rose to historic high levels, because less O2 was being used to break down organic carbon into CO2

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Carboniferous decline in CO2 level caused cool, dry conditions to prevail in the late Carboniferous and early Permian period

Abrupt global climate change caused many of the giant lycophytes and pteridophytes that had dominated Carboniferous forests to go extinct

Cooler, drier Permian conditions favored extensive diversification of the first seed plants, the gymnosperms

Seed plants were better able than nonseed plants to reproduce in cooler, drier habitats

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Rise of angiosperms

Diverse gymnosperms dominated Earth’s vegetation through the Mesozoic era (248–65 mya), the Age of Dinosaurs

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One day, about 65 mya, at least one large meteorite or comet crashed near Yucatan Peninsula in Mexico

K/T event marking end of Cretaceous and beginning of Tertiary

Huge amounts of ash, smoke and haze dimmed sunlight long enough to kill many of the world’s plants

Surviving flowering plants diversified into space left

New types of animals also appeared

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Critical innovations in plant evolution

Embryos Leaves Seeds

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Embryo

Absent from charophyceans First distinctive trait acquired by land plants Embryophytes a synonym for plants 3 features

Multicellular and diploid Zygotes and embryos retained Depends on organic and mineral materials supplied

by mother plant – placental transfer tissues

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Placental transfer tissueCells specialized to

promote movement of solute from gametophyte to embryo

Finger-like cell-wall ingrowths

Dissolved sugars, amino acids, and minerals

Browning and Gunning Demonstrated That Placental Transfer Tissues Increase Plant Reproductive Fitness

Placental transfer tissues increase the rate at which radioactively labeled carbon moves through placental transfer tissues from green gametophytes into young sporophytes

In the experiment, shading the young sporophyte from using radiolabeled CO2 – their only nutrition comes from the gametophyte

22% of the organic carbon made by the gametophyte moved to the sporophyte

Much faster than in other plant tissues without transfer tissues

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Leaves

Effectively capture sunlight for photosynthesis Lycophytes produce simplest, most ancient

leaves called lycophylls or microphylla Other vascular plants have leaves with

extensively branched veins – euphylls or megaphylls Larger size provide considerable advantage Evolved in a series of steps

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Seeds

Ovule Sporangium with single spore and a very small egg-

producing gametophyte inside Enclosed by integuments

Seed plants produce 2 distinct types of spores in 2 different types of sporangia Microspores in microsporangia – male gametophytes –

pollen Megaspores in megasporangia – female gametophyte

develops and produces eggs

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Male gametophyte extends pollen tube carrying 2 sperm toward the egg

1 sperm fertilizes egg to become an embryo

Other sperm fuses with different gametophyte tissue to form endosperm

Double fertilization Seeds allow embryos access to food

supplied by older sporophyte generation

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Ecological advantages of seeds

Able to remain dormant in the soil so can wait for favorable conditions

Larger and more complex so resistant to damage and attack

Adaptations to improve dispersal Can store considerable amounts of food Sperm can reach egg without having to swim

through water

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Descent with modification

Seed plants have no replaced spores with seeds Ovules and seeds added to life history including spores Most lycophytes and pteridophytes release one type of

spore and one type of gametophyte Others produce microspores and megaspores

(heterospory) These protected gametophytes grow inside microspore

and megaspore walls – endosporic gametophytes Heterospory advantage to increase cross-fertilization Early steps to seed evolution

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Next step would be retention of megaspores instead of releasing them

Another would be only one megaspore per sporangium

Then retention of megasporangium on parental sporophyte