Chapter 29 Evolution of Land Plants. Overview Plants can be described as multicellular, eukaryotic,...

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Chapter 29

Evolution of Land Plants

Overview

Plants can be described as multicellular, eukaryotic, photosynthetic autotrophs

Four main groups: Bryophytes (seedless, non-vascular) Pteridophytes (seedless, vascular) Gymnosperms (plants with “naked” seeds) Angiosperms (flowering plants)

How did they get on land?

Land plants evolved from green algaeMany characteristics of land plants also

appear in a variety of algal cladesHowever, land plants share four key traits

only with charophyceans (Green Algae): Rose-shaped complexes for cellulose synthesis Peroxisome enzymes Structure of flagellated sperm Formation of a phragmoplast

In charophyceans a layer of a durable polymer called sporopollenin Prevents exposed zygotes from desiccating

Accumulation of traits facilitating survival on land may have opened the way to its colonization by plants

Water conservation Cuticle

A waxy layer made of polymers Seals the cell(s)

Stomata Openings on the undersurface of the leaf Allow the passage of CO2 and H2O

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

All plants have a life cycle that consists of two stages (alternation of generations) This is an alternation between two

MULTICELLULAR stages Gametophyte stage (haploid) Sporophyte stage (diploid)

Mitosis

Alternation of Generations

Spores

Mitosis

Zygote

Gametes

Haploid multicellularorganism (gametophyte)

Diploid multicellularorganism (sporophyte)

MEIOSIS FERTILIZATION

Mitosis

Spores

Mitosis

Zygote

Gametes

Mitosis

Spores

Mitosis

Zygote

Gametes

Mitosis

Spores

Mitosis

Zygote

Gametes

Mitosis

Spores

Mitosis

Zygote

Gametes

Bryophytes

Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants: Liverworts, phylum Hepatophyta Hornworts, phylum Anthocerophyta Mosses, phylum Bryophyta

In all three bryophyte phyla, gametophytes are larger and longer-living than sporophytes

Bryophyte Life Cycle

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

Bryophyte Life Cycle

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

Malegametophyte

“Bud” Spores develop intothreadlike protonemata.

Protonemata“Bud”

The haploid protonemata produce “buds” that grow into gametophytes.

Raindrop

Antheridia

Most mosses have separate male and female gametophytes, with antheridia and archegonia, respectively.

Haploid (n)

Diploid (2n)

A sperm swims through a film of moisture to an archegonium and fertilizes the egg.

Archegonia

Rhizoid

Femalegametophyte

GametophoreSpores

Sporangium

Peristome

MEIOSIS

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.

The sporophyte grows a long stalk, or seta, that emerges from the archegonium.

FERTILIZATION

(within archegonium)

Archegonium

Zygote

Embryo

Calyptra

Youngsporophyte

Attached by its foot, the sporophyte remains nutritionally dependent on the gametophyte.

The diploid zygote develops into a sporophyte embryo within the archegonium.

Capsule(sporangium)

Maturesporophytes

Capsule withperistome (SEM)

Femalegametophytes

Ecological/Economic Importance

Sphagnum, or “peat moss,” forms extensive deposits of partially decayed organic material known as peat

Sphagnum plays an important role in the Earth’s carbon cycle

Ancestralgreen alga

Origin of land plants(about 475 mya)

Origin of vascular plants(about 420 mya)

Origin of seed plants(about 360 mya)

Land plants

Vascular plants

Seed plantsSeedless vascular plantsBryophytes

Seedless Vascular Plants

Bryophytes and bryophyte-like plants were the prevalent vegetation during the first 100 million years of plant evolution

Began to diversify during the Carboniferous period

Dominate most landscapes today

Vascular plants have two types of vascular tissue: xylem and phloem

Xylem conducts most of the water and minerals Includes dead cells called tracheids Lignified (strengthened with a polymer called lignin)

Phloem consists of living cells Distributes sugars, amino acids, and other organic products

Evolved roots and leavesThese two adaptations increase the

surface area through which to take in resources.

Seedless Vascular Plant Life Cycle

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

Antheridium

Haploid (n)Diploid (2n)

Younggametophyte

Sporangium

MEIOSIS

FERTILIZATION

Archegonium

Zygote

Newsporophyte

Maturesporophyte

Sporangium

Gametophyte

Fiddlehead

Sporophylls are modified leaves with sporangia Most seedless vascular plants are

homosporous, producing one type of spore that develops into a bisexual gametophyte

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

Sporophylls are modified leaves with sporangia Most seedless vascular plants are

homosporous, producing one type of spore that develops into a bisexual gametophyte

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

These adaptations have allowed seedless vascular plants to grow to greater heights and to take advantage of new environments Accelerated photosynthesis

Increased removal of CO2

Formed the first forests Swamp lands were created

• Formed thick layers of peat• Turned to coal over millions of years

Chapter 29 Evolution of Land Plants. Overview Plants can be described as multicellular, eukaryotic,...

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