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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
How did they get on land?
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
How did they get on land?
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
How did they get on land?
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
Mitosis
Zygote
Gametes
Haploid multicellularorganism (gametophyte)
Diploid multicellularorganism (sporophyte)
MEIOSIS FERTILIZATION
Mitosis
Alternation of Generations
Spores
Mitosis
Mitosis
Zygote
Gametes
Haploid multicellularorganism (gametophyte)
Diploid multicellularorganism (sporophyte)
MEIOSIS FERTILIZATION
Mitosis
Alternation of Generations
Spores
Mitosis
Mitosis
Zygote
Gametes
Haploid multicellularorganism (gametophyte)
Diploid multicellularorganism (sporophyte)
MEIOSIS FERTILIZATION
Mitosis
Alternation of Generations
Spores
Mitosis
Mitosis
Zygote
Gametes
Haploid multicellularorganism (gametophyte)
Diploid multicellularorganism (sporophyte)
MEIOSIS FERTILIZATION
Mitosis
Alternation of Generations
Spores
Mitosis
Mitosis
Zygote
Gametes
Haploid multicellularorganism (gametophyte)
Diploid multicellularorganism (sporophyte)
MEIOSIS FERTILIZATION
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
Sperm
Antheridia
Most mosses have separate male and female gametophytes, with antheridia and archegonia, respectively.
Egg
Haploid (n)
Diploid (2n)
Key
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)
Seta
Foot
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
Liv
erw
ort
s
Ho
rnw
ort
s
Mo
ss
es
Ly
co
ph
yte
s
Pte
rop
hy
tes
Gy
mn
o-
sp
erm
s
An
gio
-s
pe
rms
Ch
aro
ph
yc
ea
ns
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
Seedless Vascular Plants
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
Seedless Vascular Plants
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
Seedless Vascular Plant Life Cycle
Spore
Sperm
Antheridium
Egg
Haploid (n)Diploid (2n)
Key
Younggametophyte
Sorus
Sporangium
MEIOSIS
FERTILIZATION
Archegonium
Zygote
Newsporophyte
Maturesporophyte
Sporangium
Gametophyte
Fiddlehead
Seedless Vascular Plant Life Cycle
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
Seedless Vascular Plant Life Cycle
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
Seedless Vascular Plants
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