Chapter 25

Seedless PlantsChapter 25

Origin of Land Plants

• All green algae and the land plants shared a common ancestor a little over 1 BYA– Kingdom Viridiplantae – Not all photoautotrophs are plants

• Red and brown algae are photoautotrophs but not classified as plants

• A single species of freshwater green algae gave rise to the entire terrestrial plant lineage

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Ancestral alga

Chlorophytes Charophytes Liverworts HornwortsMosses Lycophytes Gymnosperms Angiosperms

Seed plantsEuphyllophytes

Bryophytes

Land plantsStreptophyta

Green plants

Green algaeGreen algae

Red Algae

Tracheophytes

Ferns + Allies

Focus on Land Plants

•Adaptations to terrestrial life – Protection from desiccation (“drying out”)

• all land plants have at least some adaptations for this!– Alternation of generations

• all land plants have this lifecycle!

– Vascular tissue allows plants to grow taller• Xylem and phloem to conduct water and food• Only “vascular” plants have this!

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Spore

Spore

nn

nn

Spore mother cell

2n

Sporangia

Sporophyte(2n)

2n

2n Zygote

Embryo

Egg

Sperm

MEIOSIS

MITOSIS

FERTILIZATIONn

2n

Gametophyte(n)

Alternation of generations (also called the haplodiplontic cycle)

• Multicellular diploid stage– The whole plant in this stage is called the sporophyte– All the cells in the sporophyte plant are diploid at first.– A few cells called sporocytes are located in the organ

called the sporangium– Only the sporocytes go through meiosis to produce haploid

cells called spores– The sporocytes are found in the sporangium. When

meiosis is complete, the spores (daughter cells of meiosis) are still in the sporangium.

– Then the sporangium cracks open and releases the spores

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© Edward S. Ross

Sporophyte

Gametophyte


n2n

2n

2n

1n

1n

Sperm

Sporangium

Antheridia

Egg

Archegonia

Gametophytes

Spores

Rhizoids

Female

Male

Zygote

MITOSIS

FERTILIZATION

Maturesporophyte

Developingsporophyte inarchegonium

Parentgametophyte

MITOSIS

MIEIO

SIS

Germinatingspores

Notice that the words “leaf,” stem,” and “root” are not used with bryophytes

• Multicellular haploid stage – The plant in this stage is called the gametophyte– When a haploid spore is released from the sporangium, it

lands on moist soil and germinates (ie it divides by mitosis) until the whole gametophyte is grown

– A few cells in the organ called the gametangium go through a “special” mitosis to form gametes.

– Sperm will leave their gametangium and swim to the eggs in the female gametangium

– Gametes fuse to form diploid zygote– Zygotes grows by mitosis into the next sporophyte plant

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Alternation of generations (also called the haplodiplontic cycle)

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© Edward S. Ross

Sporophyte

Gametophyte


n2n

2n

2n

1n

1n

Sperm

Sporangium

Antheridia

Egg

Archegonia

Gametophytes

Spores

Rhizoids

Female

Male

Zygote

MITOSIS

FERTILIZATION

Maturesporophyte

Developingsporophyte inarchegonium

Parentgametophyte

MITOSIS

MIEIO

SIS

Germinatingspores

Notice that the words “leaf,” stem,” and “root” are not used with bryophytes

Bryophytes – seedless nontracheophytes• Closest living descendants

of the first land plants• Called “nontracheophytes”

because they lack tracheids– (often called “nonvascular,”

but that’s not exactly accurate)

• Symbiotic relationships between fungi and plants are important for absorption

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Cha

roph

ytes

Live

rwor

ts

Mos

ses

Hor

nwor

ts

Trac

heop

hyte

s

• Simple, but highly adapted to diverse terrestrial environments

• 24,700 species in 3 clades

– Liverworts

– Mosses

– Hornworts

• Gametophyte – the most noticeable part, and the photosynthetic part– Sporophytes are small and dependent

• Require water for sexual reproduction11

Tracheophyte Plants

• Cooksonia, the first vascular land plant– Appeared about 420 MYA

• Only a few centimeters tall– No roots or leaves– Homosporous – only 1 type of

spore

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SporangiaCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Vascular tissues• Xylem

– Consists of dead cells called tracheids that form a tube– Conducts water and dissolved minerals upward from

the roots• Phloem

– Consists of living cells– Conducts sucrose and hormones throughout the plant

• Vascular tissues develop in sporophyte but not gametophyte, so gametophyte is smaller

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Tracheophytes• Vascular plants include seven extant phyla

grouped in three clades1. Lycophytes (club mosses) 2. Pterophytes (ferns, whisk ferns, and horsetails)3. Seed plants

• Gametophyte has been reduced in size relative to the sporophyte during the evolution of tracheophytes

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• 40 million years between appearance of vascular tissue and true leaves– Stems, then roots

• Provide transport and support

– Then leaves• Increase surface area for photosynthesis

• Seeds - didn’t evolve until Mesozoic Era– Highly resistant to desiccation– Contain an embryo plus a food supply– Lycophytes and pterophytes do not have seeds

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Ancestral alga

Chlorophytes Charophytes Liverworts HornwortsMosses Lycophytes Gymnosperms Angiosperms

Chlorophyll a and bPlasmodesmata

CuticleAntheridia and archegoniaMulticellular embryo

Stomata

EuphyllsSeeds

FlowersFruits

Dominant sporophyteStems, roots, leaves

Ferns + Allies

Vascular tissue

Bryophytes are seedless nontracheophytes

Club mosses and ferns are seedless tracheophytes

Antheridia are the male gametangia; archegonia are the female gametangia

Lycophytes – misnamed “club moss”

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Pterophytes•Whisk ferns

•Horsetails

•Ferns (see next slide)

Seedless Tracheophytes

Ferns• Most abundant group of

seedless vascular plants– About 11,000 species

• Coal formed from forests 300 mya

• The larger sporophyte and much smaller gametophyte are both photosynthetic

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MEIOSIS

n

2n

Archegonium

Archegonium

Antheridium

Antheridium

Egg

Sperm

Embryo

1n

Gametophyte

Rhizome

Sporangium

Spores

Rhizoids

Gametophyte

MITOSIS

Undersideof leaf frond

Maturesporangium

Sorus (clusterof sporangia)

Adultsporophyte

Maturefrond

Leaf of youngsporophyte

MITOSIS

Zygote2n

FERTILIZATION

Fern reproduction• Produce distinctive sporangia in clusters

called “sori” on the back of the fronds• Diploid spore mother cells (also called

sporocytes) in sporangia produce haploid spores by meiosis

• Every spore germinates into bisexual gametophyte (contains archegonia and antheridia on same gametophyte plant)

• Flagellated sperm21

Education

Chapter 25