Land plants evolved from green algae

• Green algae called charophyceans are the closest relatives of land plants

• Comparisons of both nuclear and chloroplast genes point to charophyceans as the closest living relatives of land plants

Morphological and Biochemical 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 charophyceans:

– Rose-shaped complexes for cellulose synthesis

– Peroxisome enzymes

– Structure of flagellated sperm

– Formation of a phragmoplast

10 mm

40 µm

Chara,a pondorganism(LM).

Coleochaete orbicularis, a disk-shaped charophycean (LM).

Adaptations Enabling the Move to Land

• In charophyceans a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out

• The accumulation of traits that facilitated survival on land may have opened the way to its colonization by plants

• Many adaptations emerged after land plants diverged from their charophycean relatives

Defining the Plant Kingdom

• 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

Viridiplantae

Streptophyta

Plantae

Red algae Chlorophytes Charophyceans Embryophytes

Ancestral alga

Derived Traits of Plants

• 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

ApicalMeristemof shoot

Developingleaves

Shoot Root

Apical meristem

100 µm 100 µm

Apical Meristems

Mitosis

Alternation of Generations

Spores

Mitosis

Mitosis

Zygote

Gametes

Haploid multicellularorganism (gametophyte)

Diploid multicellularorganism (sporophyte)

MEIOSIS FERTILIZATION

Walled SporesProduced in Sporangia

MulticellularGametangia

Multicellular,Dependent Embryos

Longitudinal section ofSphagnum sporangium (LM)

Spores

Sporangium

Sporophyte

Gametophyte

Sporophyte and sporangiumof Sphagnum (a moss)

Archegonia and antheridiaof Marchantia (a liverwort)

Male gametophyte Antheridiumwith sperm

Female gametophyte

Archegoniumwith egg

Maternaltissue

Embryo

2 µm 10 µm

Wallingrowths

Placentaltransfercell

• Additional derived traits such as a cuticle and secondary compounds evolved in many plant species

The Origin and Diversification of Plants

• Fossil evidence indicates that plants were on land at least 475 million years ago

• Fossilized spores and tissues have been extracted from 475-million-year-old rocks

• Those ancestral species gave rise to a vast diversity of modern plants

• Land plants can be informally grouped based on the presence or absence of vascular tissue

Fossilized spores. Unlike the spores of most living plants, which are single grains, these spores found in Oman are in groups of four (left; one hidden) and two (right).

Fossilized sporophyte tissue. The spores were embedded in tissue that appears to be from plants.

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

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Ho

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ort

s

Mo

ss

es

Ly

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Pte

rop

hy

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Gy

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An

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Ch

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The life cycles of mosses and other bryophytesare dominated by the gametophyte stage

• Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants:

– Liverworts, phylum Hepatophyta

– Hornworts, phylum Anthocerophyta

– Mosses, phylum Bryophyta

– Mosses are most closely related to vascular plants

Bryophyte Gametophytes

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

• 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

• Some mosses have conducting tissues in the center of their “stems” and may grow vertically

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

Bryophyte Sporophytes

• 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

• Hornwort and moss sporophytes have stomata

Gametophore offemale gametophyte

Marchantia polymorpha,a “thalloid” liverwort

Foot

Seta

Sporangium

500 µm

Marchantia sporophyte (LM)

Plagiochiladeltoidea,a “leafy”liverwort

An Anthroceroshornwort species

Sporophyte

Gametophyte

Polytrichumcommune,hairy capmoss

Sporophyte

Gametophyte

Ecological and Economic Importance of Mosses

• 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

A peat bog.

Gametophyte Sporangium attip of sporophyte

Livingphoto-syntheticcells

Dead water-storing cells

100 µm

Closeup of Sphagnum.Note the “leafy”Gametophytes and their offspring, the sporophytes.

Sphagnum “leaf” (LM). The combination of living photosynthetic cells and dead water-storing cells gives the moss its spongy quality.

“Tolland Man,” a bog mummy dating from 405–100 B.C. The acidic, oxygen-poor conditions produced by Sphagnum can preserve human or animal bodies for thousands of years.

Ferns and other seedless vascular plants formed the first forests

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

• Vascular plants began to diversify during the Carboniferous period

• Vascular plants dominate most landscapes today

Origins and Traits of Vascular Plants

• Fossils of the forerunners of vascular plants date back about 420 million years

• These early tiny plants had independent, branching sporophytes

• They lacked other derived traits of vascular plants

Life Cycles with Dominant Sporophytes

• 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

Spore

Sperm

Antheridium

Egg

Haploid (n)Diploid (2n)

Key

Younggametophyte

Sorus

Sporangium

MEIOSIS

FERTILIZATION

Archegonium

Zygote

Newsporophyte

Maturesporophyte

Sporangium

Gametophyte

Fiddlehead

Transport in Xylem and Phloem

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

• Xylem conducts most of the water and minerals and includes dead cells called tracheids

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

Evolution of Roots

• Roots are organs that anchor vascular plants

• They enable vascular plants to absorb water and nutrients from the soil

• Roots may have evolved from subterranean stems

Evolution of Leaves

• Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis

• Leaves are categorized by two types:

– Microphylls, leaves with a single vein

– Megaphylls, leaves with a highly branched vascular system

• According to one model of evolution, microphylls evolved first, as outgrowths of stems

Vascular tissue

Microphylls

Megaphylls

Sporophylls and Spore Variations

• 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

Classification of Seedless Vascular Plants

• There are two phyla of seedless vascular plants:

– Lycophyta includes club mosses, spike mosses, and quillworts

– Pterophyta includes ferns, horsetails, and whisk ferns and their relatives

Selaginella apoda,a spike moss

Isoetesgunnii,a quillwort

Diphasiastrum tristachyum, a club moss

Strobili(clusters ofsporophyllis)

Psilotumnudum, a whiskfern

Equisetumarvense, fieldhorsetail

Vegetative stem

Strobilus onfertile stem

Athyrium filix-femina, lady fern

Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts

• Giant lycophytes thrived for millions of years in moist swamps

• Surviving species are small herbaceous plants

Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives

• Ferns are the most diverse seedless vascular plants, with more than 12,000 species

• They are most diverse in the tropics but also thrive in temperate forests

• Some species are even adapted to arid climates

The Significance of Seedless Vascular Plants

• The ancestors of modern lycophytes, horsetails, and ferns grew to great heights during the Carboniferous, forming the first forests

• These forests may have helped produce the global cooling at the end of the Carboniferous period

• The decaying plants of these Carboniferous forests eventually became coal

Animations and Videos

• Life Cycle of a Moss – 1

• Life Cycle of a Moss – 2

• Life Cycle of a Fern

• Chapter Quiz Questions – 1

• Chapter Quiz Questions - 2

Which of the following are the closest algal relatives of land plants?

• psilophytes

• charophytes

• chrysophytes

• bacillariophytes

• rhodophytes

Which of the following are the closest algal relatives of land plants?

• psilophytes

• charophytes

• chrysophytes

• bacillariophytes

• rhodophytes

The relationship between a gametophyte and a sporophyte in a liverwort is like the relationship between

• a brother and a sister.

• a grandparent and a grandchild.

• an uncle and a nephew.

• a parent and a child.

• two cousins.

The relationship between a gametophyte and a sporophyte in a liverwort is like the relationship between

• a brother and a sister.

• a grandparent and a grandchild.

• an uncle and a nephew.

• a parent and a child.

• two cousins.

Plants that evolved vascular tissue are more advanced than plants without vascular tissue. One of the consequences is that vascular tissue enabled plants to

• reproduce via spores.

• store water.

• grow taller.

• develop stomata.

• support large gametophytes.

Plants that evolved vascular tissue are more advanced than plants without vascular tissue. One of the consequences is that vascular tissue enabled plants to

• reproduce via spores.

• store water.

• grow taller.

• develop stomata.

• support large gametophytes.

One thing you should be able to conclude from this figure is that

• gametophytes have fewer chromosomes than sporophytes do.

• gametophytes evolved before sporophytes.

• gametophytes grow from sporophytes.

• gametophyte cells come about by mitosis; sporophyte cells come about by meiosis.

One thing you should be able to conclude from this figure is that

• gametophytes have fewer chromosomes than sporophytes do.

• gametophytes evolved before sporophytes.

• gametophytes grow from sporophytes.

• gametophyte cells come about by mitosis; sporophyte cells come about by meiosis.

Stomata are found in every group of sporophyte plants except the liverworts. According to the hypothesis that stomata evolved only once among the bryophytes, this is evidence that

• liverworts resemble the most primitive plants.

• liverworts don’t need to exchange gases with the atmosphere.

• liverworts have lost the ability to make stomata.

• liverworts are able to fix nitrogen.

• gametophytes are more important in liverworts.

Stomata are found in every group of sporophyte plants except the liverworts. According to the hypothesis that stomata evolved only once among the bryophytes, this is evidence that

• liverworts resemble the most primitive plants.

• liverworts don’t need to exchange gases with the atmosphere.

• liverworts have lost the ability to make stomata.

• liverworts are able to fix nitrogen.

• gametophytes are more important in liverworts.

Which of the following plants have a sporophyte that is nutritionally dependent on the photosynthetic gametophyte?

• ferns

• mosses

• whisk ferns

• horsetails

Which of the following plants have a sporophyte that is nutritionally dependent on the photosynthetic gametophyte?

• ferns

• mosses

• whisk ferns

• horsetails

Which of the following evolutionary innovation of seed plants enabled them to outcompete ferns and other seedless plants that dominated through the end of the Carboniferous period?

• heterospory

• reduced, dependent gametophytes

• vascular systems

• flowers

Which of the following evolutionary innovation of seed plants enabled them to outcompete ferns and other seedless plants that dominated through the end of the Carboniferous period?

• heterospory

• reduced, dependent gametophytes

• vascular systems

• flowers

a) early forests contributed to a large drop in CO2 levels.

b) CO2 removed from the air was stored in marine rocks.

c) seedless vascular plants forming the first forests became coal.

d) plants reduce the rate at which chemicals such as calcium and magnesium are released from socks into soil.

Seedless vascular plants affected Earth and its other life in all of the following ways except

a) early forests contributed to a large drop in CO2 levels.

b) CO2 removed from the air was stored in marine rocks.

c) seedless vascular plants forming the first forests became coal.

d) plants reduce the rate at which chemicals such as calcium and magnesium are released from socks into soil.

Seedless vascular plants affected Earth and its other life in all of the following ways except

a) liverworts

b) mosses

c) hornworts

d) bryophytes

e) lycophytes

Which of the following are not considered a nonvascular plant?

a) liverworts

b) mosses

c) hornworts

d) bryophytes

e) lycophytes

Which of the following are not considered a nonvascular plant?

a) Sporophylls—modified leaves that bear sporangia

b) Strobili—cone-like structures formed from groups of sporophylls in lycophytes and gymnosperms

c) Sori—clusters of sporangia produced by fern sporophylls

d) Megaspores—develop into male gametophytes

Which term is correctly matched with its definition?

a) Sporophylls—modified leaves that bear sporangia

b) Strobili—cone-like structures formed from groups of sporophylls in lycophytes and gymnosperms

c) Sori—clusters of sporangia produced by fern sporophylls

d) Megaspores—develop into male gametophytes

Which term is correctly matched with its definition?

Scientific Skills Exercises

Researchers set up experimental and control microcosms, or small artificial ecosystems, to measure the release of minerals from rocks. First, they placed rock fragments of volcanic origin, either granite or andesite, into small glass containers. Then they made a suspension of water and macerated (chopped and crushed) moss of the species Physcomitrella patens. They added this mixture of water and moss to the experimental microcosms. For the control microcosms, they filtered out the moss and just added the water.

After 130 days, they measured the amounts of various minerals found in the water in the control microcosms and in the water and moss in the experimental microcosms.

How did the setup of the control and experimental microcosms differ?

a) Control microcosms did not contain living moss, while experimental microcosms did contain living moss.

b) Control microcosms contained minerals in the water, while experimental microcosms did not contain minerals in the water.

c) Control microcosms did not contain rock of volcanic origin, while experimental microcosms did contain rock of volcanic origin.

d) Control microcosms contained water, while experimental microcosms did not contain water.

How did the setup of the control and experimental microcosms differ?

a) Control microcosms did not contain living moss, while experimental microcosms did contain living moss.

b) Control microcosms contained minerals in the water, while experimental microcosms did not contain minerals in the water.

c) Control microcosms did not contain rock of volcanic origin, while experimental microcosms did contain rock of volcanic origin.

d) Control microcosms contained water, while experimental microcosms did not contain water.

Why did the researchers add filtrate from which macerated moss had been removed to the control microcosms?

a) to control for the possibility that the filtrate contained material derived from moss

b) to control for the possibility that the filtrate included nutrients released from granite or andesite

c) to provide replication

d) to control for the possibility that the filtrate contained living moss cells

Why did the researchers add filtrate from which macerated moss had been removed to the control microcosms?

a) to control for the possibility that the filtrate contained material derived from moss

b) to control for the possibility that the filtrate included nutrients released from granite or andesite

c) to provide replication

d) to control for the possibility that the filtrate contained living moss cells

The bar graphs show the mean total amount of each element weathered from rocks in the control and experimental microcosms.

Overall, what is the effect of moss on the chemical weathering of rock?

a) Moss increases the chemical weathering of rock.

b) Moss does not increase the chemical weathering of rock.

c) It is not clear whether or not moss increases the chemical weathering of rock.

Overall, what is the effect of moss on the chemical weathering of rock?

a) Moss increases the chemical weathering of rock.

b) Moss does not increase the chemical weathering of rock.

c) It is not clear whether or not moss increases the chemical weathering of rock.

Why were mineral nutrients released in the control microcosms?

a) The release of mineral nutrients in the control microcosms indicates the extent of abiotic weathering of rock.

b) The release of mineral nutrients in the control microcosms indicates the extent of biotic weathering of rock.

c) The release of mineral nutrients in the control microcosms was due to organic acids released by mosses.

d) There was no release of mineral nutrients in the control microcosms.

Why were mineral nutrients released in the control microcosms?

a) The release of mineral nutrients in the control microcosms indicates the extent of abiotic weathering of rock.

b) The release of mineral nutrients in the control microcosms indicates the extent of biotic weathering of rock.

c) The release of mineral nutrients in the control microcosms was due to organic acids released by mosses.

d) There was no release of mineral nutrients in the control microcosms.

Review the earlier bar graph. How does the moss’s chemical weathering effect on granite compare to the weathering effect on andesite?

a) Moss increased the release of all three elements (calcium, magnesium, and potassium) from both granite and andesite.

b) The results are not conclusive about the effect of moss on the chemical weathering of granite and andesite.

c) Moss did not alter the release of calcium, magnesium, and potassium from granite and andesite.

d) Moss decreased the release of all three elements (calcium, magnesium, and potassium) from both granite and andesite.

Review the earlier bar graph. How does the moss’s chemical weathering effect on granite compare to the weathering effect on andesite?

a) Moss increased the release of all three elements (calcium, magnesium, and potassium) from both granite and andesite.

b) The results are not conclusive about the effect of moss on the chemical weathering of granite and andesite.

c) Moss did not alter the release of calcium, magnesium, and potassium from granite and andesite.

d) Moss decreased the release of all three elements (calcium, magnesium, and potassium) from both granite and andesite.

Based on their experimental results, the researchers added weathering of rock by nonvascular plants to simulation models of the Ordovician climate. The new models predicted decreased CO2 levels and global cooling sufficient to produce glaciations in the late Ordovician period. Which of the following assumptions did the researchers make in using results from their experiments in climate simulation models?

a) They assumed the modern moss P. patens increased the availability of mineral nutrients by breaking down granite and andesite.

b) They assumed that the effect of Ordovician mosses on the chemical weathering of rock was similar to the weathering caused by the modern moss P. patens.

c) They assumed that chemical weathering by mosses reduced atmospheric CO2 levels, producing dramatic climatic cooling in the late Ordovician period.

d) They assumed the modern moss P. patens increased the weathering of rock.

Based on their experimental results, the researchers added weathering of rock by nonvascular plants to simulation models of the Ordovician climate. The new models predicted decreased CO2 levels and global cooling sufficient to produce glaciations in the late Ordovician period. Which of the following assumptions did the researchers make in using results from their experiments in climate simulation models?

a) They assumed the modern moss P. patens increased the availability of mineral nutrients by breaking down granite and andesite.

b) They assumed that the effect of Ordovician mosses on the chemical weathering of rock was similar to the weathering caused by the modern moss P. patens.

c) They assumed that chemical weathering by mosses reduced atmospheric CO2 levels, producing dramatic climatic cooling in the late Ordovician period.

d) They assumed the modern moss P. patens increased the weathering of rock.

How do these experimental results and models demonstrate that life has profoundly changed Earth?

a) The results show that early nonvascular plants could have caused enough chemical weathering of rock to increase atmospheric CO2 levels.

b) The results show that nonvascular and vascular plants utilize rock-based minerals, including calcium, magnesium, and potassium.

c) The results show that mineral nutrients are released by the weathering of rock in the absence of living cells.

d) The results show that early nonvascular plants could have caused enough chemical weathering of rock to reduce atmospheric CO2 levels.

How do these experimental results and models demonstrate that life has profoundly changed Earth?

a) The results show that early nonvascular plants could have caused enough chemical weathering of rock to increase atmospheric CO2 levels.

b) The results show that nonvascular and vascular plants utilize rock-based minerals, including calcium, magnesium, and potassium.

c) The results show that mineral nutrients are released by the weathering of rock in the absence of living cells.

d) The results show that early nonvascular plants could have caused enough chemical weathering of rock to reduce atmospheric CO2 levels.