Chapter 29

Plant Diversity IHow Plants

Colonized Land

Land Ho! The Greening of Earth

• For the first 3 billion years of Earth’s history, the land was lifeless• Cyanobacteria existed on land about 1.2 billion years ago• ~500 million years ago plants, fungi and animals joined them• DiversificationOver 290,00 species of plants inhabit Earth today

Plant Evolution

• Land plants evolved from green algae

• Charophyceans (a type of green algae) are the closest relatives of land plants

Chara, a pond organism

(a)10 mm

Coleochaete orbicularis(b)40 µm

Accumulation of adaptations to the terrestrial environment allowed the first land plants to live permanently above the waterline

Defining the Plant Kingdom

• Systematists– Are currently debating the boundaries of the

plant kingdomPlantae

Streptophyta

Viridiplantae

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 (gametes produced here)

– Multicellular dependent embryos

Apicalmeristemof shoot

Developingleaves

100 µm

Apical meristemof root

Root 100 µmShoot

Apical meristems and alternation of generations

Haploid multicellularorganism (gametophyte)

Mitosis Mitosis

Gametes

Zygote

Diploid multicellularorganism (sporophyte)

Alternation of generations: a generalized scheme

MEIOSIS FERTILIZATION

2n2n

n

n

nn

nSpores

Mitosis

ALTERNATION OF GENERATIONS

.

Generations

Gametophyte (haploid)

Sporophyte (diploid)

APICAL MERISTEMSLocalized regions of cell division at tips of roots and shoots

Walled spores; multicellular gametangia; and multicellular, dependent embryos

WALLED SPORES PRODUCED IN SPORANGIA

MULTICELLULAR GAMETANGIA (where gametes are produced)

MULTICELLULAR, DEPENDENT EMBRYOS

SporesSporangium

Longitudinal section ofSphagnum sporangium

SporophyteGametophyte

Sporophyte and sporangium of Sphagnum (a moss)

Female gametophyte

Archegoniumwith egg

Antheridiumwith sperm

Malegametophyte

Archegonia and antheridia of Marchantia (a liverwort)

EmbryoMaternal tissue

Wall ingrowths

Placental transfer cell

Embryo and placental transfer cell of Marchantia

Female gametangia (archegonia)

Male gametangia (antheridia)

Embryo retained within tissues of female parent. The parental tissues provide the developing embryo with nutrients.

Land plants are also known as EMBRYOPHYTES

Fossilized spores and tissues– Have been extracted from 475-million-year-

old rocks

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).

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

Diversity of Modern Plants

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

An overview of land plant evolution

Bryophytes(nonvascular plants) Seedless vascular plants Seed plants

Vascular plants

Land plants

Origin of seed plants(about 360 mya)

Origin of vascular plants (about 420 mya)

Origin of land plants(about 475 mya)

Ancestralgreen alga

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Bryophytes• The life cycles of mosses and other

bryophytes are 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

Bryophytes…the video clip

Plant Classifications: Bryophytes." Online . Encyclopædia Britannica Online. 14  Jan.  2008  <http://www.britannica.com/eb/art-68471>.

Bryophyte Gametophytes

• In all three bryophyte phyla– Gametophytes are larger and longer-living

than sporophytes

Moss Life Cycle…The Movie

The life cycle of a moss

Maturesporophytes

Youngsporophyte

Malegametophyte

Raindrop

Sperm

Key

Haploid (n)Diploid (2n)

Antheridia

Femalegametophyte

Egg

Archegonia

FERTILIZATION

(within archegonium)Zygote

Archegonium

Embryo

Femalegametophytes

Gametophore

Foot

Capsule(sporangium)

Seta

Peristome

Spores

Protonemata

“Bud”

“Bud”

MEIOSIS

Sporangium

Calyptra

Capsule with peristome

Rhizoid

Maturesporophytes

Spores develop intothreadlike protonemata.1

The haploidprotonemataproduce “buds”that grow intogametophytes.

2 Most mosses have separatemale and female gametophytes,with antheridia and archegonia,respectively.

3

A sperm swimsthrough a film ofmoisture to anarchegonium andfertilizes the egg.

4

Meiosis occurs and haploidspores develop in the sporangiumof the sporophyte. When thesporangium lid pops off, theperistome “teeth” regulategradual release of the spores.

8

The sporophyte grows along stalk, or seta, that emergesfrom the archegonium.

6

The diploid zygotedevelops into a sporophyte embryo withinthe archegonium.

5

Attached by its foot, thesporophyte remains nutritionallydependent on the gametophyte.

7

Bryophyte gametophytes• 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

Bryophyte Sporophytes

• Bryophyte sporophytes– Grow out of archegonia– Are the smallest and simplest of all extant plant groups– Consist of a foot, a seta (an elongated stalk), and a

sporangium

• Hornwort and moss sporophytes– Have stomata (like vascular plants) (an opening for gas exchange)

Bryophyte diversityLIVERWORTS (PHYLUM HEPATOPHYTA)

HORNWORTS (PHYLUM ANTHOCEROPHYTA) MOSSES (PHYLUM BRYOPHYTA)

Gametophore offemale gametophyte

Marchantia polymorpha,a “thalloid” liverwort

Foot

Sporangium

Seta

500

µm

Marchantia sporophyte (LM)

Plagiochiladeltoidea,a “leafy”liverwort

An Anthoceroshornwort species

Sporophyte

Gametophyte

Polytrichum commune,hairy-cap moss

Sporophyte

Gametophyte

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

Ecological and Economic Importance of Mosses

• Sphagnum, or “peat moss”– Forms extensive deposits of partially decayed organic material known

as peat– Peatlands play an important role as carbon reservoirs an help stabilize

atmospheric carbon dioxide levels.

GametophyteSporangium attip of sporophyte

Livingphoto-syntheticcells

Dead water-storing cells

100 µm

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

(b)

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

(c)

Peat being harvested from a peat bog(a)

(d)

Origin and Diversity of Vascular Plants

• 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 evolve during the Carboniferous

period– Fossils of the forerunners of vascular plants

date back about 420 million years

Seedless Vascular Plants• These early tiny plants

– Had branching sporophytes that weren’t dependent on gametopyhtes for growth, but lacked other derived traits of vascular plants

Dichotomous (Y-shaped) branching made possible multiple sporangia. This evolutionary development allowed for greater spore production.

Also increased survival in the face of herbivory. If some sporangia were eaten others might survive.

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

Tracheophytes-Pterophyta (ferns)…the video clip

fern." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 13 Jan. 2008  <http://www.britannica.com/eb/article-9110635>.

Fern Life Cycle…The Movie

The life cycle of a fern

Fern sperm use flagellato swim from the antheridia to eggs in the archegonia.

4

Sporangia release spores.Most fern species produce a singletype of spore that gives rise to abisexual gametophyte.

1 The fern sporedevelops into a small,photosynthetic gametophyte.

2 Although this illustration shows an egg and sperm from the same gametophyte, a variety of mechanismspromote cross-fertilizationbetween gametophytes.

3

On the undersideof the sporophyte‘sreproductive leavesare spots called sori.Each sorus is acluster of sporangia.

6

A zygote develops into a newsporophyte, and the young plantgrows out from an archegoniumof its parent, the gametophyte.

5

MEIOSIS

Sporangium

Sporangium

Maturesporophyte

Newsporophyte

Zygote

FERTILIZATION

Archegonium

Egg

Haploid (n)Diploid (2n)

Spore Younggametophyte

Fiddlehead

Antheridium

Sperm

Gametophyte

Key

Sorus

Transport in Xylem and Phloem• Vascular plants have two types of vascular tissue

– Xylem and phloem• Xylem

– Conducts most of the water and minerals– Includes dead cells called tracheids. Their cell walls remain to provide the internal ‘pipe

system’.– Cell walls are strengthened by the polymer lignin. This allows vascular plants to grow

to greater heights than bryophytes• Phloem

– Distributes sugars, amino acids, and other organic products– Consists of living cells

Evolution of Roots

• Roots– Are organs that anchor vascular plants– Enable vascular plants to absorb water and

nutrients from the soil– 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 for photosynthesis

• Leaves are categorized by two types– Microphylls, leaves with a single vein– Megaphylls, leaves with a highly branched vascular

system

Evolution of Leaves

• According to one model of evolution– Microphylls evolved first, as outgrowths of

stems

Vascular tissue

Microphylls may have originated as small stemoutgrowths supported by single, unbranched strandsof vascular tissue.

(a) Megaphylls, which have branched vascular systems, may have evolved by the fusion of branched stems.

(b)

Sporophylls and Spore Variations

• Sporophylls– Are modified leaves with

sporangia (may look like a “photosynthetic leaf” may not)

• 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

• Seedless vascular plants form two phyla– Lycophyta, including club mosses, spike

mosses, and quillworts. Are small herbaceous plants

– Pterophyta, including ferns, horsetails, and whisk ferns and their relatives. Ferns are the most diverse seedless vascular plants

Groups of seedless vascular plantsLYCOPHYTES (PHYLUM LYCOPHYTA)

PTEROPHYTES (PHYLUM PTEROPHYTA)

WHISK FERNS AND RELATIVES HORSETAILS FERNS

Isoetesgunnii,a quillwort

Selaginella apoda,a spike moss

Diphasiastrum tristachyum, a club moss

Strobili(clusters ofsporophylls)

Psilotumnudum,a whiskfern

Equisetumarvense,fieldhorsetail

Vegetative stem

Strobilus onfertile stem

Athyrium filix-femina, lady fern

Significance of Seedless Vascular Plants• The ancestors of modern lycophytes, horsetails, and ferns grew to

great heights during the Carboniferous, forming the first forests

The growth of these early forests may have helped produce the major global cooling that characterized the end of the Carboniferous period.

They decayed and eventually became coal