Plant Diversity I How Plants Colonized Land

Chapter 29

Plants vs. Algae • Land plants evolved from Charophytes (Chara)

green algae

• 4 key traits plants share with Charophytes (Morphological and Biochemical Evidence):

1. Rose-shaped complexes for cellulose synthesis2. Peroxisome enzymes3. Structure of flagellated sperm4. Formation of a phragmoplast

5 key traits in nearly all land plants but are absent in the charophytes:

1. apical meristems 2. multicellular dependent embryos3. alternation of generations 4. walled spores produced in sporangia5. multicellular gametangia

females – archegonia & males – antheridia

Fig. 29-5e

Apicalmeristemof shoot

Developingleaves

Apical meristems

Apical meristemof root Root100 µm 100 µmShoot

Fig. 29-5b

EmbryoMaternal tissue

Wall ingrowthsPlacental transfer cell(outlined in blue)

Embryo (LM) and placental transfer cell (TEM)of Marchantia (a liverwort)

2 µm

10 µm

Fig. 29-5a

Gametophyte(n)

Gamete fromanother plant

n

n

Mitosis

Gamete

FERTILIZATIONMEIOSIS

Mitosis

Sporen

n

2n Zygote

Mitosis

Sporophyte(2n)

Alternation of generations

Fig. 29-5c

SporesSporangium

Sporophyte

Longitudinal section ofSphagnum sporangium (LM)

Gametophyte

Sporophytes and sporangia of Sphagnum (a moss)

Fig. 29-5d

Female gametophyte

Malegametophyte

Antheridiumwith sperm

Archegoniumwith egg

Archegonia and antheridia of Marchantia (a liverwort)

Fig. 29-7

Origin of land plants (about 475 mya)1

2

3

1

2

3

Origin of vascular plants (about 420 mya)

Origin of extant seed plants (about 305 mya)

ANCES-TRALGREENALGA

Liverworts

Hornworts

Mosses

Lycophytes (club mosses,spike mosses, quillworts)

Pterophytes (ferns,horsetails, whisk ferns)

Gymnosperms

Angiosperms

Seed plantsSeedlessvascularplants

Nonvascular

plants(bryophytes)

Land plants

Vascular plants

Millions of years ago (mya)500 450 400 350 300 50 0

Characteristics of all land plants:• eukaryotic, multicellular, autotrophic• cell walls made mostly of cellulose• chlorophylls a & b• Domain Eukarya, Kingdom Plantae

In many plants, additional terrestrial adaptations, such as vascular tissues and secondary compounds, also evolved.

Nonvascular plants

1. Represented by three phyla:a. phylum Hepatophyta – liverwortsb. phylum Anthocerophyta – hornwortsc. phylum Bryophyta - mosses

Liverworts

HornwortsMoss

Fig. 29-9a

ThallusGametophore offemale gametophyte

Marchantia polymorpha,a “thalloid” liverwort

Marchantia sporophyte (LM)

Sporophyte

FootSeta

Capsule(sporangium)

500

µm

• gametophyte - mass of green, branched, one-cell-thick filaments• sporophytes are smaller; only present part of the time• spores germinate in favorable habitats

sporophyte

gametophyte

sporophyte

gametophyte

2. The gametophyte is the dominant generation in the life cycle

Fig. 29-8 The life cycle of a moss.

KeyHaploid (n)Diploid (2n) Protonemata

(n)

“Bud”

“Bud”

Malegametophyte(n)

Femalegametophyte (n)

Gametophore

Rhizoid

Spores

Sporedispersal

Peristome

SporangiumMEIOSIS Seta

Capsule(sporangium)

Foot

Maturesporophytes

Capsule withperistome (SEM)

Femalegametophytes

2 m

m

Raindrop

Sperm

Antheridia

Egg

Archegonia

FERTILIZATION(within archegonium)

Zygote(2n)

Embryo

Archegonium

Youngsporophyte(2n)

3. Bryophyte sporophytes disperse enormous numbers of spores

• sporophytes remain attached to gametophyte throughout the lifetime– depends on the gametophyte for sugars, amino acids, minerals and

water.

• sporangium (site of meiosis and spore production) can generate over 50 million spores.

4. Bryophytes provide many ecological and economic benefits

• distributed worldwide• common and diverse in moist forests and wetlands• Some common in extreme environments (mountaintops,

tundra, and deserts)

• Sphagnum, a wetland moss, is especially abundant and widespread.• forms extensive deposits of undecayed organic material,

called peat• Wet regions dominated by Sphagnum or peat moss are known

as peat bogs

Fig. 29-11

(a) Peat being harvested

(b) “Tollund Man,” a bog mummy

Bog People

THE ORIGIN OF VASCULAR PLANTS1. Two conducting tissues of the vascular system

A. Xylem – Dead tissue, water-conducting

B. Phloem –Living tissue, food-transporting

2. Water-conducting cells are strengthened by lignin and provide structural support

3. Sporophyte generation is dominant in vascular plants.

Seedless vascular plants4. Two modern phyla:

a. phylum Lycophyta – club mossesb. phylum Pterophyta - ferns, whisk ferns, and horsetails

Club moss

Horsetail

Whisk fern

Fern

Fig. 29-15a

Lycophytes (Phylum Lycophyta)

Selaginella apoda,a spike moss

Isoetesgunnii,a quillwort

Strobili(clusters ofsporophylls)

2.5 cm

Diphasiastrum tristachyum, a club moss1 cm

Fig. 29-15e

Pterophytes (Phylum Pterophyta)Athyriumfilix-femina,lady fern

Vegetative stem

Strobilus onfertile stem

1.5

cm

25 c

m

2.5

cm

Psilotumnudum,a whiskfern

Equisetumarvense,fieldhorsetail

5. Most seedless vascular plants are homosporous, producing one type of spore that develops into a bisexual gametophyte• both archegonia (female sex organs) and antheridia (male sex

organs) • Eg., ferns

sporophyteSingle type of spore

Bisexual

gametophyte

eggs

sperm

Fig. 29-13 The life cycle of a fern.

KeyHaploid (n)Diploid (2n)

MEIOSISSporedispersal

Sporangium

SporangiumMaturesporophyte(2n)

Sorus

Fiddlehead

Spore(n)

Younggametophyte

Maturegametophyte(n) Archegonium

Egg

Antheridium

Sperm

FERTILIZATIONNewsporophyte

Gametophyte

Zygote(2n)

6. seedless vascular plants are most common in damp habitats 7. ferns produce clusters of sporangia, called sori, on the back

of leaves

Seedless vascular plants formed vast “coal forests” during the Carboniferous period• These plants left not only living representatives

and fossils, but also fossil fuel in the form of coal.