How Plants Colonized LandHow Plants Colonized Land

Chapter 29Chapter 29

Overview:

• Origins of Mulitcellularity• Quick look at over all diversity in Plant Kingdom• Characteristics of Plants

– Special features, structures

• Origin from green algae– Link to Charophyceans– Evidence to support– Dates

• Alternation of generations– Origin of embryophytes

• Bryophyte (moss) lifecycle – in Lab• Pteridophyte (fern) lifecycle – in Lab

Multicellularity

• Arose independently several times in Eukarya• Caused another new wave in evolution• Origins in simpler colonial forms – Volvox• Cellular specialization and Division of labor• Escape cell size limitations

– Membrane area to cytoplasm volume ratio

• Multicellularity solves ratio limits Fig 6.7

The Plant Kingdom• Origins over 475 MYA• 10 Divisions• 4 Basic lifecycles• Green algae that

evolved onto land• Evolved becoming

more terrestrial, independent from water

• Then coevolved with pollinators, dispersal

• 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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Figure 29.7

Major events:

• Living on land, spores• Vascular tissues• Pollen, Seeds• Flowers & Fruits

The Plant Kingdom? Who is in / out?

Plantae

Streptophyta

Viridiplantae

Red algae Chlorophytes Charophyceans Embryophytes

Ancestral alga

• We’ll see all but the hornworts in lab

Table 29.1

• Green Algae (Charophyceans) gave rise to:• Mosses (475 MYA) and liverworts, which gave rise

to:• Ferns (420 MYA) and related plant groups which

gave rise to:• Gymnosperms (360 MYA) Conifers and related plant

groups which gave rise to:• Angiosperms (140 MYA) (flowering plants) which

have form two groups:– Monocots– Eudicots

Evolutionary Sequence

Charophyceans- plant’s green algae ancestor

Chara ColeochaeteModern examples of charophyceans

Original Traits Derived Traits

• Found in ancestral species and new species

• Not found in ancestral species, new to daughter species

Older terms: primitive and advanced traits

What’s new in Plants:Some Derived Characteristics of Plants

• Growth by divisions in Apical Meristems• Multicellular dependent embryos• Alternation of generations • Spores• Multicellular gametangia• Cuticle• Transport tissues• Secondary compounds

Growth by cell divisions in Apical Meristems

• Localized regions of cell division• In shoot tips, roots

Multicellular dependent

diploid embryos

• Land plants are called embryophytes• Haploid charophyceans

Spores

• Walls of resistant sporopollenin• Dry out and travel in the wind• Zygotes of Charophyceans protected by

sporopollenin – precursor to spore walls?• Disperse then grow into gametophyte plants

Multicellular gametangia

Archegonia Antheridia

Make egg at base of produce many sperm

Vase like column cells that swim to egg

All haploid tissues, gametes form by mitosis

Cuticle• Waxy covering layer prevents water loss, and

microbial attack• Stomata allow for gas exchange• Thicker layers in plants adapted to arid

conditions

Transport tissues

• Xylem carries water up from the roots to the leaves

• Phloem carries a sugary solution through out the plant

Secondary Compounds

• Metabolic side branches off common pathways

• Provide benefit to plant ( defense, etc.)• Can be used by People as Flavorings, drugs

insecticides, etc.

APICAL MERISTEMS Apicalmeristemof shoot

Developingleaves

100 µm

Apical meristems of plant shoots and roots. The light micrographs are longitudinal sections at the tips of a shoot and root.

Apical meristemof root

Root 100 µmShoot

Haploid multicellularorganism (gametophyte)

Mitosis Mitosis

Gametes

Zygote

Diploid multicellularorganism (sporophyte)

Alternation of generations: a generalized scheme

MEIOSIS FERTILIZATION

2n2n

n

n

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nSpores

Mitosis

ALTERNATION OF GENERATIONS

WALLED SPORES PRODUCED IN SPORANGIA

MULTICELLULAR GAMETANGIA

MULTICELLULAR, DEPENDENT EMBRYOS

SporesSporangium

Longitudinal section ofSphagnum sporangium (LM)

SporophyteGametophyte

Sporophyte and sporangium of Sphagnum (a moss)

Female gametophyteArchegoniumwith egg

Antheridiumwith sperm

Malegametophyte

Archegonia and antheridia of Marchantia (a liverwort)

EmbryoMaternal tissue

2 µm

Wall ingrowthsPlacental transfer cell

10 µm

Embryo and placental transfer cell of Marchantia

Origin of plants

• How did the sporophyte generation come about?

• Preadaptations may have lead the charyophyceans onto land

Hypothesis for origin of alternation of generations

• Delayed meiosis maximizes output of sexual reproduction.– More spores can be produced per fertilization event

• Adapting to drying conditions with fewer surviving spores

• Lineages separate before sporophytes evolved much

Preadaptation

• Evolutionary adaptation that was selected for under one set of conditions

• This trait then gives an advantage for a new situation with different conditions.

Preadaptation

• Evolutionary adaptation that was selected for under one set of conditions

• This trait then gives an advantage for a new situation with different conditions.

Feathers arose first for insulation, then helped with flight

Adaptation to shallow waters preadapted Charophyceans to life on land

• Adapted to periodic drying during low tides, droughts– Leads to cuticle?

• Adapted to higher light intensities– Leads to common chloroplast structure?

• Zygote protected from drought within archegonia with a layer of sporopollenin– Leads to spore wall?

Alternation of Generations

• Separate multicellular haploid and diploid phases – (2n) Sporophyte make spores by meiosis– (n) Gametophyte makes gametes by mitosis

• Sperm and egg (moss & fern)• Pollen and Ovule (gymnosperm & angiosperm)

• The sporophyte and gametophyte are very different in morphology– Vascular tissues only appear in sporophyte phase

• Sporophyte becomes more dominant in new plant groups

• Charophyceans lack sporophyte phase

Charophycean life cycle

Characteristics that Plants share with the green algae group Charophyceans

• Autotrophic Multicellular Eukaryote• Have cell walls made of cellulose

– Made by rosette cellulose-synthesizing complexes– 20-26% of wall material, closest match in algae

• Chloroplast similarities– have chlorophyll a & b, use β-carotene as accessory– Thylakoids stacked in grana– Chloroplast DNA comparisons

• Peroxisome enzymes• Cell plate formation by phragmoplast• Nuclear membrane breaks down during mitosis• Sperm ultrastructure - biflagellate• Gene sequences – rRNA, Cytoskeleton proteins

Switch to sporophyte dominance

Figure 29.13 Hypotheses for the evolution of leaves

Vascular tissue

Microphylls, such as those of lycophytes, may have originated as small stem outgrowths supported by single, unbranched strands of vascular tissue.

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

(b)

What’s new in Mosses?

• Spores / sporangia• Sporophyte phase• Upright growth on land• Cuticle• Multicellular gametangia

The Bryophytes• Bryophytes are represented by three divisions:

– Division Hepatophyta - liverworts– Division Anthocerophyta - hornworts– Division Bryophyta – mosses

Liverworts and hornworts are believed to be more similar to what early plants were like.

Bryophyte lifecycle: moss• Haploid dominant• No vascular

tissues• Filamentous

protonema stage• Swimming sperm• Disperse by

spores• Dependent

sporophyte• Dioecious

gametophytes• No true leaves• Rhizoids, not roots

What’s new in Ferns?

• Vascular tissue• True roots and stems and leaves• Stomata• Sporophyte (2n) dominate

Pteridophytes evolved over 400 MYASeedless, Vascular plants (having Xylem &

Phloem). Today represented by two divisions:Pterophyta: Ferns, Horsetails (Equisetum)Lycophyta: Club mossCooksonia, an extinct plant over 400 million

years old, is the earliest known vascular plant.

The branched sporophytes were up to 50cm tall with small lignified cells, much like the xylem cells of modern pteridophytes.

Vascular tissue

• Allows plants to grow taller• More support by lignified xylem tracheids• Can pull water up from soil

– Can tolerate soil that is drier on the surface

• Form parts of true leaves and roots.• Only found in diploid tissue

– Lead to sporophyte dominance?

Fern Lifecycle

• Diploid dominate• Gametophyte still independent, short lived,

– monoecious in fern (Pteridophyta)– dioecious in club moss (Lycophyta)

• Sporophyte – in Lycophyta is Monecious– in Pteridiophyta is Dioecious

• Spores disperse plant• Sporophyte perennial, monoecious

Figure 29.23 The life cycle of a fern

LYCOPHYTES (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

Carboniferous forest based on fossil evidence