THE EVOLUTION OF PLANTS
John C. Studenroth

What do we mean when we speak of "plants" or "algae?

These terms span four of the five kingdoms of living things!

KINGDOM MONERA: prokaryotes

Bacteria: includes photosynthetic & chemosynthetic types
Cyanobacteria: formerly called "bluegreen algae'

KINGDOM PROTISTA: unicellular eukaryotes

Protozoa: includes some photosynthetic types, even though "animal-like"

important in zooplankton

Algal protists: e.g., unicellular algae

important in phytoplankton

KINGDOM FUNGI: very diverse group of eukaryotic heterotrophs

important as symbionts, e.g., lichens, mycorrhizae

KINGDOM PLANTAE: multicellular eukaryotic autotrophs

The "higher algae" or "lower plants"

Division Chlorophyta: green algae, about 7,000 species
Division Phaeophyta: brown algae, about 1,500 species
Division Rhodophyta: red algae, about 4,000 species

The nonvascular land plants: some 23,600 species

Division Bryophyta: mosses, liverworts, hornworts

The vascular land plants:

Division Tracheophyta:

Lower (seedless) vascular plants, about 13,240 species

e.g., club mosses, horsetails, ferns

Gymnosperms, about 720 species

e.g., conifers, cycads, Ginkgo

Angiosperms, about 275,000 species!

about 6x the number of species of all other plant groups combined!

What has a century of paleobotany told us about the appearance of these four kingdoms over time?

PRECAMBRIAN ERA: 3.8 to 0.6 billion years ago

Evidence of first bacterial cells about 3.5 b.y. ago (MONERA)
Evidence of first cyanobacteria about 2.8 b.y. ago (MONERA)
Evidence of first eukaryotes about 1.45 b.y. ago (PROTISTA)
Evidence of primitive fungi (FUNGI)

PALEOZOIC ERA: 600 to 225 million years ago

CAMBRIAN PERIOD: 600 to 500 millon years ago

algae dominant (PROTISTA)

ORDOVICIAN PERIOD: 500 to 430 million years ago

marine algae dominant
some modern groups of algae and fungi
first land plants (?) in Upper Ordovician (PLANTAE)

SILURIAN PERIOD: 430 to 395 million years ago

algae dominant
vascular plants appear
tracheids found in as yet unidentified sterile stems
Division Rhyniophyta, ancestors to our Division Tracheophyta

DEVONIAN PERIOD: 395 to 345 million years ago

major diversification of vascular plants in Lower Devonian Division
Zosterophyllophyta, ancestors to our Class Lycopsida?
Division Trimerophyta, ancestors to the "progymnosperms and to our

Classes Filicopsida (ferns) and Sphenopsida (horsetails)?

heterospory and microphyllous leaves in Lower Devonian
evidence of secondary growth in Middle Devonian
arborescence, seeds, progymnosperms, and megaphyllous leaves in Upper Devonian

CARBONIFEROUS PERIOD: 345 to 280 million years ago

club mosses (Lycopsida) and horsetails (Sphenopsida) dominant in the
forests of ferns (Filicopsida) and socalled 'seed ferns (actually gymnosperms,
many bizarre forms that are now extinct

PERMIAN PERIOD: 280 to 225 million years ago

first conifers, cycads, Ginkgos
a time of major extinctions

MESOZOIC ERA: 225 to 65 million years ago

TRIASSIC PERIOD: 225 to 195 million years ago

forests of gymnosperms and ferns dominate

JURASSIC PERIOD: 195 to 135 million years ago
similar to Triassic (ferns, cycads, etc.)

CRETACEOUS PERIOD: 135 to 65 million years ago

angiosperms originate and spread
gymnosperms decline
another time of major extinctions

CENOZOIC ERA: 65 million years ago to present

TERTIARY PERIOD: 65 to 2.5 million years ago

rise of monocotyledons
grasslands develop
forests decline

QUATERNARY PERIOD: 2.5 million years ago to present

extinction of many species
decline of woody plants
rise of herbaceous plants
many desert forms appear

How does it all fit together?

For all macroevolutionary schemes

A few problems for all varieties of creationists, too!

The biggest jump of all

Our ongoing knowledge explosion

Indirect evidence for cyanobacteria (i.e., stromatolites) from 3.5 billion years ago,

yet these organisms were doing "modern" photosyntehsis!

A smaller jump, but too big for us to emulate with random mutations

A quick look at some striking examples

Just how far does microevolution go in the botanical world?

BIBLIOGRAPHY

- history of paleobotany with biographic emphasis.

- a good introductory textbook of paleobotany.

- cited several times by Anderson & Coffin in Fossils in Focus; for many years the standard college textbook.

- a nifty little paperback, part of the "Fundamentals of Botany" series.

- since land plants appear 1.5 billion years after first algae, transition to land is seen as extremely difficult, yet "convergent" adaptations to life on land occurred independently in several plant groups.

- suggests a simple modification of the life cycle of an extinct green alga as the likely origin of the first land plants.

- argues from microfossil assemblages of spores in late Ordovician Period that early phases of terrestrial plant life mav have occurred this far back.

New York: Academic Press.

Burlington, NC: Carolina Biological Supply Co.

- one of barriers to survival on dry land initially would be lack of soil (of which a key component is decayed plant matter!); provides evidence for a conjectured symbiotic relationship between roots of early vascular plants and fungi.

- palynology = pollen analysis, reconstructing ancient floras based on observations on pollen grains and spores in ancient peat and sedimentary deposits. The resistance to decay of the outer coats of pollen grains, plus their distinctive sculpturing makes possible both qualitative and quantitative estimates of species occurring thousands of years ago.