Evolution of plants

By GVR prasad Dept of geology,univ. Of delhi

Age of the Bacteria• Prokaryotes dominated the Earth for most of its

history. Multicellular eukaryotes are less than 1 billion years old.

• Prokaryotic fossils are very small, and consist mainly of fossilized cell walls. Some structures formed by mats of bacteria are found toady and also fossilized from 2 billion years ago. The cyanobacteria (blue-green algae) form cell walls that fossilize nicely.

• Traces of organic compounds can also be found, and

attributed to living cells because they contain unusual ratios of carbon and sulfur isotopes.

• Oldest traces of life: 3.5 – 3.8 billion years old. Very

few rocks available of this age or older. • Bacteria can live under a much wider variety of

conditions than eukaryotes. They use many different sources of energy and carbon, and they can grow at very high temperatures, high pressures, acidic conditions, etc. Large eukaryotes need much more stable conditions to survive.

LAND COLONIZATION

• Earth’s early atmosphere lacked O2• Oxygenic photosynthesis evolved 2.5 billion years ago

– O2 accumulated in the atmosphere– Ozone (O3) formed

• Increasing O2 levels precipitated evolutionary change– Mechanisms to tolerate O2 arose– Mechanisms to use O2 evolved

• Ozone protected life from ultraviolet light– Life previously remained primarily beneath the surface of

water and mud– This protection enabled organisms to colonize land

LAND COLONIZATION

• Cyanobacteria were likely the first living cells to colonize the land

• Green algae and fungi together colonized the land later

• Green algae were ancestral to all plants

MAJOR PLANT GROUPS

• Four major periods of plant evolution– New structures evolved, adaptive radiations followed–Origin of plants from

• aquatic ancestors• Diversification of • vascular plants• Origin of seeds• Emergence of • flowering plants

BRYOPHYTES

• Nonvascular plants• <19,000 species• Previously grouped together in a single division

–Bryophyta• Now separated into three divisions

–Bryophyta (mosses), Hepatophyta (liverworts), Anthocerophyta (hornworts)• Still, the term “bryophyte” encompasses all

three of these divisions of nonvascular plants

Liverwort

Liverwort

Hornwort Moss

• Furthermore, the green algae are one of the few plant groups –to have made the transition from salt water to

freshwater• The evolution of terrestrial vascular plants from

an aquatic plant (probably of green alga ancestry) –was accompanied by various modifications –that allowed them to occupy this new and harsh

environment

Transitions from Salt to Freshwater to Land

• Most experts agree –that the ancestors of land plants first evolved in a

marine environment, –then moved into a freshwater environment –and finally on to land

• In this way the differences in osmotic pressures –between salt and freshwater were overcome while the

plant was still in the water• The higher land plants are composed of two

major groups, –the nonvascular and vascular plants

Marine, then Fresh, then Land

• The earliest land plants from the Middle to Late Ordovician –were probably small and bryophyte-like in their

overall organization – but not necessarily related to bryophytes

• The evolution of vascular tissue in plants was an important step –as it allowed for the transport of food and water

• Probable vascular plant megafossils –and characteristic spores indicate – to many paleontologists –that the evolution of vascular plants occurred well

before the Middle Silurian

Earliest Land Plants

• When plants made the transition from water to land, –they faced the same problems that animals did– desiccation, – support, – and the effects of gravity

• Plants adapted evolving a variety of structural features –that were fundamental to the subsequent radiations

and diversification that occurred during the Silurian, Devonian, and later periods

Plant Evolution

• The problem of desiccation –was circumvented by the evolution of cutin, – an organic compound found in the outer-wall layers of plants

• Cutin also provides additional resistance –to oxidation, the effects of ultraviolet light, and the

entry of parasites• Roots evolved in response to

–the need to collect water and nutrients from the soil and to help anchor the plant in the ground

• The evolution of leaves –from tiny outgrowths on the stem or from branch

systems • provided plants with

–an efficient light-gathering system for photosynthesis

Problems of Desiccation and Oxidation

• An interesting parallel can be seen between seedless vascular plants and amphibians

• When they made the transition from water to land, –they had to overcome the same problems such a

transition involved• Both groups,

–while successful, nevertheless required a source of water in order to reproduce

• In the case of amphibians, –their gelatinous egg had to remain moist

• while the seedless vascular plants –required water for the sperm to travel through to

reach the egg

Parallel between Seedless Vascular Plants and Amphibians

• From this simple beginning, –the seedless vascular plants –evolved many of the major structural features –characteristic of modern plants such as – leaves, – roots, – and secondary growth

• These features did not all evolve simultaneously –but rather at different times, – a pattern known as mosaic evolution

Seedless Vascular Plants Evolved

• The ancestor of terrestrial vascular plants –was probably some type of green algae

• While no fossil record of the transition –from green algae to terrestrial vascular plants exists,

comparison of their physiology reveals a strong link• Primitive seedless vascular plants

– such as ferns –resemble green algae in their pigmentation, –important metabolic enzymes, –and type of reproductive cycle

Ancestor of Terrestrial Vascular Plants

• Besides the primary function –of transporting water and nutrients throughout a

plant, vascular tissue also provides some support for the plant body

• Additional strength that acts to counteract gravity is derived –from the organic compounds lignin and cellulose,

which are found throughout a plant's walls

Vascular Tissue Also Gives Strength

OVERVIEW

• Nonvascular plants–Lack tissue systems to conduct water

• Vascular plants–Possess tissue systems to conduct water–Seedless vascular plants–Gymnosperms– Seed-bearing vascular plants–Angiosperms– Flower- and seed-bearing vascular plants

VASCULAR PLANTS

Adaptations of vascular plants• Differentiated bodies

–Subterranean root systems (water & minerals)–Aerial stems and leaves (photosynthesis)

• Vascular tissue–Xylem (water & minerals)–Phloem (organic nutrients)

• Lignin–Cell wall component providing mechanical support

Roots, Stems, and Leaves

• Life on land favored water conserving features– Cuticle: Waxy layer that restricts evaporation– Stomata: Openings across the cuticle

Vascular Tissues

In vascular plants, a system of vascular tissue reinforced by lignin distributes materials through leaves, stems, and roots of sporophytesXylem: Distributes water and mineralsPhloem: Distributes products of photosynthesis

Ancient Carbon Treasures• In the Carboniferous, plants with lignin-reinforced tissues flourished, died, and became compacted into coal, a nonrenewable fossil fuel

–Lepidodendron, a Carboniferous lycophyte

Energy from the Carboniferous Forest

PLANT ANCESTRY

• Plants represent a monophyletic group– Evolved from a common ancestor– Who was this common ancestor?

• Multiple lines of evidence indicate that land plants evolved from a group of green algae termed Charophytes

–What is the evidence?• Homologous chloroplasts

–Only green algae possess the accessory pigments chlorophyll b and beta-carotene present in plants

• Biochemical similarity–Of all green algae, charophytes possess a cell wall

cellulose composition most similar to that of plants

Evolution of Land Plants

• Land plants (embryophytes) evolved from a lineage of green algae (charophytes) after the ozone layer made life on land possible

PLANT ANCESTRY• Genetic relationships• Molecular analysis of

highly conserved –genes show charophytes

to be the green algae most closely related to plants

SEEDLESS VASCULAR PLANTS

• The earliest known vascular land plants –are small Y-shaped stems –assigned to the genus

Cooksonia –from the Middle Silurian

of Wales and Ireland• Upper Silurian and

Lower Devonian species are known from – Scotland, New York State,

and the Czech Republic, • These earliest plants

were –small, simple, leafless

stalks –with a spore-producing

structure at the tip (sporangia)

• It also had a resistant cuticle

• and produced spores typical of vascular plants

• These plants probably lived in moist environments such as mud flats

• This specimen is 1.49 cm long

CooksonisaDominated forest landscapes of Carboniferous period360 - 290 million years agoThree living divisionsLycophytes, Horsetails, Ferns

Early Devonian Plants• Reconstruction of an• Early Devonian landscape

Dawsonites /

Protolepidodendron\

- Bucheria

showing some of the earliest land plants

Bargwanthia

Rhynia

LYCOPHYTES• Division Lycophyta• Evolved in Devonian period

–Prevalent in Carboniferous period–Woody tree lineage– Became extinct near end of Carboniferous period–Herbaceous lineage– Represented today by ~1,000 species– e.g., Club mosses & ground pines

• (Which, incidentally, are neither mosses nor pines)

– Many species are epiphytes• Use another species as substrate (not a parasite)

– Many species grow on temperate forest floors

Club Moss

Ground Pine

HORSETAILS

• Division Sphenophyta– Sometimes considered part of

Division Pterophyta• Ancient lineage of seedless

vascular plants– Dates back to Devonian period– Prevalent during Carboniferous

• Modern survivors include ~15 species in the genus Equisetum– Most common in Northern

hemisphere– Generally found in damp locations,

streambanks

WHISK FERNS• Division Pterophyta

– Formerly Division Psilophyta– Molecular analysis has indicated

its close relatedness with ferns• Simple body structure evolved

secondarily– Ancestors were more complex

• Lacks roots present in ancestor– Possess subterranean rhizomes

• Small outgrowths of stems are likely reduced leaves

FERNS

• Division Pterophyta

• Ancient ancestry–Origins in Devonian period–Prevalent in Carboniferous period

• Currently most prevalent seedless vascular plant–>12,000 species exist today–Most diverse in tropics

•Leaves (called fronds) are compound–Divided into multiple leaflets

•Frond grows as tip (“fiddlehead”) unfurls

•Some leaves are specialized sporophylls–Sporangia on underside

• In addition to the diverse seedless vascular plant flora of the Late Devonian, –another significant floral event took place

• The evolution of the seed at this time –liberated land plants from their dependence on moist

conditions –and allowed them to spread over all parts of the land

Evolution of Seeds

• Whereas the Early Devonian landscape –was dominated by relatively small, –low-growing, –bog-dwelling types of plants,

• the Late Devonian –witnessed forests of large tree-size plants up to 10 m

tall

Early and Late Devonian Plants

• Seedless vascular plants require moisture –for successful fertilization –because the sperm must travel to the egg –on the surface of the gamete-bearing plant – gametophyte –to produce a successful spore-generating plant – sporophyte

• Without moisture, the sperm would dry out before reaching the egg

Seedless Vascular Plants Require Moisture

• In this way the need for a moist environment –for the gametophyte generation is solved

• The significance of this development • is that seed plants,

– like reptiles, –were no longer restricted –to wet areas –but were free to migrate –into previously unoccupied dry environments

Gymnosperms Free to Migrate

• Before seed plants evolved, –an intermediate evolutionary step was necessary

• This was the development of heterospory, –whereby a species produces two types of spores–a large one (megaspore) – that gives rise to the female gamete-bearing plant –and a small one (microspore) – that produces the male gamete-bearing plant

• The earliest evidence of heterospory –is found in the Early Devonian plant –Chaleuria cirrosa, – which produced spores of two distinct sizes

Heterospory, an Intermediate Step

• Reconstruction of early heterosporous–plant from New Brunswick, Canada

An Early Devonian Plant

Chaleuria cirrosa Spore of Chaleuria cirrosa

• The appearance of heterospory –was followed several million years later –by the emergence of progymnosperms– Middle and Late Devonian plants with fern-like reproductive

habit and a gymnosperm anatomy–which gave rise in the Late Devonian –to such other gymnosperm groups as – the seed ferns and conifer-type seed plants

Evolution of Conifer Seed Plants

• While the seedless vascular plants –dominated the flora of the Carboniferous coal-

forming swamps, • the gymnosperms

–made up an important element of the Late Paleozoic flora,

– particularly in the non-swampy areas

Plants in Swamps Versus Drier Areas

• The rocks of the Pennsylvanian Period Late Carboniferous–are the major source of the world's coal

• Coal results from –the alteration of plant remains –accumulating in low swampy areas

• The geologic and geographic conditions of the Pennsylvanian –were ideal for the growth of seedless vascular plants, –and consequently these coal swamps had a very

diverse flora

Late Carboniferous and Permian Floras

COAL FORESTS

• Seedless vascular plants were widespread during the Carboniferous period–360 – 290 million years ago

• Formed the fossil fuel coal–Less extensive coal beds were also formed during

other geological periods–Most continents were flooded by shallow swamps–Dead plants did not completely decay–“Peat” formed–Heat and pressure converted peat to coal

• It is evident from the fossil record –that whereas the Early Carboniferous flora –was similar to its Late Devonian counterpart, –a great deal of evolutionary experimentation was

taking place –that would lead to the highly successful Late

Paleozoic flora – of the coal swamps and adjacent habitats

• Among the seedless vascular plants, –the lycopsids and sphenopsids –were the most important coal-forming groups –of the Pennsylvanian Period

Coal-Forming Pennsylvanian Flora

• The lycopsids were present during the Devonian,

–chiefly as small plants, • but by the Pennsylvanian,

–they were the dominant element of the coal swamps,

–achieving heights up to 30 m in such genera as Lepidodendron and Sigillaria

• The Pennsylvanian lycopsid trees are interesting –because they lacked branches except at

their top

Lycopsids

• The leaves were elongate and similar to the individual palm leaf of today

• As the trees grew, –the leaves were replaced from the top, –leaving prominent and characteristic rows or spirals

of scars on the trunk• Today, the lycopsids are represented by small

temperate-forest ground pines

Lycopsids

• The sphenopsids, the other important coal-forming plant group, –are characterized by being jointed and having

horizontal underground stem-bearing roots–many of these plants, such as Calamites, average 5 to

6 m tall• Living sphenopsids include the horsetail

– Equisetum–and scouring rushes

• Small seedless vascular plants and seed ferns –formed a thick undergrowth or ground cover beneath

these treelike plants

Sphenopsids

• Not all plants were restricted to the coal-forming swamps

• Among those plants occupying higher and drier ground were some of the cordaites

–a group of tall gymnosperm trees

–that grew up to 50 m

–and probably formed vast forests

• A cordaite forest from the Late Carboniferous

Cordaites were a group of gymnosperm trees that grew up to • 50 m tall

A Cordaite Forest

• Another important non-swamp dweller was Glossopteris, the famous plant so abundant in Gondwana, –whose distribution is cited as critical evidence that

the continents have moved through time

Glossopteris

• The floras that were abundant –during the Pennsylvanian persisted into the Permian,

but because of climatic and geologic changes resulting from tectonic events,

they declined in abundance and importance• By the end of the Permian,

–the cordaites became extinct, while the lycopsids and sphenopsids were reduced to mostly small, creeping forms

• Those gymnosperms –with life styles more suited to the warmer and drier

Permian climates –diversified and came to dominate the Permian,

Triassic, and Jurassic landscapes

Climatic and Geologic Changes

SEED PLANTS

Advent of the Seed• Spores are the resistant stage in the life cycle of

bryophytes and seedless vascular plants–Able to withstand harsh environments–Seeds are also able to resist harsh environments

• Spores are the means by which bryophytes and seedless vascular plants disperse offspring–Able to be dispersed over great distances–Seeds became important in dispersing offspring

GYMNOSPERMS

• Probably descended from progymnosperms–Seeds had evolved by the end of Devonian period

–Adaptive radiation in Carboniferous and early Permian produced the gymnosperm divisions

–Largely replaced seedless vascular plants– Better adapted to drier (Pangean) climate –Seed Plant Clades

• Gymnosperms–Monophyletic group of flower-less seed plants

• Angiosperms–Monophyletic group of flowering seed plants

• Gymnosperms and angiosperms appear to have evolved from separate ancestors in the extinct progymnosperm group

GYMNOSPERMS• Four divisions currently exist

–Ginkgophyta (ginkgo)–Cycadophyta (cycads)–Gnetophyta (gnetophytes)–Coniferophyta (conifers)

Coal Forests

Seedless vascular plants were widespread during the Carboniferous period360 – 290 million years agoFormed the fossil fuel coalLess extensive coal beds were also formed during other geological periods

Most continents were flooded by shallow swampsDead plants did not completely decay“Peat” formed, Heat and pressure converted peat to coal

• Reconstruction of a Pennsylvanian coal swamp–with its characteristic vegetation

Pennsylvanian Coal Swamp

Amphibian Eogyrinus

SEED PLANTS

• Swamps began to dry up at the end of the Carboniferous period–290 million years ago–Pangea supercontinent formation hotter and

dryer continental interiors–Flora and fauna changed dramatically

• Seed plants had already existed, but rose to prominence after this environmental change

• By the end of the Permian, –about 90% of all marine invertebrate species were

extinct, –compared with more than two-thirds of all

amphibians and reptiles• Plants, on the other hand,

–apparently did not experience as great a turnover as animals did

Fewer Losses for Plants at PTB

• The Jurassic landscape was dominated by –seedless vascular plants, – particularly ferns, –as well as gymnosperms – such as conifers, – tree ferns, – and cycads

Mesozoic Plants

Mesozoic FloraTerrestrial Plants - Gymnosperms

Phyla: Cycadophyta, ConiferophytaMesozoic - "The Age of Cycads"Characteristics

Preservation:Cycadeoids, Wollemi Pines

seedsno true flowers

DIVISION CYCADOPHYTA

• A new group of gymnosperms – known as cycads made its

appearance during the Triassic• These palm-like plants

– became widespread – and now exist in tropical – and semi-tropical areas

• Superficially resemble palms• True palms are angiosperms• Seeds develop on the surface of sporophylls

– Specialized reproductive leaves

– Packed together to form cones

DIVISION GNETOPHYTA

• Consists of three genera very different in appearance

–Welwitschia– Giant strap-like leaves–Gnetum– Grow in tropics as trees or– vines–Ephedra– “Mormon tea”– Shrub of American deserts

Land Plants Gymnosperms• Cycads

– Cylindrical trunks and large-fernlike leaves– Dominant during Jurassic, common until Cretaceous

• Conifers– most modern conifer types– dominated Cretaceous forests as cycads declined

• Ginkgoes– Common in Mesozoic forests– Single surviving species like Mesozoic ancestors

Cycad: Sago palmGingko biloba

DIVISION CONIFEROPHYTA

• “Conifers”• Largest of the four

gymnosperm genera– ~550 species– e.g., Pines, firs, spruces,

larches, yews, junipers, cedars, cypresses, redwoods, etc.

– A few of these species dominate vast forested regions of the Northern Hemisphere

• Reproductive structures are cones– Cluster of scale-like

sporophylls

Pine Cones

Male pollen cone

Female ovulate cone

Sequoias

• The long dominance of seedless plants and gymnosperms – ended during the Early Cretaceous, – perhaps the Late Jurassic, – when many were replaced – by angiosperms

• Studies of fossil and living gymnosperms – show that some have a close relationship with angiosperms, – but precise ancestors remain obscure

Angiosperms

ANGIOSPERM RADIATION

• Radiation of angiosperms marks the transition from the Mesozoic era to the Cenozoic era–Earliest angiosperms found are 130 million years old

• Adaptive radiation made angiosperms the dominant plants on Earth by the end of the Cretaceous 65 million years ago–Adaptive radiation followed a period of

environmental disturbance

• A measure of the angiosperms' success is –that today with 250,000 to 300,000 species –they account for more than 90% of all land plant

species, –and they occupy some habitats in which other land

plants do poorly or cannot exist

• Archaefructus sinensis – from Lower Cretaceous rocks

in China – is among the oldest known

angiosperms

Fossil Angiosperms

Reconstruction of Archaefructus sinensis

Keys to Angiosperm Success

• Short life cycles and rapid growth

• Specialized reproductive structures (flowers)

• Specialized pollination and dispersal structures–Wind and animal pollinators–Fruits that float or stick–Seeds that survive animal digestive tracts

Specialized Angiosperm Structures

• A flower is a specialized reproductive shoot

• Seeds develop inside the ovaries (chambers that enclose ovules) of flowers

• After fertilization, an ovary becomes a fruit

Flower Structures

Pollination and Coevolution

• Pollinators–Animals (such as insects that feed on pollen)

move pollen grains from male parts of one flower to female parts of another

• Coevolution–Over time, plants and their animal pollinators

jointly evolved; changes in one exerts selection pressure on the other

COEVOLUTION• Plants have influenced the

evolution of animals• Animals have influenced

the evolution of plants–e.g., Plant-herbivore

coevolution–e.g., Plant-pollinator

coevolution

COEVOLUTION• Plant-pollinator coevolution is responsible for the

diversity of flowers–What does the pollinator gain?– Nectar, pollen, etc.–What does the plant gain?– Cross-pollination

Adaptive Radiation of Angiosperms

MAJOR PLANT GROUPS

• The Devonian Period was a time of rapid evolution for land plants

Major Events in the Evolution of Land Plants

Major events were the appearance of

leaves

heterospory secondary growth

and emergence of seeds

3a. Plants - early events

Precambrian/Cambrian

Ordovician

Silurian

Green scum near water. Bacteria and algaeLimited surface stabilisationand humus buildup.

Turf of mosses (bryophytes)and liverworts - non-vascularplants a few cm in height.Origin of spores

Origin of vascular plants, eg. Rhynia.Tens of cm height,extensive cover.

3b. Plants - first forests

Devonian

Carboniferous

Secondary wood by mid-Devonian. Deep root systems, soil development. Mainly seedless plants (eg ferns)Altered weathering/ CO2

Widespread,denseforests in a range ofclimatic bands andtopographic niches.Large scale coal production.Rise of seed plants - gymnospermseg Seed ferns, conifers,cycadsIce age??

Archaeopteris- a progymnosperm

4. Invertebrates

LowerPalaeozoicturf

UpperPalaeozoicforests

PostPalaeozoicmigrations

Millepedes, spiders,other primitive arthropods

Slugs, snails, worms,scorpions

May have needed leaf litterMain period of colonization

Insects evolve

Crabs, woodlice, amphipods

Mainly preadapted forms

Summary

• The earliest fossil record of land plants –is from Middle to Upper Ordovician rocks

• These plants were probably small and bryophyte-like in their overall organization

• The evolution of vascular tissue –was an important event in plant evolution as it

allowed food and water to be transported throughout the plant

–and provided the plant with additional support• The ancestor of terrestrial vascular plants

–was probably some type of green algae –based on such similarities – as pigmentation, – metabolic enzymes, – and the same type of reproductive cycle

Summary

• The earliest seedless vascular plants –were small, leafless stalks with spore-producing

structures on their tips• From this simple beginning,

–plants evolved many of the major structural features characteristic of today's plants

• By the end of the Devonian Period, –forests with tree-sized plants up to 10 m had

evolved

Summary

• The Late Devonian also witnessed – evolution of flowerless seed plants (gymnosperms)– whose reproductive style freed them – from having to stay near water

• The Carboniferous Period was a time – of vast coal swamps, – where conditions were ideal for seedless vascular plants

• With the onset of more arid conditions during the Permian, – the gymnosperms became the dominant element of the

world's flora– By Cretaceous period angiosperms became the most

dominant element of terrestrial vegetation