Plant Evolution

The Plant Kingdom

• Nearly all are multicelled

• Vast majority are photoautotrophs

– Energy from sun

– Carbon dioxide from air

– Minerals dissolved in water

Setting the Stage for Plants

• Earth’s atmosphere was originally oxygen free

• Ultraviolet radiation bombarded the surface

• Photosynthetic cells produced oxygen and allowed formation of a protective ozone layer

Invading the Land

• Cyanobacteria were probably the first to

spread into and up freshwater streams

• Later, green algae and fungi made the

journey together

• Every plant is descended from species of

green algae

Adaptations to Land

• Root systems

• Shoot systems

• Vascular tissues

• Waxy cuticle for water conservation

Evolutionary Trend in Plant Life Cycles

• Algae and bryophytes put most energy into making gametophytes

• Land plants put energy into structures that produce spores and retain, nourish, and protect gametes

Evolutionary Trend

sporophyte’s

importancegametophyte’s importance

green algae bryophytes ferns gymnosperms angiosperms

zygote only, no sporophyte

Milestones in Plant Evolution

charophytes bryophytes lycophytes horsetails ferns cycads ginkgos conifers gnetophytes flowering plants

seed plants

plants with true leaves

vascular plants

land plants

(closely related groups)

Pollen

• Pollen grains are sperm-bearing male gametophytes that develop from microspores

• Allows transfer of sperm to egg without water

• Can drift on air currents, or be carried by pollinators

Seeds

• Ovules are female reproductive structures that become seeds

• Consist of:– Female gametophyte with egg cell

– Nutrient-rich tissue

– Jacket of cell layers that will form seed coat

Nonvascular Plants

• Bryophytes

• Include 24,000 species of:

Liverworts

Hornworts

Mosses

Bryophytes

• Small, nonvascular, nonwooody

• Gametophyte dominates life cycle; has leaflike, stemlike, and rootlike parts

• Usually live in wet habitats

• Flagellated sperm require water to reach eggs

Moss Life Cycle

Fertilization MeiosisDiploid Stage

Haploid Stage

malegametophytetip

femalegametophytetip

Zygote

Development of mature sporophyte (still attached to gametophyte)

Male gametophyte

Female gametophyte

Spores released

Egg

Sperm

Marchantia: A Liverwort

• Reproduces asexually by way of

gemmae cups

• Sexual reproduction

• Gametophytes are male or female

• Gametes are produced on elevated

structures

Vascular Plants

• Majority of plants

• Have internal tissues that carry water

and solutes

• Two groups

– Seedless vascular plants

– Seed-bearing vascular plants

Seedless Vascular Plants

• Produce spores but no seeds

• Main groups:

Lycophytes

Horsetails

Ferns

Seedless Vascular Plants

• Like bryophytes:

– Live in wet, humid places

– Require water for fertilization

• Unlike bryophytes:

– Sporophyte is free-living and

has vascular tissues

Ferns

• 12,000 species, mostly tropical

• Most common sporophyte structure– Perennial underground stem (rhizome)– Roots and fronds arise from rhizome– Young fronds are coiled “fiddleheads”– Spores form on lower surface of some

fronds

Fern Life Cycle

The sporophyte (still attached to the gametophyte) grows, develops

zygote fertilization

Diploid Stage

Haploid Stage

egg

sperm

egg-producing structure

sperm-producing structure

mature gametophyte (underside)

Spores develop

meiosis

Sori

Spores are released

Spore germinates

rhizome

gametophyte

Seed-Bearing Vascular Plants

• Gymnosperms arose first– Cycads– Ginkgos– Conifers

• Angiosperms arose later– Monocots– Dicots

Seed-Bearing Plants

• Microspores that give rise

to pollen grains

• Megaspores inside ovules

• More water-conserving

than seedless vascular

plants

Special Traits of Seed-Bearing Plants

• Pollen grains– Arise from megaspores– Develop into male gametophytes– Can be transported without water

• Seeds– Embryo sporophyte inside nutritive tissues

and a protective coat– Can withstand hostile conditions

Gymnosperms

• Plants with “naked seeds”

• Seeds don’t form inside an ovary

Conifers Ginkgos

Cycads

Conifer Characteristics

• Woody trees or shrubs

• Most are evergreen

• Bear seeds on exposed cone scales

• Most produce woody cones

Cycads

• Only 130 living species

• Two species of Zamia are native to U.S.

• Palmlike appearance

• Pollen-bearing and seed-bearing cones on different plants

Ginkgos

• Only surviving species, Ginkgo biloba, is native to China

• Deciduous tree with fan-shaped leaves

• Trees are male or female

• Female trees produce seeds covered with a fleshy, foul-smelling coat

Pine Cones

• Woody scales of a “pine cone” are the

parts of where megaspores formed

and developed into female

gametophytes

• Male cones, where microspores and

pollen are produced, are not woody

PineLife

Cycle Sporophyte

Female cone

Male cone

Ovule

Fertilization Meiosis

MicrosporesMegaspores

Pollen sac

Egg View insideovule

Pollen tube

Seed

Angiosperms

• Flowering plants

• Dominant land plants (260,000 species)

• Defining feature: Ovules and (after fertilization) seeds are enclosed in an ovary

• Two classes: Monocots and dicots

Plants and Pollinators

• Pollen:– Sperm packed inside a nutritious package

– Transferred first by wind currents

– Later transferred by insects

• Plants that attracted insect pollinators with flowers had a reproductive advantage

Pollen Formation

• Each anther has four pollen sacs

• Inside the pollen sacs, cells undergo

meiosis and cytoplasmic division to form

microspores

• Microspores undergo mitosis to form

pollen grains

Double Fertilization

• A pollen tube grows down through the ovary tissue

• It carries two sperm nuclei

• When pollen tube reaches an ovule, it penetrates embryo sac and deposits two sperm

• One fertilizes the egg, other fuses with both nuclei of endosperm mother cell

Endosperm Formation

• Occurs only in angiosperms

• Fusion of a sperm nucleus with the two nuclei

of the endosperm mother cell produces a

triploid (3n) cell

• This cell will give rise to the endosperm, the

nutritive tissue of the seed

Seeds and Fruits

• The seed is the mature ovule

• The fruit is the mature ovary

wall of ovary

ovule

Structure of a Seed

• Protective seed coat is derived from

integuments that enclosed the ovule

• Nutritious endosperm is food reserve

• Embryo has one or two cotyledons

– Monocot has one

– Eudicot has two

Seed Formation

• Fertilization of the egg produces a diploid sporophyte zygote

• The zygote undergoes mitotic divisions to become an embryo sporophyte

• Seed: A mature ovule, which encases an embryo sporophyte and food reserves inside a protective coat

Seed Dispersal

• Fruit structure is adapted to mode of dispersal

• Some modes of seed dispersal:

– Wind currents

– Water currents

– Animals

Double Fertilization

• Distinctive feature of angiosperms

• Male gametocyte delivers two sperm to an ovule

• One fertilizes egg; other fertilizes a cell that gives rise to endosperm that supports embryo

Flower Structure

stamen (microspores form here)

carpel (megaspores form here)

petal

sepal

ovule in an ovary

Flowering Plant Life

Cycle Double fertilization Meiosis Meiosis

Microspores

Female gametophyte

Pollination

Mitosis without cytoplasmic division

Two sperm enter ovule

Diploid

Haploid

Deforestation

• Result of demand for wood as fuel and lumber; cultivation of land for agriculture

• Greatest occurrence in Brazil, Indonesia, Columbia, and Mexico