Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr

Chapter 21Plant Evolution

(Sections 21.5 - 21.8)

21.5 History of the Vascular Plants

• The oldest fossils of vascular plants are spores that date to about 450 million years ago (late Ordovician period)

• Early vascular plants stood only a few centimeters high and had a simple branching pattern, with no leaves or roots

• Fossil Cooksonia

From Tiny Branchers to Coal Forests

• By the early Devonian, taller species with a more complex branching pattern were common worldwide

From Tiny Branchers to Coal Forests

• Forests of giant seedless vascular plants thrived during the Carboniferous period – heat and pressure transformed the remains of these forests to coal

• coal • Fossil fuel formed over millions of years by compaction

and heating of plant remains

A Carboniferous “Coal Forest”

Rise of the Seed Plants

• Cycads and ginkgos were among the earliest gymnosperm lineages

• Early angiosperms such as magnolias evolved while dinosaurs walked on Earth

Reproductive Traits of Seed Plants

• Seed plant sporophytes have pollen sacs, where microspores form and develop into male gametophytes (pollen grains)

• Sporophytes also have ovules, where megaspores form and develop into female gametophytes

Key Terms

• pollen sac • Of seed plants, reproductive structure in which sperm-

bearing gametophytes (pollen grains) develop

• microspore • Haploid spore formed in pollen sacs of seed plants;

develops into a sperm-producing gametophyte (a pollen grain)

Key Terms

• megaspore • Haploid spore formed in ovule of seed plants• Develops into an egg-producing gametophyte

• ovule • Of seed plants, reproductive structure in which egg-

bearing gametophyte develops• After fertilization, matures into a seed

How a Seed Forms

Fig, 21.17, p. 333

Seed (embryo

sporophyte in mature ovule)

pollination and fertilization

Male gametophyte (pollen grain)

released

develops into

Microspore in the pollen sac of

a sporophyte

develops into

Female gametophyte

in ovule

Megaspore in the ovule of a sporophyte

How a Seed Forms

Pollination

• A sporophyte releases pollen grains, but holds onto its eggs

• Wind or animals can deliver pollen from one seed plant to the ovule of another

• pollination• Delivery of a pollen grain to the egg-bearing part of a seed

plant

Time Line for Plant Evolution

Fig, 21.13, p. 332

Millions of years ago (mya)

Tertiary416

Ordovician Silurian Devonian Carboniferous Permian Triassic JurassicCretaceous66146200251299359443488

Bryophytes evolve, diversify;seedless vascular plants evolve.

Diversification of seedless vascular plants.

Giant horsetails, club mosses relatives in swamp forests. Conifers arise late in period.

Ginkgos, cycads evolve. Most horsetails and club mosses die off by end of the period.

Adaptive radiations offerns, cycads, conifers; by start of Cretaceous, conifers the are dominant trees.

Flowering plants appear in the early Cretaceous, undergo adaptive radiation, and become dominant.

First treelike plants (fern relatives), first seed plants.

Time Line for Plant Evolution

21.6 Gymnosperms—Plants With Naked Seeds

• Gymnosperms are vascular seed plants that produce seeds on the surface of ovules• Seeds are “naked” (not inside a fruit)• Includes conifers, cycads, ginkgos, and gnetophytes

• gymnosperm • Seed plant that does not make flowers or fruits

The Conifers

• In conifers, ovules form on the surfaces of woody cones

• Conifers typically have needlelike or scalelike leaves , and tend to be resistant to drought and cold

• conifer • Gymnosperm with nonmotile sperm and woody cones• Examples: pines, redwoods

A Conifer

• Conifers include the long-lived bristlecone pines

• One of these trees is now 4,600 years old

Cycads

• An Australian cycad with its fleshy seeds

• cycad • Tropical or subtropical

gymnosperm with flagellated sperm, palmlike leaves, and fleshy seeds

Ginkgo biloba

• Ginkgo’s fleshy seeds and fan-shaped leaves

• ginkgo • Deciduous gymnosperm

with flagellated sperm, fan-shaped leaves, and fleshy seeds

• One species: Ginkgo biloba

Gnetophytes

• Ephedra: Yellow structures on stems are pollen-bearing cones

• gnetophyte • Shrubby or vinelike

gymnosperm, with nonmotile sperm

A Representative Life Cycle: Ponderosa Pine

• Inside the ovule, a megaspore forms by meiosis and develops into a female gametophyte

• Male cones hold pollen sacs where microspores develop into pollen grains

• Pollen grains are released; pollination occurs when one lands on an ovule, and the pollen grain germinates

• It takes about a year for a pollen tube to grow through ovule tissue and deliver sperm to the egg

A Representative Life Cycle: Ponderosa Pine (cont.)

• When fertilization finally occurs, it produces a zygote

• The zygote develops into an embryo sporophyte that, along with tissues of the ovule, becomes a seed

• The seed is released, germinates, and grows and develops into a new sporophyte

Life Cycle of a Conifer: Ponderosa Pine

Fig, 21.19, p. 335

Pollen grain matures into male gametophyte. Two nonflagellated sperm nuclei form as pollen tube grows through ovule tissue.

sperm-producing cell

Megaspores form by meiosis; one develops into the female gametophyte.

Pollination: wind deposits pollen grain on seed cone.

(view inside ovule)

pollen tube

One sperm nucleus fertilizes the egg, forming a zygote.

Ovule develops into a mature seed.

Seed is released, germinates, and the embryo grows and develops into a new sporophyte. nutritive

tissue

surface view of pollen cone scale

section through pollen-producing sac (red cut)

A pollen cone has many scales, each housing a pollen sac.

surface view of seed cone scale ovule

section through one ovule (the red “cut” in the diagram to the left)

A seed cone has many scales, each with two ovules on its upper surface

Microspores form by meiosis, develop into pollen grains.

seed coat

embryo

female gametophyte

eggs

MeiosisFertilization Haploid StageDiploid Stage

1

2

3

45

6

7

8

9

Time Line for Plant Evolution

One sperm nucleus fertilizes the egg, forming a zygote.

Seed is released, germinates, and the embryo grows and develops into a new sporophyte. nutritive

tissue

seed coat

embryo7

9

Ovule develops into a mature seed. Fertilization

8

Megaspores form by meiosis; one develops into the female gametophyte.

section through pollen-producing sac (red cut)

ovule

section through one ovule (the red “cut” in the diagram to the left)

Microspores form by meiosis, develop into pollen grains.

MeiosisHaploid StageDiploid Stage

24

Pollen grain matures into male gametophyte. Two nonflagellated sperm nuclei form as pollen tube grows through ovule tissue.

sperm-producing cell

(view inside ovule)

pollen tube

female gametophyte

eggs

5

6

Pollination: wind deposits pollen grain on seed cone.

surface view of pollen cone scale

A pollen cone has many scales, each housing a pollen sac.

A seed cone has many scales, each with two ovules on its upper surface

1

3

surface view of seed cone scale

Fig, 21.19, p. 335

Stepped Art

Time Line for Plant Evolution

Fig, 21.19.1, p. 335

Time Line for Plant Evolution

Fig, 21.19.3, p. 335

Time Line for Plant Evolution

Fig, 21.19.9, p. 335

Time Line for Plant Evolution

ANIMATION: Pine life cycle

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Key Concepts

• Gymnosperms • Seed plants make pollen grains that allow fertilization to

occur even in dry times • They also make eggs in an ovule that develops into a seed• Gymnosperms such as pine trees are seed plants with

“naked” seeds – their seeds do not form inside an ovary

21.7 Angiosperms—The Flowering Plants

• Angiosperms are seed plants that make flowers and fruits

• angiosperms • Largest seed plant lineage• Only group that makes flowers and fruits

• flower • Specialized reproductive shoot of a flowering plant

• fruit • Mature flowering plant ovary; encloses a seed or seeds

Flowers

• A flower consists of modified leaves arranged in concentric whorls of sepals and petals

• The stamens of a flower produce pollen.

• Eggs form in the female part of the flower (carpel)

• An ovary at the base of the carpel holds one or more ovules

Key Terms

• stamen • Male reproductive part of a flower

• carpel• Female reproductive part of a flower

• ovary • Of flowering plants, floral chamber that encloses ovule

A Cherry Flower

Fig, 21.20, p. 336

sepal (all sepals combined form flower’s calyx)

petal (all petals combined form the flower’s corolla)

receptacle

ovule(forms within ovary)

style

stamen carpel

filament anther stigma ovary

A Cherry Flower

Pollinators

• Many flowering plants coevolved with pollinators

• pollinator • Animal that moves

pollen, facilitating pollination

Fruits and Seeds

• After pollination, the flower’s ovary becomes a fruit that contains one or more seeds

• A flowering plant seed includes an embryo sporophyte and endosperm, a nutritious tissue

• A variety of dispersal-related traits help disperse seeds to new habitats where they can thrive

Major Lineages of Flowering Plants

• Two major lineages differ in seed structure and other traits:• Monocots include orchids, palms, lilies, and grasses• Eudicots include most herbaceous (nonwoody) plants

such as tomatoes, cabbages, roses, poppies, most flowering shrubs and trees, and cacti

Key Terms

• monocots • Lineage of angiosperms with one cotyledon• Do not produce true wood• Includes grasses, orchids, and palms

• eudicots • Lineage of angiosperms with two cotyledons• Includes herbaceous plants, woody trees, and cacti

Monocot Life Cycle (Lilium)

1. Sporophyte dominates the life cycle

2. Pollen forms inside pollen sacs of stamens

3. Eggs develop in an ovule within an ovary

4. Pollination occurs; a tube grows from the pollen grain into the ovule, delivering two sperm

5. Double fertilization occurs

6. The resulting seed grows into a sporophyte

Double Fertilization

• Double fertilization occurs in all flowering plant life cycles• One sperm fertilizes the haploid egg• One fertilizes a diploid cell, yielding a triploid cell that

divides to form endosperm, which nourishes the embryo sporophyte

• endosperm • Nutritive triploid tissue in angiosperm seeds

Life Cycle of Lily (Lilium)

Fig, 21.21, p. 337

female gametophyte inside ovule

ovary that holds many ovules

pollen tube delivering two sperm to an ovule

egg

cell from which endosperm will form

pollen grain (the male gametophyte)

sperm (n)

MeiosisHaploid (n) PhaseDouble fertilization

cell in ovule that will give rise to a megaspore

ovules inside ovary

pollen sac, where each one of many cells will give rise to microspores

seed

embryo (2n)

seedling

pollen tubesperm (n)

Diploid (2n) Phase Meiosis

seed coat

endosperm (3n)

ovules inside ovary

The resulting seed will grow into a new sporophyte.

Double fertilization occurs in all flowering plant life cycles. One sperm fertilizes the haploid egg. The other fertilizes a diploid cell, yielding a triploid cell that divides to form endosperm, a tissue that nourishes the embryo sporophyte.

Pollination occurs and a tube grows from the pollen grain into the ovule, delivering two sperm.

Eggs develop in an ovule within an ovary.

Pollen forms inside pollen sacs of stamens.

A sporophyte dominates this life cycle.

1

2

3

4

5

1

2

3

4

5

6

6

Life Cycle of Lily (Lilium)

female gametophyte inside ovule

egg

cell from which endosperm will form

pollen grain (the male gametophyte)

MeiosisHaploid (n) Phase

Meiosis

2

3

cell in ovule that will give rise to a megaspore

ovules inside ovary

pollen sac, where each one of many cells will give rise to microspores

Diploid (2n) Phase

1

pollen tube delivering two sperm to an ovule

sperm (n)

Double fertilization

pollen tubesperm (n)

4

ovary that holds many ovules

seed

embryo (2n)

seedling

seed coat

endosperm (3n)5

6

Fig, 21.21, p. 337

Stepped Art

Life Cycle of Lily (Lilium)

ANIMATION: Monocot life cycle

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ANIMATION: Flower parts

21.8 Ecological and Economic Importance of Angiosperms

• Dominant plants in most land habitats, flowering plants are ecologically important – and essential to human existence

• Angiosperms feed and shelter animals, and they provide us with food, fabric, oils, medicines, drugs, and more…

Flowering Plants Feed Animals

• Angiosperms provide food and shelter for a variety of animals

• This hummingbird is sipping nectar, which is mainly sucrose, from a columbine flower

Angiosperms as Crop Plants

• Nutrient-rich endosperms of angiosperm seeds are staples of human diets throughout the world

• Mechanized harvesting of wheat, a monocot

Commercial Products

• Angiosperms supply us with fiber, wood, and oils

• Field of cotton, a eudicot

Medicines and Drugs

• Some flowering plants make secondary metabolites that we use as medicines or as mood-altering drugs

• secondary metabolite • Chemical that has no known role in an organism’s normal

metabolism; often deters predation

Secondary Metabolites

• Aspirin, digitalis, caffeine, nicotine, pyrethrums, opium

• Marijuana (a eudicot) growing illegally in Oregon

Key Concepts

• Angiosperms • Angiosperms, or flowering plants, are the most recently

evolved seed plants • They alone make flowers, and their seeds form inside a

floral ovary that develops into a fruit • Angiosperms are the most widely dispersed and diverse

group of plants

Speaking for the Trees (revisited)

• Every atom of carbon in a tree was taken up from the air in the form of carbon dioxide

• A tree is about 20 percent carbon by weight, so enormous amounts of carbon are stored in forests

• Burning forests to make way for agriculture or other uses adds carbon dioxide to the air, reduces carbon uptake, and contributes to global warming

Releasing Carbon