Cecie Starr Christine Evers Lisa Starr www.cengage.com/biology/starr

Chapter 21 Plant Evolution

(Sections 21.5 - 21.8) Albia Dugger • Miami Dade College

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

develop into female gametophytes form and

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

How a Seed Forms

Microspore in the pollen sac of a sporophyte develops into Male gametophyte (pollen grain) released pollination and fertilization Seed (embryo sporophyte in mature ovule) Megaspore in the ovule of a sporophyte develops into Female gametophyte in ovule Fig, 21.17, p. 333

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

Time Line for Plant Evolution

Bryophyt es evolve, diversify; seedless vascular plants evolve.

Diversificati on of seedless vascular plants.

First treelike plants (fern relatives), first seed 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 of ferns, cycads, conifers; by start of Cretaceous, conifers the are dominant trees. Flowering plants appear in the early Cretaceous, undergo adaptive radiation, and become dominant.

Ordovician Silurian Devonian Carboniferous 488 443 416 359 299 Permian 251 Triassic JurassicCretaceous 200 146 66 Tertiary Millions of years ago (mya) Fig, 21.13, p. 332

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

Time Line for Plant Evolution 1

A seed cone has many scales, each with two ovules on its upper surface section through one ovule (the red “cut” in the diagram to the left) surface view of seed cone scale ovule

3

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

section through pollen producing sac (red cut) surface view of pollen cone scale

9

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

seed coat embryo nutritive tissue

7

One sperm nucleus fertilizes the egg, forming a zygote.

6 8

Ovule develops into a mature seed.

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

Fertilization Diploid Stage Haploid Stage Meiosis (view inside ovule) eggs

5

Pollination: wind deposits pollen grain on seed cone.

4

Microspores form by meiosis, develop into pollen grains.

female gametophyte

2

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

Fig, 21.19, p. 335

Time Line for Plant Evolution 1

A seed cone has many scales, each with two ovules on its upper surface section through one ovule (the red “cut” in the diagram to the left) surface view of seed cone scale

3

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

ovule section through pollen producing sac (red cut) surface view of pollen cone scale

9

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

seed coat embryo nutritive tissue

7

One sperm nucleus fertilizes the egg, forming a zygote.

6 8

Ovule develops into a mature seed.

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

Fertilization Diploid Stage Haploid Stage Meiosis eggs

5

wind (view inside ovule) Pollination: pollen grain on seed cone.

4

Microspores form by meiosis, develop into pollen grains.

female gametophyte

2

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

Stepped Art Fig, 21.19, p. 335

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

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

A Cherry Flower

stamen filament anther stigma carpel style ovary petal (all petals combined form the flower’s corolla) sepal (all sepals combined form flower’s calyx) ovule (forms within ovary) receptacle Fig, 21.20, p. 336

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)

6

seedling seed coat embryo (2n) endosperm (3n)

5

seed Double fertilization Diploid (2n) Phase Haploid (n) Phase pollen sac, where each one of many cells will give rise to microspores Meiosis

2

ovules inside ovary

4

pollen tube sperm (n) sperm (n) pollen tube delivering two sperm to an ovule

1 Life Cycle of Lily (

Lilium)

ovules inside ovary cell in ovule that will give rise to a megaspore Meiosis pollen grain (the male gametophyte) cell from which endosperm will form egg

3

ovary that holds many ovules female gametophyte inside ovule

1

A sporophyte dominates this life cycle.

2

Pollen forms inside pollen sacs of stamens.

3

Eggs develop in an ovule within an ovary.

4

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

5

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.

6

The resulting seed will grow into a new sporophyte.

Fig, 21.21, p. 337

1 Life Cycle of Lily (

Lilium)

6

seedling Diploid (2n) Phase Haploid (n) Phase

5

seed coat embryo (2n) endosperm (3n) seed Double fertilization

4

pollen tube sperm (n) sperm (n) pollen tube delivering two sperm to an ovule pollen sac, where each one of many cells will give rise to microspores Meiosis

2

ovules inside ovary cell in ovule that will give rise to a megaspore Meiosis pollen grain (the male gametophyte) cell from which endosperm will form egg

3

ovary that holds many ovules female gametophyte inside ovule Stepped Art Fig, 21.21, p. 337

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