Chapter 30

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 30

Plant Diversity II: The Evolution of Seed Plants

Evolution of Plants

Floweringplants

Cone-bearingplants

Ferns andtheir relatives

Mosses andtheir relatives

Green algaeancestor

Flowers; SeedsEnclosed in Fruit

Seeds

Water-Conducting(Vascular) Tissue

Vascular w/ SeedsAngiospermsVascular w/ Seeds

GymnospermsVascular SeedlessPterophytaNon-Vascular

Bryophytes


Overview: Feeding the World• Seeds changed the course of plant evolution

– Enabling their bearers to become the dominant producers in most terrestrial ecosystems

Figure 30.1


Overview of Seed Plant Evolution

1. Reduction of gametophyte continued with the evolution of seed plants.

2. Seeds became an important means of dispersing offspring.

3. Pollen eliminated the liquid-water requirement for fertilization.

4. The two clades of seed plants are gymnosperms and angiosperms.

Figure 30.1 Three variations on gametophyte/sporophyte relationships


Two Groups of Seed Plants

1. Gymnosperms – “naked seed” plants – most ancient of seed plants; produce cones with seeds

Conifers

Cycads

Ginkos

Gnetophytes

2. Angiosperms – covered seed plants - these produce flowers….then fruits that have the seeds inside – fruits provide seed protection and better guarantees seed dispersal when eaten by animals

Grasses

Flowering trees and shrubs

All flowers

Seed coat

Embryo

Storedfood supply

Seed

Wing

A

B

Section 22-4The Structure of a Seed

Go to Section:

Seed: embryo of plant that is wrapped in a protective covering and surrounded by a food supply.

Presence of a seed allows for reproduction free of water.

Figure 30.2 From ovule to seed

a) A fleshy megasporangium is surrounded by protective layers of tissue called integument.

b) A megaspore develops into a multicellular female gametophyte. The micropyle, the only opening through the integument, allows entry of the pollen grain. The pollen grain contains a male gametophyte, which develops a pollen tube that discharges sperm.

c) Fertilization initiates the transformation of the ovule into a seed, which consists of a sporophyte embryo, a food supply, and a protective seed coat derived from the integument.


Ovules and Production of Eggs

• An ovule consists of

– A megasporangium, megaspore, and protective integuments

Figure 30.3a

(a) Unfertilized ovule. In this sectional view through the ovule of a pine (a gymnosperm), a fleshy megasporangium is surrounded by a protective layer of tissue called an integument. (Angiosperms have two integuments.)

Integument

Spore wall

Megasporangium(2n)

Megaspore (n)


Pollen and Production of Sperm

• Microspores develop into pollen grains

– Which contain the male gametophytes of plants

• Pollination

– Is the transfer of pollen to the part of a seed plant containing the ovules


Pollen and the Production of Sperm• If a pollen grain germinates

– It gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule

Figure 30.3b

(b) Fertilized ovule. A megaspore develops into a multicellular female gametophyte. The micropyle,the only opening through the integument, allowsentry of a pollen grain. The pollen grain contains amale gametophyte, which develops a pollen tubethat discharges sperm.

Spore wall

Male gametophyte(within germinatingpollen grain) (n)

Femalegametophyte (n)

Egg nucleus (n)

Dischargedsperm nucleus (n)

Pollen grain (n)Micropyle


Seed Plant Characteristics

• In addition to seeds, the following are common to all seed plants

– Reduced gametophytes• Egg and sperm are protected in ovules and pollen grains.

– Heterospory• Plants in which the sporophyte produces two kinds of spores that

develop into either male or female gametophytes.

– Ovules• Structure that develops in the plant ovary and contains the female

gametophyte.

– Pollen

• Structure that contains immature male gametophyte.


The Evolutionary Advantage of Seeds

• A seed

– Is a sporophyte embryo, along with its food supply, packaged in a protective coat

– Develops from the whole ovule

Figure 30.3c

Gymnosperm seed. Fertilization initiatesthe transformation of the ovule into a seed,which consists of a sporophyte embryo, a food supply, and a protective seed coat derived from the integument.

(c)

Seed coat(derived fromIntegument)

Food supply(femalegametophytetissue) (n)

Embryo (2n)(new sporophyte)


Advantages of Reduced Gametophytes• The gametophytes of seed plants

– Develop within the walls of spores retained within tissues of the parent sporophyte

– Because of this, the delicate female gametophytes do not have to cope with many environmental stresses

• The gametophyte and the embryo produced after fertilization are sheltered from drought and UV radiation by their enclosure in the moist reproductive tissues of the parental sporophyte generation.

• This arrangement also makes it possible for the gametophytes to obtain nutrients from their parents.


Gametophyte/Sporophyte Relationships

Figure 30.2a–c

Sporophyte dependent on gametophyte (mosses and other bryophytes).

(a) Large sporophyte and small, independent gametophyte (ferns and other seedless vascular plants).

(b)

Microscopic femalegametophytes (n) inovulate cones(dependent)

Sporophyte (2n),the flowering plant(independent)

Microscopic malegametophytes (n)inside these partsof flowers(dependent)

Microscopic malegametophytes (n)in pollen cones(dependent) Sporophyte (2n)

(independent)

Microscopic femalegametophytes (n)inside these partsof flowers(dependent)

Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms).

(c)

Gametophyte(n)

Gametophyte(n)

Sporophyte(2n)

Sporophyte(2n)


Heterospory: The Rule Among Seed Plants

• Seed plants evolved from plants that had megasporangia

– Which produce megaspores that give rise to female gametophytes

• Seed plants evolved from plants that had microsporangia

– Which produce microspores that give rise to male gametophytes


Evolutionary Advantage of Pollen• Pollen, which can be dispersed by air or animals

– Eliminated the water requirement for fertilization

– Is light weight, so it can be carried away by wind or animals after their release from the microsporangium

• In seed plants, the use of resistant, far-traveling, airborne pollen to bring gametes together is a terrestrial adaptation that led to even greater success and diversity of plants on land.

– As opposed to bryophytes and pterophytes which have flagellated sperm that must swim through a film of water to reach egg cells in archegonia.


Gymnosperms – Cone Bearers• Gymnosperms bear “naked” seeds, typically on cones

• Among the gymnosperms are many well-known conifers

– Or cone-bearing trees, including pine, fir, and redwood

• The gymnosperms include four plant phyla

– Cycadophyta

– Gingkophyta

– Gnetophyta

– Coniferophyta


Figure 30.4

Gnetum

Ephedra

Ovulate cones

Welwitschia

PHYLUM GNETOPHYTA

PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA

Cycas revoluta

Exploring Gymnosperm Diversity


Exploring Gymnosperm Diversity

Figure 30.4

Douglas fir

Pacificyew

Common juniper

Wollemia pine

Bristlecone pine Sequoia

PHYLUM CYCADOPHYTA


A Closer Look at the Life Cycle of a Pine

• Key features of the gymnosperm life cycle include

– Dominance of the sporophyte generation, the pine tree

– The development of seeds from fertilized ovules

– The role of pollen in transferring sperm to ovules

– Seeds typically require wind for dispersal


Figure 30.6

Ovule

Megasporocyte (2n)

IntegumentLongitudinalsection ofovulate cone

Ovulatecone

Pollencone

Maturesporophyte(2n)

Longitudinalsection ofpollen cone

Microsporocytes(2n)

Pollengrains (n)(containing malegametophytes)

MEIOSIS

Micropyle

Germinatingpollen grain

Megasporangium

MEIOSIS

SporophyllMicrosporangium

Survivingmegaspore (n)

Germinatingpollen grain

ArchegoniumIntegumentEgg (n)

Femalegametophyte

Germinatingpollen grain (n)

Dischargedsperm nucleus (n)

Pollentube

Egg nucleus (n)FERTILIZATION

Seed coat(derived fromparentsporophyte) (2n)

Food reserves(gametophytetissue) (n)

Embryo(new sporophyte)(2n)

Seeds on surfaceof ovulate scale

Seedling

Key

Diploid (2n)Haploid (n)

The Life Cycle of a Pine

A pollen cone contains many microsporangia held in sporophylls. Each microsporangium contains microsporocytes (microspore mothercells). These undergo meiosis, giving rise tohaploid microspores that develop into pollen grains.

3

In mostconifer species,

each tree hasboth ovulate

and pollencones.

1

A pollen grainenters throughthe micropyleand germinates,forming a pollentube that slowlydigeststhrough themegasporangium.

4

While thepollen tubedevelops, themegasporocyte(megasporemother cell)undergoes meiosis,producing fourhaploid cells. Onesurvives as amegaspore.

5

The female gametophytedevelops within the megasporeand contains two or threearchegonia, each with an egg.

6

By the time the eggs are mature,two sperm cells have developed in thepollen tube, which extends to thefemale gametophyte. Fertilization occurswhen sperm and egg nuclei unite.

7

Fertilization usually occurs more than a year after pollination. All eggs

may be fertilized, but usually only one zygote develops into an embryo. The

ovule becomes a seed, consisting of an embryo, food supply, and seed coat.

8

An ovulate cone scale has twoovules, each containing a mega-sporangium. Only one ovule is shown.

2

Figure 30.3 Winged seed of a White Pine (Pinus strobus)


Female vs. Male Pine Cones

• Female pine cones are the large, ‘normal’ cones – ovulate cone

• Male pine cones are the clusters of small, worm-like structures at the tips of the branches – pollen cone

Evolution of Plants

Floweringplants

Cone-bearingplants

Ferns andtheir relatives

Mosses andtheir relatives

Green algaeancestor

Flowers; SeedsEnclosed in Fruit

Seeds

Water-Conducting(Vascular) Tissue

Vascular w/ SeedsAngiospermsVascular w/ Seeds

GymnospermsVascular SeedlessPterophytaNon-Vascular

Bryophytes


Angiosperms – Flower and Fruit Producers

• The reproductive adaptations of angiosperms include flowers and fruits

• Angiosperms

– Are commonly known as flowering plants

– Are seed plants that produce the reproductive structures called flowers and fruits

– Are the most widespread and diverse of all plants


Characteristics of Angiosperms

• The key adaptations in the evolution of angiosperms

– Are flowers and fruits

• The flower

– Is an angiosperm structure specialized for sexual reproduction

• Fruits– Typically consist of a mature ovary


Angiosperms – Flowering Plants

• Angiosperm means “enclosed seed”

• Angiosperms have unique reproductive organs known as flowers– Flowers attract pollinators, which makes spreading seeds more

efficient than the wind pollination of most gymnosperms

• Flowers contain ovaries, which surround and protect the seeds

– After pollination, the ovary develops into a fruit, which protects the seed and aids dispersal

– Fruit is a thick wall of tissue and another reason why angiosperms are successful – the fruit attracts herbivores – which eat the fruit and then spread the seeds


Flowers• A flower is a specialized shoot with modified

leaves– Sepals, which enclose the flower

– Petals, which are brightly colored and attract pollinators

– Stamens, which produce pollen

– Carpels, which produce ovules

Figure 30.7

Anther

Filament

StigmaStyle

Ovary

Carpel

Petal

ReceptacleOvule

Sepal

Stamen


Fruits

Figure 30.8a–e

(b) Ruby grapefruit, a fleshy fruitwith a hard outer layer andsoft inner layer of pericarp

(a) Tomato, a fleshy fruit withsoft outer and inner layersof pericarp

(c) Nectarine, a fleshyfruit with a soft outerlayer and hard innerlayer (pit) of pericarp

(e) Walnut, a dry fruit that remains closed at maturity

(d) Milkweed, a dry fruit thatsplits open at maturity

Figure 30.15 Relationship between a pea flower and a fruit (pea pod)


Fruit Adaptations Enhance Seed Dispersal• Can be carried by wind, water, or animals to new

locations, enhancing seed dispersal

Figure 30.9a–c

Wings enable maple fruits to be easily carried by the wind.

(a)

Seeds within berries and other edible fruits are often dispersed in animal feces.

(b)

The barbs of cockleburs facilitate seed dispersal by allowing the fruits to “hitchhike” on animals.

(c)


Fruit types

• Depends on their developmental origin:

– Simple fruit – comes from a single ovary

• ex. Cherry, soybean pod

– Aggregate fruit – single flower with several carpels

• ex. Blackberry

– Multiple fruit – develops from an inflorescence (group of flowers tightly clustered together)

• ex. pineapple

Table 30.1 Classification of Fleshy Fruits


The Angiosperm Life Cycle

• In the angiosperm life cycle

– Double fertilization occurs when a pollen tube discharges two sperm into the female gametophyte within an ovule

– One sperm fertilizes the egg, while the other combines with two nuclei in the center cell of the female gametophyte and initiates development of food-storing endosperm

• The endosperm nourishes the developing embryo


Figure 30.10

Key

Mature flower onsporophyte plant(2n)

Ovule withmegasporangium (2n)

Female gametophyte(embryo sac)

Nucleus ofdevelopingendosperm

(3n)

Dischargedsperm nuclei (n)

Pollentube

Male gametophyte(in pollen grain)

Pollentube

Sperm

Survivingmegaspore(n)

Microspore (n) Generative cell

Tube cell

Stigma

OvaryMEIOSIS

MEIOSIS

Megasporangium(n)

Pollengrains

EggNucleus (n)

Zygote (2n)

Antipodal cellsPolar nucleiSynergidsEgg (n)

Embryo (2n)

Endosperm(foodSupply) (3n)

Seed coat (2n)

Seed

FERTILIZATION

Haploid (n)

Diploid (2n)

Anther

Sperm(n)

Pollentube

Style

MicrosporangiumMicrosporocytes (2n)

GerminatingSeed

Anthers contain microsporangia.Each microsporangium contains micro-sporocytes (microspore mother cells) thatdivide by meiosis, producing microspores.

1 Microspores formpollen grains (containingmale gametophytes). Thegenerative cell will divideto form two sperm. Thetube cell will produce thepollen tube.

2

In the megasporangiumof each ovule, themegasporocyte divides bymeiosis and produces fourmegaspores. The survivingmegaspore in each ovuleforms a female gametophyte(embryo sac).

3

After pollina-tion, eventuallytwo sperm nucleiare discharged ineach ovule.

4

Double fertilization occurs. One spermfertilizes the egg, forming a zygote. Theother sperm combines with the two polarnuclei to form the nucleus of the endosperm,which is triploid in this example.

5

The zygotedevelops into an

embryo that ispackaged alongwith food into aseed. (The fruit

tissues surround-ing the seed are

not shown).

6

When a seedgerminates, the

embryo developsinto a mature

sporophyte.

7

The Angiosperm Life Cycle


Angiosperm Diversity• The two main groups of angiosperms

– Are monocots and dicots

• Basal angiosperms

– Are less derived and include the flowering plants belonging to the oldest lineages

• Magnoliids

– Share some traits with basal angiosperms but are more closely related to monocots and eudicots


Section 22-5

Go to Section:

Comparison of Monocots and Dicots


Exploring Angiosperm Diversity

Figure 30.12

Amborella trichopoda Water lily (Nymphaea “Rene Gerard”)

Star anise (Illicium floridanum)

BASAL ANGIOSPERMS

HYPOTHETICAL TREE OF FLOWERING PLANTS

MAGNOLIIDS

Am

bore

lla

Wat

er li

lies

Star

ani

sean

d re

lativ

es

Mag

nolii

ds

Mon

ocot

s

Eudi

cots

Southern magnolia (Magnoliagrandiflora)


Exploring Angiosperm Diversity

Figure 30.12

Orchid(Lemboglossumfossii)

MonocotCharacteristics

Embryos

Leafvenation

Stems

Roots

Pollen

Flowers

Pollen grain withone opening

Root systemUsually fibrous(no main root)

Vascular tissuescattered

Veins usuallyparallel

One cotyledon Two cotyledons

Veins usuallynetlike

Vascular tissueusually arranged

in ring

Taproot (main root)usually present

Pollen grain withthree openings

Zucchini(CucurbitaPepo), female(left) andmale flowers

Pea (Lathyrus nervosus,Lord Anson’sblue pea), a legume

Dog rose (Rosa canina), a wild rose

Pygmy date palm (Phoenix roebelenii)

Lily (Lilium“Enchant-ment”)

Barley (Hordeum vulgare), a grass

Anther

Stigma

Californiapoppy(Eschscholziacalifornica)

Pyrenean oak(Quercuspyrenaica)

Floral organsusually in

multiples of three

Floral organs usuallyin multiples of

four or fiveFilament Ovary

EudicotCharacteristics

MONOCOTS EUDICOTS


Evolutionary Links Between Angiosperms and Animals

• Pollination of flowers by animals and transport of seeds by animals are two important relationships in terrestrial ecosystems

Figure 30.13a–c

(a) A flower pollinated by honeybees. This honeybee is harvesting pollen and nectar (a sugary solution secreted by flower glands) from a Scottish broom flower. The flower has a tripping mechanism that arches the stamens over the beeand dusts it with pollen, some ofwhich will rub off onto the stigmaof the next flower the bee visits.

(c) A flower pollinated by nocturnal animals. Some angiosperms, such as this cactus, depend mainly on nocturnal pollinators, including bats. Common adaptations of such plants include large, light-colored, highly fragrant flowers that nighttime pollinators can locate.

(b) A flower pollinated by hummingbirds.The long, thin beak and tongue of this rufous hummingbird enable the animal to probe flowers that secrete nectar deep within floral tubes. Before the hummer leaves, anthers will dust its beak and head feathers with pollen. Many flowers that are pollinated by birds are red or pink, colors to which bird eyes are especially sensitive.


are categorized as

that completetheir life cycle in



Section 22-5

Plants

Annuals Biennials Perennials

1 growingseason 2 years

More than2 years

Go to Section:


The Human Connection to Seed Plants

• Human welfare depends greatly on seed plants

• No group is more important to human survival than seed plants

• Humans depend on seed plants for

– Food

– Wood

– Many medicines

Table 30.2 A Sampling of Medicines Derived from Plants

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