chapter 30(2)

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 30Chapter 30

Plant Diversity II: The Evolution of Seed Plants


• 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


• Concept 30.1: The reduced gametophytes of seed plants are protected in ovules and pollen grains

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

– Reduced gametophytes

– Heterospory

– Ovules

– Pollen


Advantages of Reduced Gametophytes

• The gametophytes of seed plants

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


• 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


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


• 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


• Pollen, which can be dispersed by air or animals

– Eliminated the water requirement for fertilization


The Evolutionary Advantage of Seeds

• A seed

– Develops from the whole ovule

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

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)


• Concept 30.2: 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


• Exploring Gymnosperm Diversity

Figure 30.4

Gnetum

Ephedra

Ovulate cones

Welwitschia

PHYLUM GNETOPHYTA

PHYLUM CYCADOPHYTA PHYLUM GINKGOPHYTA

Cycas revoluta


• Exploring Gymnosperm Diversity

Figure 30.4

Douglas fir

Pacificyew

Common juniper

Wollemia pine

Bristlecone pine Sequoia

PHYLUM CYCADOPHYTA


Gymnosperm Evolution

• Fossil evidence reveals that by the late Devonian

– Some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants

Figure 30.5


• Gymnosperms appear early in the fossil record

– And dominated the Mesozoic terrestrial ecosystems

• Living seed plants

– Can be divided into two groups: gymnosperms and angiosperms


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


Figure 30.6

Ovule

Megasporocyte (2n)

Integument

Longitudinalsection 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


• Concept 30.3: 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


Flowers

• The flower

– Is an angiosperm structure specialized for sexual reproduction


• 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

Stigma

Style

Ovary

Carpel

Petal

ReceptacleOvule

Sepal

Stamen


Fruits

• Fruits

– Typically consist of a mature ovary

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


• 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)


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


• The life cycle of an angiosperm

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

Microsporangium

Microsporocytes (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


Angiosperm Evolution

• Clarifying the origin and diversification of angiosperms

– Poses fascinating challenges to evolutionary biologists

• Angiosperms originated at least 140 million years ago

– And during the late Mesozoic, the major branches of the clade diverged from their common ancestor


Fossil Angiosperms• Primitive fossils of 125-million-year-old

angiosperms

– Display both derived and primitive traits

Figure 30.11a, b

Carpel

Stamen

Archaefructus sinensis, a 125-million-year-old fossil.

(a)

Artist’s reconstruction of Archaefructus sinensis

(b)

5 cm


An “Evo-Devo” Hypothesis of Flower Origins

• In hypothesizing how pollen-producing and ovule-producing structures were combined into a single flower

– Scientist Michael Frohlich proposed that the ancestor of angiosperms had separate pollen-producing and ovule-producing structures


Angiosperm Diversity

• The two main groups of angiosperms

– Are monocots and eudicots

• 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


• 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

bo

rell

a

Wat

er l

ilie

s

Sta

r an

ise

and

rel

ativ

es

Mag

no

liid

s

Mo

no

cots

Eu

dic

ots

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.


• Concept 30.4: Human welfare depends greatly on seed plants

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


Products from Seed Plants

• Humans depend on seed plants for

– Food

– Wood

– Many medicines

Table 30.1


Threats to Plant Diversity

• Destruction of habitat

– Is causing extinction of many plant species and the animal species they support

chapter 30(2)

Technology

pearson education

benjamin cummings pollen

n megaspore n

sporophyte seed plants

benjamin cummings ovules

benjamin cummings concept

benjamin cummings heterospory

benjamin cummings overview