chapter 30: plant diversity ii: the evolution of seed...

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

Chapter 30: Plant Diversity II: The Evolution of Seed Plants - Feeding the World

• Seeds changed the course of plant evolution, enabling their bearers to

become the dominant producers in most terrestrial ecosystems


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 – seed development is protected

– Heterospory – male and female gametophytes develop separately

– Ovules

– Pollen

Sporophyte

(2n)

Gametophyte

(n)

Sporophyte dependent on

gametophyte (mosses

and other bryophytes)

Gametophyte

(n)

Sporophyte

(2n)

Large sporophyte and

small, independent game-

tophyte (ferns and other

seedless vascular plants)

Microscopic female

gametophytes (n) in

ovulate cones

(dependent)

Microscopic male

gametophytes (n) in

inside these parts

of flowers

(dependent)

Sporophyte (2n),

the flowering plant

(independent)

Microscopic male

gametophytes (n)

in pollen cones

(dependent)

Microscopic female

gametophytes (n) in

inside these parts

of flowers

(dependent) Sporophyte (2n),

(independent)

Reduced gametophyte dependent on sporophyte (seed

plants: gymnosperms and angiosperms)

The gametophytes of seed

plants develop within the

walls of spores retained

within tissues of the parent

sporophyte – BIG

ADVANTAGE


Heterospory: The Rule Among Seed Plants

• Seed plants evolved from plants with megasporangia, which produce

megaspores that give rise to female gametophytes

• Seed plants evolved from plants with microsporangia, which produce

microspores that give rise to male gametophytes

Integument

Megasporangium

(2n)

Megaspore (n)

Unfertilized ovule Fertilized ovule

Spore wall

Male gametophyte

(within germinating

pollen grain) (n)

Micropyle

Female

gametophyte (n)

Egg nucleus (n)

Discharged

sperm nucleus (n)

Pollen grain (n)

Seed coat

(derived from

integument)

Embryo (2n)

(new sporophyte)

Gymnosperm seed

Food supply

(female

gametophyte

tissue) (n)

Ovules and Production of Eggs •An ovule consists of a megasporangium, megaspore, and one or more protective

integuments

•Phylum Coniferphyta - Gymnosperm megaspores have one integument

(covering)

•Phylum Anthophyta - Angiosperm megaspores usually have two integuments

•These two phylum are most closely related to each other than the other 2 because

they both form seeds, NOT spores


Pollen and Production of Sperm

• Microspores develop into pollen grains, which contain the male

gametophytes

• Pollination is the transfer of pollen to the part of a seed plant containing

the ovules

• Pollen can be dispersed by air or animals, eliminating the water

requirement for fertilization

• If a pollen grain germinates, it gives rise to a pollen tube that

discharges two sperm into the female gametophyte within the ovule


The Evolutionary Advantage of Seeds

• A seed develops from the whole ovule

• A seed is a sporophyte embryo, along with its food supply, packaged in

a protective coat

• ADVANTAGES OF A SEED:

– A seed aids in dispersal of the embryo to different locations in an

ecosystem

– Supplies nutrients to the embryo

– Allows the embryo to remain dormant, or not growing, until

conditions are right for growth

– Allows dessication resistance, or keeps the embryo from drying

out

• Living seed plants can be divided into two groups: gymnosperms and

angiosperms


Gymnosperms bear “naked” seeds, typically on cones – NOT in fruits

• The gymnosperms include four phyla:

– Cycadophyta (cycads)

– Gingkophyta (one living species: Ginkgo biloba)

– Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia)

– Coniferophyta (conifers, such as pine, fir, and redwood)

Cycas revoluta

Gymnosperms

Gnetum. This genus includes

about 35 species of tropical trees,

shrubs, and vines, mainly native

to Africa and Asia. Their leaves

look similar to those of flowering

plants, and their seeds look

somewhat like fruits.

Gymnosperms

Ephedra. This genus includes about

40 species that inhabit arid regions

throughout the world. Known in

North America as “Mormon tea,”

these desert shrubs produce the

compound ephedrine, commonly

used as a decongestant.

Welwitschia. This genus

consists of one species

Welwitschia mirabilis, a plant

that lives only in the deserts

of southwestern Africa. Its

strap like leaves are among

the largest known.

Gymnosperms

Douglas fir. “Doug fir”

(Pseudotsuga menziesii) provides

more timber than any other North

American tree species. Some

uses include house framing,

plywood, pulpwood for paper,

railroad ties, and boxes and

crates.

Pacific yew. The

bark of Pacific yew

(Taxa brevifolia) is a

source of taxol, a

compound used to

treat women with

ovarian cancer. The

leaves of a

European yew

species produce a

similar compound,

which can be

harvested without

destroying the plants. Pharmaceutical companies are now refining

techniques for synthesizing drugs with taxol-like properties.

Gymnosperms

Bristlecone pine.

This species (Pinus

longaeva), which is

found in the White

Mountains of

California, includes

some of the oldest

living organisms,

reaching ages of

more than 4,600

years. One tree (not

shown here) is

called Methuselah

because it may be

the world’s oldest

living tree. In order

to protect the tree,

scientists keep its

location a secret.

Gymnosperms

Sequoia. This giant sequoia (Sequoiadendron giganteum), in California’s Sequoia National Park weighs about 2,500 metric tons, equivalent to about 24 blue whales (the largest animals), or 40,000 people. Giant sequoias are the largest living organisms and also some of the most ancient, with some estimated to be between 1,800 and 2,700 years old. Their cousins, the coast redwoods (Sequoia sempervirens), grow to heights of more than 110 meters (taller than the Statue of Liberty) and are found only in a narrow coastal strip of northern California.

Gymnosperms

Common juniper. The “berries” of the common juniper (Juniperus communis), are actually ovule-producing cones consisting of fleshy sporophylls.

Gymnosperms

Wollemia pine. Survivors of a confer group once known only from fossils, living Wollemia pines (Wollemia nobilis), were discovered in 1994 in a national park only 150 kilometers from Sydney, Australia. The species consists of just 40 known individuals two small groves. The inset photo compares the leaves of this “living fossil” with actual fossils.

Gymnosperms

Ovulate

cones

Gymnosperms bear “naked” seeds


Gymnosperm Evolution

• Fossil evidence reveals that

by the late Devonian period

some plants, called

progymnosperms, had

begun to acquire some

adaptations that characterize

seed plants

• Gymnosperms appear early

in the fossil record and

dominated the Mesozoic

terrestrial ecosystems


A Closer Look at the Life Cycle of a Pine

• Key features of the gymnosperm life cycle:

– Dominance of the sporophyte generation

– Development of seeds from fertilized ovules

– The transfer of sperm to ovules by pollen

• The life cycle of a pine is an example

Mature sporophyte (2n)

Pollen cone

Microsporocytes (2n)

Longitudinal section of pollen cone

Ovulate cone

Longitudinal section of ovulate cone

Micropyle

Ovule

Key

Haploid (n)

Diploid (2n)

Megasporocyte (2n)

Integument

Megasporangium

Germinating pollen grain Pollen

grains (n) (containing male gametophytes)

MEIOSIS

Microsporangium Sporophyll

MEIOSIS

Germinating pollen grain

Female gametophyte

Archegonium

Egg (n)

Germinating pollen grain (n)

Integument

Discharged sperm nucleus (n)

Pollen tube

Egg nucleus (n) FERTILIZATION

Embryo (new sporophyte) (2n)

Food reserves (gametophyte tissue) (n)

Seed coat (derived from parent sporophyte) (2n)

Seedling

Seeds on surface of ovulate scale

Surviving megaspore (n)

A Closer Look at the Life Cycle of a Pine


The reproductive adaptations of angiosperms include flowers and fruits

• Angiosperms are flowering plants

• These seed plants have reproductive structures called flowers and

fruits

• They are the most widespread and diverse of all plants

• All angiosperms are classified in a single phylum, Anthophyta

• The name comes from the Greek anthos, flower


Flowers

• The flower is an angiosperm structure specialized for sexual

reproduction

• A flower is a specialized shoot with up to four types of modified leaves:

– Sepals, which enclose the flower

– Petals, which are brightly colored and attract pollinators

– Stamens, which produce pollen

– Carpels, which produce ovules

Stamen

Filament

Anther

Stigma Carpel

Style

Ovary

Petal

Receptacle

Ovule

Sepal

Structure of Flowers

Tomato, a fleshy fruit with soft outer and inner layers of pericarp

Ruby grapefruit, a fleshy fruit with a hard outer layer and soft inner layer of pericarp

Milkweed, a dry fruit that splits open at maturity

Walnut, a dry fruit that remains closed at maturity

Nectarine, a fleshy fruit with a soft outer layer and hard inner layer (pit) of pericarp

Fruits

•A fruit typically

consists of a mature

ovary but can also

include other flower

parts

•Fruits protect seeds

and aid in their

dispersal

•Seeds are made

from the ovule,

while the fruit part

comes from the

ovary

•Mature fruits can be

either fleshy or dry

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

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

The barbs of cockleburs facilitate seed dispersal by allowing these fruits to hitchhike on animals.

•Various fruit

adaptations help

disperse seeds

•Seeds can be carried

by wind, water, or

animals to new

locations

Dispersal of 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 central cell of the female gametophyte and initiates development

of food-storing, triploid endosperm

• The endosperm nourishes the developing embryo

Anther

Mature flower on sporophyte plant (2n)

Key

Haploid (n)

Diploid (2n)

Microsporangium

Microsporocytes (2n)

MEIOSIS

Microspore (n)

MEIOSIS

Ovule with megasporangium (2n) Male

gametophyte (in pollen grain)

Ovary

Generative cell

Tube cell

Megasporangium (n)

Surviving megaspore (n)

Female gametophyte (embryo sac)

Antipodal cells

Polar nuclei Synergids

Eggs (n)

Pollen tube

Sperm (n)

Pollen grains

Pollen tube

Style

Stigma

Pollen tube

Sperm

Eggs nucleus (n)

Discharged sperm nuclei (n)

Germinating seed

Zygote (2n)

FERTILIZATION

Nucleus of developing endosperm (3n)

Embryo (2n)

Endosperm (food supply) (3n)

Seed coat (2n)

Seed

The Angiosperm Life Cycle


Angiosperm Evolution

• Clarifying the origin and diversification of angiosperms poses

fascinating challenges to evolutionary biologists

• Angiosperms originated at least 140 million years ago

• During the late Mesozoic, the major branches of the class diverged

from their common ancestor

• Primitive fossils of 125-million-year-old angiosperms display derived

and primitive traits

• Archaefructus sinensis, for example, has anthers and seeds but lacks

petals and sepals

Carpel

Stamen

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

5 cm

Artist’s reconstruction of Archaefructus sinensis

Archaefructus sinensis


An “Evo-Devo” (Evolution/Development) Hypothesis of Flower Origins

• Scientist Michael Frohlich hypothesized how pollen-producing and

ovule-producing structures were combined into a single flower

• He 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

• A third group, the basal angiosperms are less derived and include the flowering plants belonging to the oldest lineages

• A fourth group, the Magnoliids share some traits with basal angiosperms but are more closely related to monocots and eudicots

Amborella trichopoda Water lily (Nymphaea “Rene Gerald”)

Star anise (Illicium floridanum)

BASAL ANGIOSPERMS

Southern magnolia (Magnolia grandiflora)

HYPOTHETICAL TREE OF FLOWERING PLANTS

MAGNOLIIDS

Am

bo

rella

Sta

r an

ise

an

d r

ela

tives

Wate

r lilies

Mag

no

liid

s

Mo

no

co

ts

Eu

dic

ots

Magnoliids

Orchid (Lemboglossum rossii)

Monocot

Characteristics

One cotyledon

Embryos

Two cotyledons

Eudicot

Characteristics

California poppy (Eschscholzia california)

EUDICOTS MONOCOTS

Pygmy date palm (Phoenix roebelenii)

Veins usually

parallel

Vascular tissue scattered

Stems

Pyrenean oak (Quercus pyrenaica)

EUDICOTS MONOCOTS

Leaf

venation

Vascular tissue usually arranged

in ring

Veins usually

netlike

Monocots vs. Eudicots

Dog rose (Rosa canina), a wild rose

Root system usually fibrous (no main root)

Roots

Lily (Lilium “Enchantment”)

EUDICOTS MONOCOTS

Taproot (main root) usually present

Barley (Hordeum vulgare), a grass

Floral organs usually in

multiples of three

Pollen

Zucchini (Cucurbita Pepo), female (left), and male flowers

EUDICOTS MONOCOTS

Pollen grain with three openings

Stigma

Ovary

Anther

Filament

Floral organs usually in multiples of

four or five

Pollen grain with one opening

Flowers

Pea (Lathyrusner vosus, Lord Anson’s blue pea), a legume


Differences between monocots and eudicots

• Monocot embryos have one cotyledon, eudicots have two

• Monocot leaf veins are usually parallel, while eudicot leaves usually have netlike meshwork of veins

• Vascular tissue in monocots is scattered, while eudicot vascular tissue is usually arranged in a ring

• Monocots usually have a fibrous root system, while eudicots usually have a main taproot

• Monocot pollen grains usually have one opening, while eudicot pollen grains usually have three openings

• Monocot flower organs are usually in multiples of three, while eudicot flower organs are usually in multiples of four or five


Evolutionary Links Between Angiosperms and Animals

• Pollination of flowers by animals and transport of seeds by animals are

two important relationships in terrestrial ecosystems

A flower pollinated by honeybees.

A flower pollinated by hummingbirds.

A flower pollinated by nocturnal animals.


Human welfare depends greatly on seed plants

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

plants

• Plants are key sources of food, fuel, wood products, and medicine

• Our reliance on seed plants makes preservation of plant diversity

critical

• Most of our food comes from angiosperms

• Six crops (wheat, rice, maize, potatoes, cassava, and sweet potatoes)

yield 80% of the calories consumed by humans

• Modern crops are products of relatively recent genetic change resulting

from artificial selection

• Many seed plants provide wood

• Secondary compounds of seed plants are used in medicines


LOTS of medicine comes from seed plants


Threats to Plant Diversity

• Destruction of habitat is causing extinction of many plant species

• Loss of plant habitat is often accompanied by loss of the animal

species that plants support

chapter 30: plant diversity ii: the evolution of seed...

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