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Chapter 30. Plant Diversity II Seed Plants. Evolution of Seed Plants (angiosperms and gymnosperms). Seeds changed the course of plant evolution Enabling their bearers to become the dominant producers in most terrestrial ecosystems Seed plants arose about 360 million years ago.

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chapter 30
Chapter 30

Plant Diversity II

Seed Plants

evolution of seed plants angiosperms and gymnosperms
Evolution of Seed Plants (angiosperms and gymnosperms)

Seeds changed the course of plant evolution

  • Enabling their bearers to become the dominant producers in most terrestrial ecosystems
  • Seed plants arose about 360 million years ago
key terrestrial adaptations for success of seed plants
Key Terrestrial Adaptations for Success of Seed Plants
  • Reduced gametophytes
  • Heterospory
  • Ovules
  • Pollen
  • Seeds
advantages of reduced gametophytes
Advantages of Reduced Gametophytes
  • The gametophytes of seed plants
    • Microscopically small (miniaturization of gametophytes)
    • Develop within the walls of spores retained within tissues of the parent sporophyte.
    • Receive nutrients and protection from environmental stress, such as drought, from sporophyte tissue.
    • The reduced gametophytes of seed plants are protected in ovules and pollen grains
slide5

(a)

Sporophyte dependent

on gametophyte

(mosses and other

bryophytes).

(b)

Large sporophyte and

small, independent

gametophyte (ferns

and other seedless

vascular plants).

(c)

Reduced gametophyte dependent on sporophyte

(seed plants: gymnosperms and angiosperms).

  • Gametophyte/sporophyte relationships

Sporophyte

(2n)

Sporophyte

(2n)

Bryophyte

Seedless Vascular Plant

Gametophyte

(n)

Gametophyte

(n)

Microscopic female

gametophytes (n) in

ovulate cones

(dependent)

Microscopic female

gametophytes (n)

inside these parts

of flowers

(dependent)

Microscopic male

gametophytes (n)

inside these parts

of flowers

(dependent)

Gymnosperm

Angiosperm

Microscopic male

gametophytes (n)

in pollen cones

(dependent)

Sporophyte (2n),

the flowering plant

(independent)

Sporophyte (2n)

(independent)

seed plants are heterosporous
Seed Plants are Heterosporous
  • megaspores give rise to female gametophytes (which develop within the ovule)
  • microspores give rise to male gametophytes (which develop in the pollen grain)
ovules and production of eggs

Integument

Spore wall

Megasporangium

(2n)

Megaspore (n)

(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.)

Ovules and Production of Eggs
  • An ovule consists of
    • A megasporangium, megaspore, and protective integuments
pollen and production of sperm
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
  • Evolution of pollen which can be dispersed by air or animals contributed greatly to the success and diversity of seed plants
    • Eliminated the water requirement for fertilization that is required by bryophytes and seedless vascular plants (remember flagellated sperm of these plants must swim through a water film to reach the egg cells)
slide9
If a pollen grain germinates
    • It gives rise to a pollen tube that discharges two sperm into the female gametophyte within the ovule

Female

gametophyte (n)

Egg nucleus (n)

Sperm of seed plants does not require water or motility because the pollen tube conveys the sperm to the egg

Spore wall

When a pollen grain germinates it grows a pollen tube.

Male gametophyte

(within germinating

pollen grain) (n)

Discharged

sperm nucleus (n)

Pollen grain (n)

Micropyle

(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.

seeds

Seed coat

(derived from

Integument)

Food supply

(female

gametophyte

tissue) (n)

Embryo (2n)

(new sporophyte)

(c)

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.

Seeds
  • When sperm fertilizes egg, the zygote forms and develops into the sporophyte embryo
  • Seeds develop from the whole ovule
  • A seed is: a sporophyte embryo, along with its food supply, packaged in a protective coat
gymnosperms
Gymnosperms
  • Gymnosperms bear “naked” seeds, typically on cones.(“NAKED” when compared to angiosperms where seeds develop in enclosed chambers called ovaries…fruits are mature ovaries)
  • Gymnosperms include many well-known conifers
    • cone-bearing trees, including pine, fir, and redwood
gymnosperm phyla
Gymnosperm Phyla
  • The gymnosperms include four plant phyla
    • Cycadophyta
    • Gingkophyta
    • Gnetophyta
    • Coniferophyta
slide13

PHYLUM CYCADOPHYTA

PHYLUM GINKGOPHYTA

Cycas revoluta

PHYLUM GNETOPHYTA

Gnetum

Welwitschia

Ovulate cones

Ephedra

Gymnosperm Diversity

Cycads: Large cones and palm-like leaves. Thrived during Mesozoic era.

Ginkgo biloba is the only extant species of this phylum

This phylum contains three genera of tropical or desert plants

gymnosperm diversity

Douglas fir

Common juniper

Wollemia pine

Pacific

yew

Sequoia

Bristlecone pine

Gymnosperm Diversity

Approx. 600 species of conifers.

Most are evergreen. (larch, tamarack, dawn redwood are deciduous)

PHYLUM Coniferophyta

gymnosperm evolution
Gymnosperm Evolution
  • Fossil evidence reveals that by the late Devonian 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
life cycle of a pine
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
life cycle of a pine18

2

An ovulate cone scale has two

ovules, each containing a mega-

sporangium. Only one ovule is shown.

8

7

6

5

4

1

3

Key

In most

conifer species,

each tree has

both ovulate

and pollen

cones.

Haploid (n)

Ovule

Diploid (2n)

A pollen grain

enters through

the micropyle

and germinates,

forming a pollen

tube that slowly

digests

through the

megasporangium.

Megasporocyte (2n)

Ovulate

cone

Integument

Longitudinal

section of

ovulate cone

Micropyle

Pollen

cone

Microsporocytes

(2n)

Megasporangium

Mature

sporophyte

(2n)

Germinating

pollen grain

Pollen

grains (n)

(containing male

gametophytes)

MEIOSIS

MEIOSIS

While the

pollen tube

develops, the

megasporocyte

(megaspore

mother cell)

undergoes meiosis,

producing four

haploid cells. One

survives as a

megaspore.

Longitudinal

section of

pollen cone

Surviving

megaspore (n)

Sporophyll

Microsporangium

A pollen cone contains many microsporangia

held in sporophylls. Each microsporangium

contains microsporocytes (microspore mother

cells). These undergo meiosis, giving rise to

haploid microspores that develop into

pollen grains.

Seedling

Germinating

pollen grain

Archegonium

Egg (n)

Integument

Female

gametophyte

Seeds on surface

of ovulate scale

Germinating

pollen grain (n)

Food reserves

(gametophyte

tissue) (n)

The female gametophyte

develops within the megaspore

and contains two or three

archegonia, each with an egg.

Seed coat

(derived from

parent

sporophyte) (2n)

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.

Discharged

sperm nucleus (n)

Pollen

tube

By the time the eggs are mature,

two sperm cells have developed in the

pollen tube, which extends to the

female gametophyte. Fertilization occurs

when sperm and egg nuclei unite.

Embryo

(new sporophyte)

(2n)

FERTILIZATION

Egg nucleus (n)

Life Cycle of a Pine
angiosperms
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 (over 250,000 species…about 90% of all plant species)
flowers

Carpel

Stigma

Anther

Style

Stamen

Ovary

Filament

Petal

Sepal

Receptacle

Ovule

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 often brightly colored and attract pollinators
    • Stamens, which produce pollen
    • Carpels, which produce ovules
fruits

(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

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

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

Fruits

Typically consist of a mature ovary

Pericarp is the thickened wall of the fruit.

fruit adaptations can enhance seed dispersal

(a)

Wings enable maple fruits

to be easily carried by the wind.

(b)

Seeds within berries and other

edible fruits are often dispersed

in animal feces.

(c)

The barbs of cockleburs

facilitate seed dispersal by

allowing the fruits to

“hitchhike” on animals.

Fruit adaptations can enhance seed dispersal
  • Fruits can be carried by wind, water, or animals to new locations, enhancing seed dispersal
the angiosperm life cycle
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
    • Some flowers self-pollinate, but most species have mechanisms that promote cross-pollination which enhances genetic variability.
  • The endosperm
    • Nourishes the developing embryo
slide25

Key

1

Anthers contain microsporangia.

Each microsporangium contains micro-

sporocytes (microspore mother cells) that

divide by meiosis, producing microspores.

2

Microspores form

pollen grains (containing

male gametophytes). The

generative cell will divide

to form two sperm. The

tube cell will produce the

pollen tube.

Haploid (n)

Diploid (2n)

Microsporangium

Anther

Microsporocytes (2n)

Mature flower on

sporophyte plant

(2n)

MEIOSIS

Microspore (n)

Generative cell

7

When a seed

germinates, the

embryo develops

into a mature

sporophyte.

Ovule with

megasporangium (2n)

Tube cell

Male gametophyte

(in pollen grain)

Ovary

Pollen

grains

MEIOSIS

Germinating

Seed

Stigma

In the megasporangium

of each ovule, the

megasporocyte divides by

meiosis and produces four

megaspores. The surviving

megaspore in each ovule

forms a female gametophyte

(embryo sac).

3

Pollen

tube

Megasporangium

(n)

Embryo (2n)

Sperm

Endosperm

(food

Supply) (3n)

6

The zygote

develops into an

embryo that is

packaged along

with food into a

seed. (The fruit

tissues surround-

ing the seed are

not shown).

Surviving

megaspore

(n)

Seed

Pollen

tube

Seed coat (2n)

Style

Antipodal cells

Polar nuclei

Synergids

Egg (n)

Female gametophyte

(embryo sac)

Pollen

tube

Zygote (2n)

Nucleus of

developing

endosperm

(3n)

Egg

Nucleus (n)

Sperm

(n)

After pollina-

tion, eventually

two sperm nuclei

are discharged in

each ovule.

4

FERTILIZATION

Double fertilization occurs. One sperm

fertilizes the egg, forming a zygote. The

other sperm combines with the two polar

nuclei to form the nucleus of the endosperm,

which is triploid in this example.

5

Discharged

sperm nuclei (n)

The Angiosperm Life Cycle

Early embryo has a rudimentary root and Cotyledons (seed leaves)

angiosperm evolution

Carpel

Stamen

5 cm

(a)

(b)

Archaefructus sinensis, a 125-million-year-

old fossil.

Artist’s reconstruction of

Archaefructus sinensis

Angiosperm Evolution
  • Angiosperms originated at least 140 million years ago
  • Primitive fossils of 125-million-year-old angiosperms
    • Display both derived and primitive traits
angiosperm diversity
Angiosperm Diversity
  • The two main groups of angiosperms
    • Are monocots (one cotyledon) and eudicots (true 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
angiosperm diversity28

BASAL ANGIOSPERMS

Amborella trichopoda

Star anise (Illicium

floridanum)

Water lily (Nymphaea

“Rene Gerard”)

HYPOTHETICAL TREE OF FLOWERING PLANTS

Monocots

Eudicots

Star anise

and relatives

Water lilies

Amborella

Magnoliids

MAGNOLIIDS

Southern magnolia (Magnolia

grandiflora)

Angiosperm Diversity
angiosperm diversity29

EUDICOTS

MONOCOTS

Monocot

Characteristics

Eudicot

Characteristics

California

poppy

(Eschscholzia

californica)

Orchid

(Lemboglossum

fossii)

Embryos

One cotyledon

Two cotyledons

Leaf

venation

Pyrenean oak

(Quercus

pyrenaica)

Veins usually

netlike

Veins usually

parallel

Pygmy date palm

(Phoenix roebelenii)

Stems

Vascular tissue

usually arranged

in ring

Lily (Lilium

“Enchant-

ment”)

Vascular tissue

scattered

Roots

Dog rose (Rosa canina), a wild rose

Root system

Usually fibrous

(no main root)

Taproot (main root)

usually present

Barley (Hordeum vulgare),

a grass

Pea (Lathyrus nervosus,

Lord Anson’sblue pea), a legume

Pollen

Pollen grain with

three openings

Pollen grain with

one opening

Flowers

Zucchini

(Cucurbita

Pepo), female

(left) andmale flowers

Anther

Floral organs

usually in

multiples of three

Floral organs usually

in multiples of

four or five

Stigma

Filament

Ovary

Angiosperm Diversity

Monocots

Eudicots

evolutionary links between angiosperms and animals

(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.

(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.

(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.

Evolutionary Links Between Angiosperms and Animals
  • Pollination of flowers by animals and transport of seeds by animals
    • Are two important relationships in terrestrial ecosystems
importance of seed plants
Importance of Seed Plants
  • Human welfare depends greatly on seed plants
    • No group is more important to human survival than seed plants. Agriculture is based almost entirely on angiosperms.
  • Humans depend on seed plants for
    • Food
    • Wood
    • Many medicines
threats to plant diversity
Threats to Plant Diversity
  • Plants are a renewable resource, but plant diversity is not.
  • Destruction of habitat
    • Is causing extinction of many plant species and the animal species they support
    • 50 million acres of tropical rainforest are cleared each year