Transcript Gymnosperm

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
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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
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The gametophytes of seed
plants develop within the
walls of spores retained
within tissues of the parent
sporophyte – BIG
ADVANTAGE
Sporophyte
(2n)
Sporophyte
(2n)
Gametophyte
(n)
Sporophyte dependent on
gametophyte (mosses
and other bryophytes)
Microscopic female
gametophytes (n) in
ovulate cones
(dependent)
Gametophyte
(n)
Large sporophyte and
small, independent gametophyte (ferns and other
seedless vascular plants)
Sporophyte (2n),
the flowering plant
(independent)
Microscopic male
gametophytes (n) in
inside these parts
of flowers
(dependent)
Microscopic male
gametophytes (n)
in pollen cones
(dependent)
Sporophyte (2n),
(independent)
Microscopic female
gametophytes (n) in
inside these parts
of flowers
(dependent)
Reduced gametophyte dependent on sporophyte (seed
plants: gymnosperms and angiosperms)
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
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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
Integument
Female
gametophyte (n)
Seed coat
(derived from
integument)
Spore wall
Egg nucleus (n)
Megasporangium
(2n)
Megaspore (n)
Unfertilized ovule
Male gametophyte
(within germinating
pollen grain) (n)
Micropyle
Fertilized ovule
Discharged
sperm nucleus (n)
Pollen grain (n)
Food supply
(female
gametophyte
tissue) (n)
Embryo (2n)
(new sporophyte)
Gymnosperm seed
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
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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
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Gymnosperms bear “naked” seeds, typically on
cones – NOT in fruits
•
The gymnosperms include four phyla:
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Cycadophyta (cycads)
–
Gingkophyta (one living species: Ginkgo biloba)
–
Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia)
–
Coniferophyta (conifers, such as pine, fir, and redwood)
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Gymnosperms
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.
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.
Gymnosperms
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.
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.
Gymnosperms
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 ovuleproducing 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 bear “naked” seeds
Ovulate
cones
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
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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
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A Closer Look
at the Life Cycle
of a Pine
Key
Haploid (n)
Diploid (2n)
Ovule
Ovulate
cone
Pollen
cone
Megasporocyte (2n)
Integument
Longitudinal
Micropyle
section of
ovulate cone
Megasporangium
Mature
sporophyte
(2n)
Microsporocytes
(2n)
MEIOSIS
Longitudinal
section of
pollen cone
Sporophyll
Microsporangium
Germinating
Pollen pollen grain
grains (n)
MEIOSIS
(containing male
gametophytes)
Surviving
megaspore (n)
Seedling
Germinating
pollen grain
Archegonium
Egg (n)
Seeds on surface
of ovulate scale
Female
gametophyte
Germinating
pollen grain (n)
Food reserves Seed coat
(gametophyte (derived from
Discharged
tissue) (n)
sperm nucleus (n)
parent
sporophyte) (2n)
Pollen
tube
Embryo
(new sporophyte)
(2n)
FERTILIZATION
Egg nucleus (n)
Integument
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
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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
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Structure of Flowers
Stigma
Stamen
Anther
Carpel
Style
Filament
Ovary
Petal
Sepal
Ovule
Receptacle
Fruits
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
•A fruit typically
consists of a mature
ovary but can also
include other flower
parts
•Fruits protect seeds
and aid in their
dispersal
Nectarine, a fleshy
fruit with a soft outer
layer and hard inner
layer (pit) of pericarp
•Seeds are made
from the ovule,
while the fruit part
comes from the
ovary
•Mature fruits can be
either fleshy or dry
Milkweed, a dry fruit that splits
open at maturity
Walnut, a dry fruit that remains
closed at maturity
Dispersal of
Fruits
Wings enable maple
fruits to be easily
carried by the wind.
•Various fruit
adaptations help
disperse seeds
•Seeds can be carried
by wind, water, or
animals to new
locations
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.
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
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Key
Haploid (n)
Diploid (2n)
The Angiosperm
Life Cycle
Microsporangium
Microsporocytes (2n)
Anther
Mature flower on
sporophyte plant
(2n)
MEIOSIS
Microspore (n)
Ovule with
megasporangium (2n) Male
gametophyte
(in pollen
grain)
Ovary
Germinating
seed
Generative cell
Pollen
grains
MEIOSIS
Stigma
Pollen
Megasporangium tube
(n)
Sperm
Surviving
megaspore
(n)
Embryo (2n)
Endosperm
(food
supply) (3n)
Seed
Pollen
tube
Seed coat (2n)
Style
Antipodal cells
Female gametophyte
Polar nuclei
(embryo sac)
Synergids
Eggs (n)
Pollen
tube
Sperm
(n)
Zygote (2n)
Nucleus of
developing
endosperm
(3n)
Eggs
nucleus (n)
FERTILIZATION
Discharged
sperm nuclei (n)
Tube cell
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
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Archaefructus sinensis
Carpel
Stamen
5 cm
Archaefructus sinensis, a
125-million-year-old fossil
Artist’s reconstruction of
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 pollenproducing and ovule-producing structures
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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
BASAL ANGIOSPERMS
Amborella trichopoda
Water lily (Nymphaea
“Rene Gerald”)
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Star anise (Illicium
floridanum)
Magnoliids
MAGNOLIIDS
Southern magnolia
(Magnolia
grandiflora)
Eudicots
Monocots
Magnoliids
Star anise
and relatives
Water lilies
Amborella
HYPOTHETICAL TREE OF FLOWERING PLANTS
Monocots vs. Eudicots
MONOCOTS
EUDICOTS
Orchid
Monocot
(Lemboglossum
Characteristics
rossii)
Eudicot
Characteristics
California
poppy
(Eschscholzia
california)
Embryos
Two cotyledons
One cotyledon
MONOCOTS
EUDICOTS
Pyrenean oak
(Quercus
pyrenaica)
Leaf
venation
Veins usually
netlike
Veins usually
parallel
Stems
Pygmy date palm (Phoenix roebelenii)
Vascular tissue
scattered
Vascular tissue
usually arranged
in ring
MONOCOTS
EUDICOTS
Lily (Lilium
“Enchantment”)
Roots
Dog rose (Rosa canina), a wild rose
Taproot (main root)
usually present
Root system
usually fibrous
(no main root)
MONOCOTS
EUDICOTS
Barley (Hordeum vulgare), a grass
Pea
(Lathyrusner
vosus, Lord
Anson’s
blue pea),
a legume
Pollen
Pollen grain with
one opening
Pollen grain with
three openings
Flowers
Anther
Stigma
Filament
Ovary
Floral organs
usually in
multiples of three
Floral organs usually
in multiples of
four or five
Zucchini
(Cucurbita
Pepo), female
(left), and
male flowers
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
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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.
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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
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LOTS of medicine comes from seed plants
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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
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