Download Lecture 6B

Lecture 6B
Angiosperms
Characteristics of Angiosperms
• commonly known as the flowering plants
– angion = “container”
– angio – refers to seeds contained in fruits and
mature ovaries
• are seed plants that produce reproductive
structures called flowers and fruits
Basal angiosperms
• some of the oldest angiosperms
• surviving plants - divided into three lineages – only about 1,000 species
• oldest lineage – Amborella trichopoda
– only found in the South Pacific – New Caledonia
– lacks vessels – found in later lineages of angiosperms
• then divided into two clades
– 1. clade including the water lilies
– 2. clade including star anise
Amborella trichopoda
Water lily (Nymphaea
“Rene Gerald”)
Star anise (Illicium
floridanum)
MAGNOLIIDS
Eudicots
Monocots
Magnoliids
Star anise
and relatives
Water lilies
Amborella
Angiosperm phylogeny
HYPOTHETICAL TREE OF FLOWERING PLANTS
Angiosperm Diversity
• The three main groups of surviving
angiosperms derived from the basal
angiosperms are:
– 1. magnoliids
– 2. monocots – embryo with one cotyledon
– 3. eudicots (dicots) – embryo with two
cotyledons
Angiosperm Diversity
• Magnoliids: 8,000 species
– e.g. magnolia, nutmeg, bay laurel, cinnamon, avocado,
black pepper trees
– share many traits with monocots and eudicots
– share some traits with basal angiosperms
• embryo with one cotyledon
• other traits:
–
–
–
–
–
–
Monocots
1. veins in leaves are usually parallel
2. vascular bundles scattered in stems
3. root system is usually fibrous
4. pollen grain with one opening
5. flower organs usually in multiples of three
6. most cannot undergo secondary (i.e. woody) growth
Dicots (Eudicots)
• former classification known as dicots
has been abandoned (too polyphyletic)
• using DNA analysis – clade was created
of “true” dicots
• embryo with two cotyledons
California
poppy
– cotyledons: store food absorbed from the
endosperm
zucchini flower
Dicots (Eudicots)
• other traits:
– 1. veins in leaves are usually netlike
– 2. vascular bundles arranged in a ring in
stems
– 3. root system is usually a taproot
– 4. pollen grain with three openings
– 5. flower organs usually in multiples of
four or five
– 6. many are perennial and undergo
secondary
(i.e. woody) growth
California
poppy
zucchini flower
• flower = angiosperm structure that is
specialized for sexual reproduction
– specialized shoot that can have up to four
rings of modified leaves or sporophylls
• in many angiosperm species – pollination is
by insects or other animals
– from flower to flower
– so pollination is more direct than by wind
– for angiosperms in dense populations –
wind is the pollinator
Flowers
• structure of a flower – 4 rings of modified
leaves called flower organs:
– 1. sepals
– 2. petals
– 3. stamens
– 4. carpels
Flowers
• 1. sepals (sterile flower organ)
Flower
Anatomy
– usually green and enclose the flower
before it opens
• 2. petals (sterile flower organ)
– interior to the sepals
– most are brightly colored – to attract
pollinators like insects
– wind pollinated have leaves that are less
Stamen Anther
Filament
colorful
Stigma
Carpel
Style
Ovary
Petal
Sepal
Ovule
Receptacle
• 3. stamens (produce spores)
– contain chambers called microsporangia
(Pollen sacs)
– pollen sacs produce microspores that
develop into pollen grains containing
the male gametophyte
– consists of a stalk called the filament
and a terminal end called the anther
Stamen
(pollen)
Flower
Anatomy
Stigma
Anther
Carpel
Style
Filament
Ovary
Petal
Sepal
Ovule
Receptacle
• 4. carpels (produce spores)
– comprised of the stigma, style and ovary
– ovary contain ovules that produce
megaspores - develop into the female
gametophyte
– some flowers have a single carpel – others
have multiple (separate or fused together)
– end of the carpel is a sticky stigma that
receives pollen
– the stigma leads to a style which leads to
the ovary at the base of the carpel
– the ovary contains one or more ovules – site
of the megaspore, the female gametophyte
& the egg
– these ovules when fertilized develop into
seeds within a fruit
Flower
Anatomy
Stigma
Stamen
Anther
Carpel
Style
Filament
Ovary
Petal
Sepal
Ovule
Receptacle
• fruits typically consists of the
mature ovary
– but can also contain other flower
parts
• the egg is fertilized within the
ovule - the embryo begins to
develop within the seed
• as seeds develop – the ovary wall
(pericarp) thickens = fruit
development
• fruits protect seeds and aid in
their dispersal
Fruits
Fruit terminology
• ovary wall = pericarp
– can be very thick and made up of
three layers
– 1. exocarp (skin of fruit)
– 2. mesocarp (flesh of fruit)
– 3. endocarp
• the ovary/fruit can be divided into
many chambers called locules
– within the locules are the ovules
– the ovules contain the egg which
when fertilized develops into the
seed
• the ovary may also be single
structure
– containing a single ovule/seed
• ovaries with multiple seeds
arrange their seeds in
specific patterns =
placentation
• see lab for patterns
• fruits can be either fleshy or dry
– fleshy = tomatoes, plums, grapes
• the pericarp becomes soft during
ripening
– dry = beans, nuts and grains
• some can split open at maturity to
release seeds
Fruits
• fruits have adapted for seed
dispersal in many ways
– many are eaten – seeds “pooped”
out
– others cling to animals – “burrs”
– e.g. dandelions and maples – fruits
function as parachutes or propellers
– e.g. coconut – dispersal by water
Fruits
Key
Haploid (n)
Diploid (2n)
Life Cycle of
Angiosperms
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
MEIOSIS
Stigma
Pollen
Megasporangium tube
(n)
Sperm
Surviving
megaspore
(n)
Pollen
tube
Style
Embryo (2n)
Endosperm
(food
supply) (3n)
Seed coat (2n)
Seed
Female gametophyte
(embryo sac)
Nucleus of
developing
endosperm
(3n)
Antipodal cells
Polar nuclei
Synergids
Eggs (n)
Pollen
tube
Sperm
(n)
Zygote (2n)
Eggs
nucleus (n)
FERTILIZATION
Discharged
sperm nuclei (n)
Tube cell
Pollen
grains
http://www.sumanasi
nc.com/webcontent/a
nimations/content/an
giosperm.html
Male Cycle:
Key
Haploid (n)
• on the anther are four
microsporangia or
pollen sacs
Diploid (2n)
Microsporangium
Anther
Microsporocytes (2n)
Mature flower on
Sporophyte plant
(2n)
MEIOSIS
– supported by a filament
Microspore (n)
Generative cell
Ovule with
megasporangium (2n)
Tube cell
Male
gametophyte
(in pollen
grain)
• each microsporangium
(2n) contains multiple
microsporocytes (2n)
– also known as
microspore mother cells
(2n)
– microsporocytes undergo
meiosis to form
microspores (n)
Ovary
MEIOSIS
Megasporangium
(n)
Surviving
megaspore
(n)
Antipodal cells
microsporangium
Female gametophyte
Polar nuclei
(embryo sac)
Synergids
Anther
Pollen
tube
Eggs (n)
Sperm
(n)
pollen grains
Male Cycle:
• each microspore develops
into a haploid pollen grain
– within the pollen grain is the
male gametophyte (n) which
is made up of a generative cell
and a tube cell
– pollen grain = generative cell +
tube cell + spore wall
– pollen dispersed and lands on
the stigma
– the tube cell elongates to form
the pollen tube
– as the tube grows - the
generative cell divides to form
2 sperm (n) = pollen
maturation
Key
Haploid (n)
Diploid (2n)
Microsporangium
Microsporocytes (2n)
Anther
Mature flower on
Sporophyte plant
(2n)
MEIOSIS
Microspore (n)
Tube cell
Male
gametophyte
(in pollen
grain)
Ovary
MEIOSIS
Antipodal cells
Female gametophyte
Polar nuclei
(embryo sac)
Synergids
Generative cell
Pollen
tube
Eggs (n)
Sperm
(n)
Female:
• there are over 15 variations in how
the female can develop - most
common:
• in each ovule of the carpel is one
megasporangium (2n) that contains
one megasporocyte
Key
Haploid (n)
Diploid (2n)
Mature flower on
Sporophyte plant
(2n)
Anther
MEIOSIS
– the megasporangium is surrounded
by two integuments – will become
the seed coat
– the integuments have an opening –
micropyle (for sperm entry)
– the megasporocyte enlargens &
divides by meiosis to produce four
megaspores (n)
– only one megaspore survives
Ovule with
megasporangium (2n)
Ovary
MEIOSIS
Megasporangium
(n)
Surviving
megaspore
(n)
Antipodal cells
Female gametophyte
(embryo sac)
Polar nuclei
Synergids
Eggs (n)
Pollen
tube
Sperm
(n)
– only one megaspore
survives (contained within
the megasporangium
– the surviving megaspore
develops into the female
gametophyte
• surviving megaspore
undergoes three mitotic
divisions (no cytokinesis) 
one large cell with 8 nuclei
• this multinucleated cell will be
partitioned off by membranes
to form a multicellular female
gametophyte OR embryo sac
Female:
Megasporangium
(n)
Integuments
Surviving
megaspore
(n)
Degenerating
megaspores
Antipodal cells
Female
gametophyte
(embryo sac)
Polar nuclei
Pollen
tube
Synergids
Eggs (n)
Sperm
(n)
– cells of the embryo sac:
• 1. antipodal cells – 3 cells of
unknown function
• 2. central cell – containing two
polar nuclei
– will form the endosperm
• 3. synergids – 2 cells at the
micropyle end, flank the egg
– guide in the pollen tube
• 4. egg
Female:
Pollination
• by numerous methods
– abiotic: wind
– by bees – 65% of all angiosperms
– by moths & butterflies – detect odors (sweet
fragrance)
– by flies – many are reddish and fleshy with a rotten
odor
– by bats – light colored petals and aromatic
– by birds – very large and brightly colored (red or
yellow) – no scent required but they produce a nectar
Pollination & Fertilization
• pollen lands on the stigma of the carpel – absorbs water and
begins to germinate
• pollen tube develops & travels down the style
• each pollen tube terminates at an ovule
– penetrates into the ovule through the micropyle at the base of the ovule
Pollen
grain
Stigma
Pollen tube
If a pollen grain
germinates, a pollen
tube grows down the
style toward the ovary.
Polar
nuclei
Egg
2 sperm
Style
Ovary
Ovule (containing
female
gametophyte, or
embryo sac)
Micropyle
Pollination & Fertilization
• following the start of tube formation – the generative cell nucleus
splits by mitosis -> 2 sperm
• the pollen tube arrives at the micropyle
• sperm are discharged into each ovule
Ovule
Polar nuclei
The pollen tube
discharges two sperm into
the female gametophyte
(embryo sac) within an
ovule.
Egg
Two sperm
about to be
discharged
Pollination & Fertilization
• double fertilization then takes place
– one sperm nucleus unites with egg nucleus  zygote
– the other sperm nucleus fuses with the 2 polar nuclei  triploid
central cell
• the triploid central cell form the endosperm
One sperm fertilizes
the egg, forming the zygote.
The other sperm combines
with the two polar nuclei of
the embryo sac’s large
central cell, forming a triploid
cell that develops into the
nutritive tissue called
endosperm.
Endosperm
nucleus (3n)
(2 polar nuclei
plus sperm)
Zygote (2n)
(egg plus sperm)
Pollination & Fertilization
• the zygote develops into an embryo that is packaged along with food (i.e.
endosperm) into the seed (embryo + endosperm + integuments/seed coat)
• fruit begins to develop around the seeds
• seed dispersal completes the life cycle
Anther
Mature flower on
sporophyte plant
(2n)
Germinating
seed
Diploid (2n)
Embryo (2n)
Endosperm
(food
supply) (3n)
Seed coat (2n)
Seed
Double Fertilization
• unique to angiosperms
• produces a triploid endosperm + a diploid
zygote
• why?
• hypothesis: synchronizes the development of
food with the development of the embryo
that needs it
– so it ensures the wasting of nutrients on infertile
ovules
Seed Development
• the seed consists of:
– the embryo
– the triploid endosperm - three nuclei
(two central nuclei, one sperm nucleus)
– the seed coat
• the endosperm – rich in starch
– usually develops before the embryo –
milky consistency at first
– cytokinesis does eventually produce
three cells each with a nucleus
– these cells produce cell walls and the
endosperm becomes solid
– in many angiosperms - the endosperm
stores nutrients that is used by the
seedling as it germinates
• the first mitotic division of the
zygote splits it into a basal cell
and a terminal cell
– the terminal cell gives rise to most
of the embryo
Embryo
Development
Zygote
• the basal cell continues to divide
to produce a suspensor
Terminal cell
Basal cell
Proembryo
Suspensor
Basal cell
Cotyledons
Shoot apex
Root apex
Suspensor
Seed coat
Endosperm
• the terminal cell produces a
spherical proembryo – attached
via the suspensor
• the proembryo develops
embryonic leaves called
cotyledons
– cotyledons also store food
– will become the first leaves of the
seedling
• the cotyledons form as “bumps”
in the proembryo
• the cotyledons elongate along
with the proembryo
• the elongated proembryo =
embryonic axis
Embryo
Development
Proembryo
Suspensor
Basal cell
Cotyledons
Shoot apex
Root apex
Seed coat
Suspensor
Endosperm
• the elongated embryo is called
the embryonic axis
• attached to it are the
cotyledons
Embryo
Development
– one cotyledon in monocots
– two cotyledons in eudicots
• top of the embryonic axis –
shoot apex
Cotyledons
– where the cotyledons attach to
Shoot apex
the axis
• bottom of the embryonic axis
Root apex
– root apex
– attaches to the suspensor
• both the shoot and root apex
contain meristematic tissue
– stem cells for development
Seed coat
Suspensor
Endosperm
• eudicot: e.g. garden bean
– embryonic axis attached to thick cotyledons
– below where cotyledons attach – the axis is
called the hypocotyl
• the hypocotyl ends as the radicle or
embryonic root
– above where the cotyledons attach - the
axis is the epicotyl
• produces the true leaves of the seedling
Seed coat
Embryo
Structure
Epicotyl
Hypocotyl
Radicle
Cotyledons
Common garden bean, a eudicot with thick cotyledons
Dicot Embryo
Structure
Embryo
Structure
• monocot: e.g. corn
– embryonic axis + one cotyledon
– single cotyledon is called a scutellum
– embryo is enclosed within 2 sheaths: a coleoptile that covers the young
shoot and a coleorhiza that covers the young root
– both these coverings aid in soil penetration during germination
Scutellum
(cotyledon)
Coleoptile
Pericarp fused
with seed coat
Endosperm
Epicotyl
Hypocotyl
Coleorhiza
Maize, a monocot
Radicle
Monocot Embryo Structure
The Mature Seed
• last stages of maturation – seed dehydrates
• embryo enters dormancy – time length varies with species
• cues from the environment are designed to ensure the seed
breaks dormancy when the conditions are optimal for
germination and seedling growth
• some cues:
–
–
–
–
–
light
moisture
intense heat – fires
intense cold
seed coats must be enzymatically digested by animals when eaten
Germination
• germination requires imbibition –
uptake of water (due to the low water
content of the dormant seed)
• first organ to emerge is the radicle
• next the shoot tip must break the soil
surface
pea seedling
2 types of germination
• Eudicots: epigeal germination (cotyledons break
the surface)
– a hook forms in the hypocotyl and growth pushes the hook
above ground – carrying the rest of the seed
– the hypocotyl straightens in response to light
– the cotyledons separate into the first leaves – “seed leaves”
– once the first true leaves form - the cotyledons shrivel and fall
http://www.youtube.com
away
/watch?v=TJQyL-7KRmw
Foliage leaves
Epicotyl
Hypocotyl
Hypocotyl
Cotyledon
Cotyledon
Hypocotyl
Radicle
Seed coat
Common garden bean
• Monocots: hypogeal germination (cotyledons remain in
the seed & underground)
– the radicle grows down out of the coleorhiza & into the soil
– the coleoptile pushes upward through the soil into the air
– the embryonic shoot tip grows straight up through a tunnel in
the coleoptile
Foliage leaves
Coleoptile
Coleoptile
Radicle
Maize
http://www.youtube.com/watch?v=iFCdAgeMGOA
Asexual Reproduction
• asexual reproduction = the development of offspring without
fusion of sperm and egg
• result is called a clone
• nearly genetically identical to the parent
• advantages: no need for a pollinator
– works well if plants are sparsely distributed
– also works well if the plant is well suited to its environment or if the environment
is unstable
– the germination of a seed is a vulnerable stage so if many seeds must be
produced this expends energy – not seen in asexual reproduction
• disadvantages: can pass on dangerous mutations
– or can perpetuate “bad” traits
• common mechanisms:
– detached vegetative fragments of the parent plant grows into a new
sporophyte = fragmentation
– roots of the aspen tree give rise to shoots that eventually become
separate shoot systems and new plants