lecture 6b
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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. - PowerPoint PPT PresentationTRANSCRIPT
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• most diverse and widespread of all plants• 250,000 species worldwide – 90% of all plants!
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 (Illiciumfloridanum)
Angiosperm phylogeny
• 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 clade diverged from
their common ancestor• Primitive fossils of 125-million-year-old angiosperms display derived and
primitive traits
HYPOTHETICAL TREE OF FLOWERING PLANTS
MAGNOLIIDS
Ambo
rella
Star
ani
sean
d re
lativ
es
Wat
er li
lies
Mag
nolii
ds
Mon
ocot
s
Eudi
cots
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
Monocots• embryo with one cotyledon• other traits:
– 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
• remaining plants were put into a lineage informally known as basal angiosperms
• embryo with two cotyledons– cotyledons: store food absorbed from the
endosperm
zucchini flower
Californiapoppy
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
zucchini flower
Californiapoppy
Flowers• 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
Flower Anatomy
Stamen
Filament
AntherStigma Carpel
Style
Ovary
Petal
ReceptacleOvule
Sepal
• 1. sepals (sterile flower organ)– usually green and enclose the
flower before it opens– emerge from the receptacle
• 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 colorful
Flower Anatomy
Stamen
Filament
AntherStigma Carpel
Style
Ovary
Petal
ReceptacleOvule
Sepal
• 3. stamens (produce spores)– contain 4 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 (pollen)
Flower Anatomy
Stamen
Filament
AntherStigma Carpel
Style
Ovary
Petal
ReceptacleOvule
Sepal
• 4. carpels (produce spores)– contain ovules that contain
megaspores - develop into the female gametophyte
– some flowers have a single carpel – others have multiple (separate or fused together)
Flower Anatomy
Stamen
Filament
AntherStigma Carpel
Style
Ovary
Petal
ReceptacleOvule
Sepal
• 4. carpels (produce spores)– 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 female gametophyte: the megaspore & the egg
– these ovules when fertilized develop into seeds within a fruit
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
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• 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 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 you may know – and find yummy!
• http://waynesword.palomar.edu/fruitid1.htm
• Simple Fruits– fruit from one ovary/one flower– A. Fleshy
• 1. Berry – grapes, tomatoes, pomegranates, avocados• 2. Pepo – thick exocarp – watermelon, cucumbers,
cantelope• 3. Hesperidium – leathery exocarp – citrus fruits• 4. Drupe – hard inner endocarp (stone) around the seed –
peach, plum, apricot, pecans, walnuts• 5. Pome – ovary surrounded by a fleshy receptacle –
apple, loquats
– B. Dry• 1. Dehiscent –split along seams
– a. Legume – 2 seam split - peas, beans (locust tree)– b. Capsule – multiple seam split (Pittisporum, Jacaranda,
Yucca)
• 2. Indehiscent – do not split– a. Achene – one seeded fruit in clusters - sunflower– b. Grain – seed coat fused with pericarp – wheat, rye, rice– c. Nuts – large, one-seeded fruit with very hard pericarps
• Aggregate Fruits– cluster of many ovaries
from a single flower– blackberries– raspberries– strawberries– rose hip
• Multiple Fruits– many ovaries formed
from a coalescence of flowers in ONE inflorescence
– pineapple– breadfruit– mulberry
Anther
Mature flower onsporophyte plant(2n)
Key
Haploid (n)Diploid (2n)
MicrosporangiumMicrosporocytes (2n)
MEIOSIS
Microspore (n)
MEIOSIS
Ovule withmegasporangium (2n) Male
gametophyte(in pollengrain)Ovary
Generative cell
Tube cell
Megasporangium(n)Survivingmegaspore(n)
Female gametophyte(embryo sac)
Antipodal cellsPolar nucleiSynergidsEggs (n)
Pollentube
Sperm(n)
Pollengrains
PollentubeStyle
StigmaPollentube
Sperm
Eggs nucleus (n)
Discharged sperm nuclei (n)
Germinatingseed
Zygote (2n)
FERTILIZATION
Nucleus ofdevelopingendosperm (3n)
Embryo (2n)Endosperm(foodsupply) (3n)
Seed coat (2n)Seed
Life Cycle of Angiosperms
http://www.sumanasinc.com/webcontent/animations/content/angiosperm.html
• on the anther are 4 microsporangia or pollen sacs
• each microsporangium (2n) contains multiple microsporocytes (2n)– microsporocytes undergo meiosis to
form microspores (n)• 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
Anther
Mature flower onSporophyte plant(2n)
Key
Haploid (n)Diploid (2n)
MicrosporangiumMicrosporocytes (2n)
MEIOSIS
Microspore (n)
MEIOSIS
Ovule withmegasporangium (2n)Male
gametophyte(in pollengrain)Ovary
Generative cell
Tube cell
Megasporangium(n)Survivingmegaspore(n)
Female gametophyte(embryo sac)
Antipodal cellsPolar nucleiSynergidsEggs (n)
Pollentube
Sperm(n)
Male Cycle:
Anthermicrosporangium
pollen grains
MEIOSIS
Ovule withmegasporangium (2n)
Ovary
Megasporangium(n)
Survivingmegaspore(n)
Female gametophyte(embryo sac) Pollen
tube
Sperm(n)
Antipodal cellsPolar nucleiSynergidsEggs (n)
• 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 megaspore mother cell (2n)– the megasporangium has an
opening – micropyle (for sperm entry)
– the megasporocyte enlarges & divides by meiosis to produce four megaspores (n)
– only one megaspore survives
Female:
MEIOSIS
Ovule withmegasporangium (2n)
Ovary
Megasporangium(n)
Survivingmegaspore(n)
Female gametophyte(embryo sac) Pollen
tube
Sperm(n)
Antipodal cellsPolar nucleiSynergidsEggs (n)
– only one megaspore survives– the surviving megaspore &
surrounding tissue develops into the female gametophyte• megaspore undergoes three
mitotic divisions (no cytokinesis) one large cell results with 8 nuclei
• this multi-nucleated cell will be partitioned off by membranes to form a multicellular female gametophyte OR embryo sac
Female:
MEIOSIS
Ovule withmegasporangium (2n)
Ovary
Megasporangium(n)Survivingmegaspore(n)
Female gametophyte(embryo sac)
Pollentube
Sperm(n)
Antipodal cellsPolar nucleiSynergidsEggs (n)
– cells of the embryo sac: • 1. antipodal cells – 3 cells of
unknown function• 2. central cell – containing 2
polar nuclei• 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
• 25% of all angiosperms– 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
Stigma
Pollen tube
2 sperm
StyleOvary
Ovule (containingfemalegametophyte, orembryo sac)
Micropyle
Polarnuclei
Egg
If a pollen graingerminates, a pollen
tube grows down the style toward the
ovary.
Pollengrain
• pollen lands on the stigma of the carpel & begins to germinate
• pollen tubes develop first– tubes travel down the style toward the ovule
• each pollen tube terminates at an ovule– penetrates into the ovule through the
micropyle at the base of the ovule• following tube formation – the
generative cell splits by mitosis -> two sperm
Pollination & Fertilization
Ovule
Polar nuclei
Egg
Two spermabout to bedischarged
The pollen tubedischarges two sperm into the female gametophyte (embryo
sac) within an ovule.
• pollen tube arrives at the micropyle • sperm are discharged into each ovule
Pollination & Fertilization
• double fertilization then takes place– one sperm nuclei unites with egg nuclei– the other sperm nuclei fuses with the 2 polar nuclei of the central cell triploid central cell
• the triploid central cell forms the endosperm
Pollination & Fertilization
One sperm fertilizesthe 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.
Endospermnucleus (3n)(2 polar nucleiplus sperm)
Zygote (2n)(egg plus sperm)
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
• there is a type of double fertilization that occurs in Phylum Gnetophyta– but this produces two embryos
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)
• the megasporangium & female gametophyte is surrounded by two integuments – will become the seed coat
• fruit begins to develop around the seeds• seed dispersal completes the life cycle
Seed Development
• the seed consists of:– the embryo – the triploid endosperm – the seed coat
• the endosperm – rich in starch; food for seedling germination– usually develops before the embryo– the triploid central cell – has three nuclei– initially has a milky consistency until cytokinesis takes place– cytokinesis three cells– these cells produce cell walls and the endosperm becomes
solid
Embryo Development
Zygote
Terminal cellBasal cell
ProembryoSuspensor
Basal cell
Cotyledons
Endosperm
Root apexSeed coat
Suspensor
Shoot apex
• 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• the basal cell continues to divide to
produce a suspensor– anchors the embryo to the parent plant– for the transfer of nutrients
Embryo Development
Cotyledons
Endosperm
Root apexSeed coat
Suspensor
Shoot apex
• the terminal cell continues to divide to form a spherical pro-embryo – attached to the parent via the suspensor
• the embryo develops embryonic leaves called cotyledons– cotyledons store food absorbed from the
endosperm prior to germination• the cotyledons form as “bumps” in the pro-
embryo– eudicot looks like a “heart”
Embryo Development
• the embryo then starts to elongate = embryonic axis
• top of the axis = shoot apex next to or between the cotyledons
• bottom of the axis = development of a root apex
The Mature Seed• embryo structure:
– eudicot: e.g. garden bean• elongated embryo (embryonic axis) attaches to
thick cotyledons at the stem apex• growing stem of the seedling below where the
cotyledons attach is called the hypocotyl– stays under the soil and ends as the roots
• root apex forms the radicle or embryonic root• above where the cotyledons attach to growing
stem is the epicotyl– becomes the stem above ground &
develops into the leaves of the plant
The Mature Seed
The Mature Seed• embryo structure:– monocot: e.g. corn
• embryonic axis + one cotyledon 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
Seed coat Epicotyl
Radicle
Hypocotyl
Cotyledons
Common garden bean, a eudicot with thick cotyledons
Seed coat
Cotyledons
Epicotyl
Radicle
Hypocotyl
Endosperm
Castor bean, a eudicot with thin cotyledons
Maize, a monocot
ColeoptileEpicotyl
Radicle
Hypocotyl
Endosperm
Pericarp fusedwith seed coat
Coleorhiza
Scutellum(cotyledon)
The Mature Seed
• last stages of maturation – seed dehydrates – water content drops to 5-15% of its weight
• 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
2 types of 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• Two types of germination:• 1. hypogeal = monocots• 2. epigeal = dicots
Epigealgermination
Foliage leavesCotyledon
Cotyledon
Hypocotyl
Hypocotyl
RadicleSeed coat
Hypocotyl
Cotyledon
Epicotyl
Common garden bean
• 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 “seed leaves”– the epicotyl develops into the first “true”
leaves – begin photosynthesis– the cotyledons shrivel and fall away
http://www.youtube.com/watch?v=TJQyL-7KRmw
Hypogealgermination
Foliage leaves
Coleoptile
RadicleMaize
Coleoptile
• Monocots: hypogeal germination (cotyledons remain in the seed & underground – e.g. nuts)– the radicle grows down from the coleorhiza into the
soil– the coleoptile pushes upward through the soil into
the air – the embryonic shoot emerges– the shoot tip grows straight up through a tunnel in
the coleoptile
http://www.youtube.com/watch?v=iFCdAgeMGOA