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)

Angiosperm phylogeny

HYPOTHETICAL TREE OF FLOWERING PLANTS MAGNOLIIDS

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: – 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 –

Monocots

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

– cotyledons: store food absorbed from the endosperm California poppy 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)

– 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 – colorful

Anther Filament

Flower Anatomy

Stigma Style Carpel Ovary Petal Receptacle Ovule Sepal

•

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

(pollen) Stamen Anther Filament

Flower Anatomy

Stigma Style Carpel Ovary Petal Receptacle Ovule Sepal

•

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

Stamen Anther Filament Petal

Flower Anatomy

Stigma Style Carpel Ovary 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) Mature flower on sporophyte plant (2n) Germinating seed Nucleus of developing endosperm (3n) Anther Embryo (2n) Endosperm (food supply) (3n) Seed coat (2n) Seed Female gametophyte (embryo sac) Zygote (2n) Eggs nucleus (n)

Life Cycle of

Ovary Antipodal cells Polar nuclei Synergids Eggs (n) Microsporangium Microsporocytes (2n) MEIOSIS Microspore (n) Ovule with megasporangium (2n) Male gametophyte (in pollen grain) MEIOSIS Megasporangium (n) Surviving megaspore (n) Stigma Pollen tube Sperm Pollen tube Style

Angiosperms

Generative cell Tube cell Pollen grains

http://www.sumanasi

nc.com/webcontent/a nimations/content/an giosperm.html

Pollen tube Sperm (n) FERTILIZATION Discharged sperm nuclei (n)

Male Cycle:

• • on the anther are four

microsporangia or pollen sacs

– supported by a filament each microsporangium

(2n) contains multiple

microsporocytes (2n)

– also known as

microspore mother cells (2n)

– microsporocytes undergo meiosis to form

microspores (n) Key Haploid (n) Diploid (2n) Mature flower on Sporophyte plant (2n)

pollen grains

Anther

microsporangium

(embryo sac) Antipodal cells Polar nuclei Synergids Eggs (n) Microsporangium Microsporocytes (2n) MEIOSIS Ovary Microspore (n) Ovule with megasporangium (2n) Male gametophyte (in pollen grain) Generative cell Tube cell MEIOSIS Megasporangium (n) Surviving megaspore (n)

Anther

Pollen tube Sperm (n)

•

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) Mature flower on Sporophyte plant (2n) Anther Female gametophyte (embryo sac) Synergids Eggs (n) Microsporangium Microsporocytes (2n) MEIOSIS Ovary MEIOSIS Microspore (n) Generative cell Male gametophyte (in pollen grain) Tube cell Pollen tube 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 – 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 Key Haploid (n) Diploid (2n) Mature flower on Sporophyte plant (2n) Female gametophyte (embryo sac) Anther Antipodal cells Polar nuclei Synergids Eggs (n) MEIOSIS Ovule with megasporangium (2n) Ovary MEIOSIS Megasporangium (n) Surviving megaspore (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

Megasporangium (n) Integuments Surviving megaspore (n) Degenerating megaspores Female gametophyte (embryo sac) Polar nuclei Synergids Eggs (n)

Female:

Antipodal cells Pollen tube 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 If a pollen grain germinates, a pollen tube grows down the style toward the ovary.

Polar nuclei Egg Stigma Pollen tube 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

The pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule.

Ovule Polar nuclei 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 

central cell triploid

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

the basal cell continues to divide to produce a suspensor

Embryo Development

Zygote 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

Development

Basal cell Cotyledons Shoot apex Root apex

Embryo

Proembryo Suspensor Seed coat Endosperm Suspensor

• • • • • the elongated embryo is called the

embryonic axis

attached to it are the cotyledons – – one cotyledon in monocots two cotyledons in eudicots

Embryo Development

top of the embryonic axis –

shoot apex

– where the cotyledons attach to the axis bottom of the embryonic axis – root apex – attaches to the suspensor both the shoot and root apex contain meristematic tissue – stem cells for development

Cotyledons Shoot apex Root apex Suspensor Seed coat 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

embryonic root radicle or

– above where the cotyledons attach - the axis is the

epicotyl

• produces the

true leaves

of the seedling

Embryo Structure

Seed coat Radicle Common garden bean, a eudicot with thick cotyledons Epicotyl Hypocotyl Cotyledons

Dicot Embryo Structure

•

monocot: e.g. corn

– – embryonic axis + one cotyledon single cotyledon is called a scutellum –

Embryo Structure

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

Maize, a monocot Scutellum (cotyledon) Coleoptile Coleorhiza Pericarp fused with seed coat Endosperm Epicotyl Hypocotyl 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 away http://www.youtube.com

/watch?v=TJQyL-7KRmw

Foliage leaves Hypocotyl Hypocotyl Cotyledon Epicotyl Cotyledon Hypocotyl Radicle Common garden bean Seed coat

•

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 Maize Radicle

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