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