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Chapter 20
Lecture Outline
Seed Plants:
Angiosperms
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Outline

Introduction

Phylum Magnoliophyta – The Flowering Plants
• Development of Gametophytes
• Pollination
• Fertilization and Development of the Seed
• Apomixis and Parthenocarpy
• Trends of Specialization and Classification in
Flowering Plants

Pollination Ecology

Herbaria and Plant Preservation
Introduction

Angiosperms = flowering plants

Seeds enclosed in carpel that resembles a
leaf that has folded over and fused at the
margins.
• Pistil composed of a single carpel, or two or more
united carpels.

Seed develops from
ovule within carpel.

Ovary becomes a fruit.
Bleeding hearts
Introduction

Phylum Magnoliophyta – The Flowering Plants

Has been divided into two large classes:
• Magnoliopsida - Dicots
–
DNA and cladistic evidence suggest that two groups of dicots
should be recognized.
• Liliopsida - Monocots

Flower is modified stem bearing modified leaves.
• Most primitive flower:
–
–
–
Long receptacle
Many spirally arranged flower parts that are separate and not
differentiated into sepals and petals
Flattened and numerous stamens and carpels
Phylum Magnoliophyta – The Flowering Plants

Heterosporous

Sporophytes dominant.

Female gametophytes wholly enclosed
within sporophyte tissue and reduced to only
a few cells.

At maturity, male gametophytes consist of a
germinated pollen grain with three nuclei.
Phylum Magnoliophyta

Development of gametophytes - Female:
• Diploid megasporocyte differentiates in ovule.
–
Undergoes meiosis and produces four haploid
megaspores.
o
Three degenerate.
• Remaining cell enlarges and nucleus divides to
produce 8 nuclei (without walls).
• Outer two layers of ovule differentiate into
integuments that later become seed coat.
–
Micropyle at one end of ovule
Phylum Magnoliophyta

Development of gametophytes - Female:
• 8 nuclei form two groups, 4 near each end of cell.
• One nucleus from each group migrates to cell middle and
form central cell.
• Cell walls form around
remaining six nuclei.
–
–
Egg and two synergids
closest to micropyle
Three antipodals at
opposite end - No apparent
function
• Female gametophyte
(megagametophyte, embryo
sac) = large sac containing
8 nuclei and 7 cells
Phylum Magnoliophyta

Development of gametophytes - Male:
• Formation of male gametophytes takes place in anthers.
• Four patches, corresponding to pollen sacs, of
microsporocyte cells differentiate in anther.
• Each
microsporocyte
undergoes
meiosis to
produce
quartet of
haploid
microspores.
Anther with microspores
Phylum Magnoliophyta

Development of gametophytes - Male:
• Microspores undergo three important changes:
–
Divide once by mitosis to form a small generative cell
inside the larger tube cell
o
Nucleus of tube cell = vegetative nucleus
–
Members of each quartet of microspores separate.
–
Wall becomes two-layered.
o Outer layer = exine
« Finely sculptured
« Contains chemicals that
may react with chemicals
in stigma
• Generative nucleus will later
divide to produce two sperm.
Pollen grain
Phylum Magnoliophyta

Pollination:
• Pollination - Transfer of pollen grains from anther
to stigma
–
Self-pollination - Pollen grains germinate on stigma of
same flower.
• Fertilization - Union of sperm and egg
• Pollination by insects, wind, water, animals or
gravity.
Phylum Magnoliophyta

Fertilization and development of the seed:
• After pollination, further development of male
gametophyte may not take place unless pollen grain is:
–
–
From a different plant of the same species.
From a variety different from that of the receiving flower.
• Pollen tube grows between cells of stigma and style
until it reaches ovule micropyle.
• Vegetative nucleus stays at tips of pollen tube, while
generative cell lags behind and divides into two sperm.
• Pollen tube enters female gametophyte, destroying
synergid in the process, and discharges sperms.
Phylum Magnoliophyta

Fertilization and development of the seed:
• Mature male
gametophyte =
germinated pollen
grain with its
vegetative nucleus
and two sperms
within tube cell
Phylum Magnoliophyta

Fertilization and development of the seed:
• Double fertilization:
–
One sperm unites with egg, forming zygote, then embryo.
–
Other sperm unites with central cell nuclei, producing
triploid endosperm nucleus that develops into endosperm
tissue.
–
o
Endosperm tissue = nutritive tissue for embryo
o
Endosperm becomes extensive part of seed in some
monocots, such as corn and other grasses.
« Wheat, rice and corn - Major source of nutrition for
humans due to nutritional quality of endosperm
o
Endosperm absorbed into cotyledons in most dicots.
Ovule becomes seed, ovary matures into fruit,
integuments harden into seed coat.
Phylum Magnoliophyta

Lifecycle of typical flowering plant:
Phylum Magnoliophyta

Fertilization and development of the seed:
• Other types of (female) gametophyte development:
–
Female gametophyte can have from 4 to 16 nuclei or
cells at maturity.
–
Endosperm may be 5x, 9x or 15x.
–
Example in lilies:
o
All four haploid megaspore nuclei produced from
megasporocyte remain functional.
o
Three of nuclei unite forming 3x nucleus and forth nucleus
remain haploid.
« Results in female gametophyte with four 3x nuclei and
four 1x nuclei
« One central cell nucleus is 3x and other is 1x.
« Fertilization results in 5x endosperm nucleus.
Phylum Magnoliophyta

Fertilization and development of the seed:
• Lily (female) gametophyte development:
Phylum Magnoliophyta

Apomixis and parthenocarpy:
• Apomixis - Without fusion of gametes but with the
normal structures otherwise being involved
–
Embryo from diploid nutritive cell or other diploid cell of
ovule, instead of from zygote.
o Results in a vegetatively propagated plant
• Parthenocarpy - Fruits develop from ovaries with
unfertilized eggs.
–
Results in seedless fruits
o Navel oranges and bananas
Phylum Magnoliophyta

Trends of specialization and classification in
flowering plants:
• First historical classifications for convenience.
• Modern botanists group plants according to
natural relationships based on evolution.
• Fossil record suggests flowering plants first
appeared about 160 million years ago during late
Jurassic.
• Flowering plants then developed during
Cretaceous and Cenozoic.
• Dominant plants today
Phylum Magnoliophyta

Trends of specialization and classification in
flowering plants:
• First pistil from leaflike structure with ovules along
margins = carpel.
–
Edges of blade rolled inward and fused together.
• Separate carpels
of primitive
flowers fused
together to form
compound pistil
consisting of
several carpels.
Phylum Magnoliophyta

Trends of specialization and classification in
flowering plants:
• Inferior ovary (epigynous flower)
- Receptacle or other flower parts
fused to ovary and grown up
around it.
–
Calyx and corolla appear to be
attached to top of ovary.
• Superior ovary (hypogynous
flower) - Ovary produced on top
of receptacle.
–
Other flower parts attached
around ovary base.
• Perigynous flowers - Flower parts
attached to corolla tube of fused
petals, creating floral tube that is
not attached to ovary.
Phylum Magnoliophyta

Trends of specialization and classification in
flowering plants:
• Complete flower - Has calyx,
corolla, stamens and pistil
• Incomplete flower - Corolla
or other flower parts missing.
• Perfect flower - Both
stamens and pistil present.
• Imperfect flower - Either
stamens or pistil missing.
–
–
Monoecious species - Male
and female imperfect flowers
on same plant.
Dioecious species - Plant
bears only male flowers and
other plants bear only female
flowers.
Male
flower
Female flower with inferior ovary
Phylum Magnoliophyta

Trends of specialization and classification in
flowering plants:
• Primitive flowering plants:
– Simple leaves.
– Flower with numerous, spirally arranged parts, not fused to
each other
– Flowers radially symmetrical = regular.
– Flowers with both
stamens and pistils
o Complete and
perfect flowers
– Superior ovary
(hypogynous flower)
– Still many plants today
whose flowers have
Magnolia
primitive features.
Phylum Magnoliophyta

Trends of specialization and classification in
flowering plants:
• Specialized flowering plants:
– Flower parts fewer and definite in number.
– Spiral arrangements compressed to whorls
– Bilaterally symmetrical flowers = irregular
– Reduction and fusion of parts
o Incomplete or imperfect flowers
– Inferior ovary
Orchid
Pollination Ecology

Pollinators have coevolved with plants.
• Twenty thousand bee species among current-day
pollinators.
• Bee-pollinated flowers:
–
–
Generally brightly colored, mostly blue or yellow
Often have lines or other distinctive markings, which may
function as honey guides to lead bees to nectar.
o
Bees see UV light (humans do not).
« Some flower markings visible only in UV light.
In ordinary light
In UV light
Pollination Ecology

Beetle-pollinated flowers:
•
Strong, yeasty, spicy or fruity odor
• White or dull in color - Beetles do have keen visual senses.
• Some do not secrete nectar, but furnish pollen or food on
petals in special storage cells.

Fly-pollinated flowers:
•
Smell like rotten meat
• Dull red or brown
Pollination Ecology

Butterfly- and moth-pollinated flowers:
•
Often have sweet fragrances
• White or yellow for night-flying moths
• Sometimes red, often blue, yellow or orange for butterflies
• Nectaries at bases of corolla tubes or spurs for long tongues.

Bird-pollinated flowers (hummingbirds and sunbirds):
•
•
•
•
•
Often bright red or yellow
Little if any odor - Birds do
not have a keen sense of
smell.
Large and part of sturdy
inflorescence
Copious amounts of nectar
- Birds highly active.
Long floral tubes
Pollination Ecology

Bat-pollinated flowers:
• Primarily in tropics
• Open at night when bats
are foraging
• Dull in color
• Large enough for bat to
insert head or consist of
ball-like inflorescence
containing large numbers
of small flowers
Pollination Ecology

Orchid flowers:
• Have pollinators among all types mentioned
• Some of adaptations between orchid flowers and
pollinators are extraordinary.
• Pollen grains produced in little sacs called pollinia
(singular: pollinium) with sticky pads at base.
• Members of Ophrys have modified petal that resembles
female bumble bee or wasp.
–
Male bees or wasps try to
copulate with flower.
o Pollinia deposited on
their head.
Ophrys
Herbaria and Plant Preservation

Herbaria (singular: herbarium) - Libraries of
dried, pressed plants, algae, and fungi,
arranged and labeled.

Methods:
• Fungi and
bryophytes dried
and stored in
small packets.
• Plant press
used for
vascular plants.
Herbaria and Plant Preservation

Methods:
• Vascular plant
specimens mounted
on 100% rag
herbarium paper.
• Specimens stored
so retrieval of
specimens is easily
accomplished.
Review

Introduction

Phylum Magnoliophyta – The Flowering Plants
• Development of Gametophytes
• Pollination
• Fertilization and Development of the Seed
• Apomixis and Parthenocarpy
• Trends of Specialization and Classification in
Flowering Plants

Pollination Ecology

Herbaria and Plant Preservation