Download Seed Plants: Angiosperms

Seed Plants: Angiosperms
Chapter 23
Outline
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Introduction
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Phylum Magnoliophyta – The Flowering Plants
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Development of Gametophytes
Pollination
Fertilization and Development of the Seed
Apomixis and Parthenocarpy
Trends of Specialization and Classification in Flowering
Plants
Pollination Ecology
Introduction
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Angiosperms = flowering plants
Seeds enclosed in carpel – resembles folded over
leaf and fused at margins
• Pistil composed of single carpel, or >2 united carpels
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Seed develops from
ovule within carpel
Ovary becomes fruit
Bleeding hearts
Introduction
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Angiosperms = Phylum Magnoliophyta
Divided into 2 large classes:
• Magnoliopsida - Dicots
− DNA and cladistic evidence suggest 2 groups of
dicots should be recognized
• Liliopsida - Monocots
Flower = 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 (= tepals)
− Flattened and numerous stamens and carpels
Phylum Magnoliophyta – The Flowering Plants
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Heterosporous
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Sporophytes dominant
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Female gametophytes wholly enclosed within
sporophyte tissue and reduced to only few cells
Male gametophytes consist of germinated pollen
grain with 3 nuclei
Phylum Magnoliophyta
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Development of gametophytes - Female:
• 2n megasporocyte differentiates in ovule
− Undergoes meiosis and produces 4 1n megaspores
3 degenerate
• Remaining cell enlarges and nucleus divides to
produce 8 nuclei (without walls)
• Outer 2 layers of ovule differentiate into integuments
that later become seed coat
− Micropyle at one end of ovule
Phylum Magnoliophyta
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Development of gametophytes – Female cont’d.:
• 8 nuclei form 2 groups, 4 near each end of cell
• 1 nucleus from each group migrates to cell middle and
form central cell
• Cell walls form around
remaining 6 nuclei
− Egg and 2 synergids
closest to micropyle
− 3 antipodals at
opposite end
• Female gametophyte =
megagametophyte or
embryo sac
Phylum Magnoliophyta
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Development of gametophytes - Male:
• Formation of male gametophytes takes place in
anthers
• 4 patches, corresponding to pollen sacs, of
microsporocyte cells differentiate in anther
• Each
microsporocyte
undergoes meiosis
to produce 4 1n
microspores
Anther with microspores
Phylum Magnoliophyta
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Development of gametophytes – Male cont’d.:
• Microspores undergo 3 changes:
− Divide once by mitosis to form small generative cell
inside larger tube cell
Nucleus of tube cell = vegetative nucleus
− Members of each quartet of microspores separate
− Wall becomes 2-layered
Outer layer (= exine)
finely sculptured and
contains chemicals that
react with chemicals in
stigma
• Generative nucleus divide to
produce 2 sperm
Pollen grain
Phylum Magnoliophyta
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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
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Fertilization and development of the seed:
• After pollination, further development of male
gametophyte may not take place unless pollen grain:
− From different plant of same species
− From variety different from that of receiving flower
• Pollen tube grows between cells of stigma and style
until reaches ovule micropyle
• Vegetative nucleus stays at tips of pollen tube, while
generative cell lags behind and divides into 2 sperm
• Pollen tube enters female gametophyte, destroying
synergid in process, and discharges sperms
Phylum Magnoliophyta
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Fertilization and development of the seed cont’d.:
• Mature male
gametophyte =
germinated pollen grain
with vegetative nucleus
and 2 sperms within
tube cell
Phylum Magnoliophyta
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Fertilization and development of the seed cont’d.:
• Double fertilization:
− 1 sperm unites with egg, forming zygote,
then embryo
− Other sperm unites with central cell nuclei,
producing 3n endosperm nucleus developing
into endosperm tissue
Endosperm tissue = nutritive tissue for
embryo
Phylum Magnoliophyta
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Fertilization and development of the seed cont’d.:
Endosperm becomes extensive part of
seed in some monocots (i.e., corn and
other grasses)
Endosperm absorbed into cotyledons in
most dicots
− Ovule becomes seed, ovary matures into
fruit, integuments harden into seed coat
Phylum Magnoliophyta
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Fertilization and development of the seed cont’d.:
• 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
Phylum Magnoliophyta
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Apomixis and parthenocarpy:
• Apomixis - without fusion of gametes but with
normal structures otherwise being involved
− Embryo from 2n nutritive cell or other 2n
cell of ovule, instead of from zygote
Results in vegetatively propagated plant
− Parthenocarpy - fruits develop from ovaries
with unfertilized eggs.
− Results in seedless fruits
Navel oranges and bananas
Phylum Magnoliophyta
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Trends of specialization and classification in
flowering plants:
• 1st historical classifications for convenience
• Modern botanists group plants according to natural
relationships based on evolution
• Fossil record suggests flowering plants 1st appeared
about 160 mya during late Jurassic
• Flowering plants developed during Cretaceous and
Cenozoic
• Dominant plants today
Phylum Magnoliophyta
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Trends of specialization and classification in flowering
plants cont’d.:
• 1st 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
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Trends of specialization and classification in flowering
plants cont’d.:
• Inferior ovary (epigynous
flower) - receptacle or other
flower parts fused to ovary
and grown up around it
• Superior ovary (hypogynous
flower) - ovary produced on
top of receptacle
• Perigynous flowers - flower
parts attached to corolla tube
of fused petals, creating floral
tube not attached to ovary
Phylum Magnoliophyta
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Trends of specialization and classification in flowering
plants cont’d.:
• Complete flower - has
calyx, corolla, stamens and
pistil
Male
flower
• Incomplete flower corolla or other flower
parts missing
• Perfect flower - both
stamens and pistil present
Female flower with inferior
ovary
Phylum Magnoliophyta
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Trends of specialization and classification in flowering
plants cont’d.:
• Imperfect flower - either
stamens or pistil missing
– Monoecious species Male
male and female
flower
imperfect flowers on
same plant
– Dioecious species plant bears only male
flowers and other
plants bear only
female flowers
Female flower with inferior
ovary
Phylum Magnoliophyta
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Trends of specialization and classification in flowering
plants cont’d.:
• Primitive flowering plants
− Simple leaves
− Flower with numerous, spirally arranged parts, not
fused to each other
• Flowers radially symmetrical = regular.
− Flowers complete
and perfect
− Superior ovary
(hypogynous
flower)
Magnolia
Phylum Magnoliophyta
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Trends of specialization and classification in flowering
plants cont’d.:
• Specialized flowering plants:
− Flower parts fewer and definite in #
− Spiral arrangements compressed to whorls
− Bilaterally symmetrical flowers = irregular
Orchid
Phylum Magnoliophyta
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Trends of specialization and classification in flowering
plants cont’d.:
• Specialized flowering plants cont’d.:
− Reduction and fusion of parts
 Incomplete or imperfect flowers
− Inferior ovary
Orchid
Pollination Ecology
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Pollinators co-evolved with plants
• 20,000 bee species among current-day pollinators
• Bee-pollinated flowers:
− Generally brightly colored, mostly blue or yellow
− Often have lines or distinctive markings, may
function as guides to lead bees to nectar
 Bees see UV light (humans do not)
In ordinary light
In UV light
Pollination Ecology
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Beetle-pollinated flowers:
• Strong, yeasty, spicy or fruity odor
• White or dull in color
• 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
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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
Pollination Ecology
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Bird-pollinated flowers (hummingbirds and sunbirds):
• Often bright red or yellow
• Little if any odor - Birds don’t have keen sense of
smell
• Large and part of sturdy inflorescence
• Copious amounts of nectar - Birds highly active
• Long floral tubes
Pollination Ecology
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Bat-pollinated flowers:
• Primarily in tropics
• Open at night when bats
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
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Orchid flowers:
• Have pollinators among all types mentioned
• Some adaptations between orchid flowers and
pollinators extraordinary.
• Pollen grains produced in little sacs called pollinia
(singular: pollinium) with sticky pads at base
Ophrys
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