Seed Plants: Angiosperms Chapter 23. Outline Introduction Phylum Magnoliophyta – The Flowering...
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Transcript of Seed Plants: Angiosperms Chapter 23. Outline Introduction Phylum Magnoliophyta – The Flowering...
Seed Plants: Angiosperms
Chapter 23
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
Introduction Angiosperms = flowering plants
Seeds enclosed in carpel – resembles folded over leaf and fused at margins• Pistil composed of single carpel, or >2 united carpels
Seed develops from ovule within carpel
Ovary becomes fruitBleeding hearts
Introduction 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
Heterosporous
Sporophytes dominant
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 Development of gametophytes - Female:• 2n megasporocyte differentiates in ovule
−Undergoes meiosis and produces 4 1n megaspores3 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 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 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 Development of gametophytes – Male cont’d.:• Microspores undergo 3 changes:
− Divide once by mitosis to form small generative cell inside larger tube cellNucleus 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 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:−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 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 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 tissueEndosperm tissue = nutritive tissue for
embryo
Phylum Magnoliophyta 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 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 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 zygoteResults in vegetatively propagated plant
−Parthenocarpy - fruits develop from ovaries with unfertilized eggs.
−Results in seedless fruitsNavel oranges and bananas
Phylum Magnoliophyta 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 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 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 Trends of specialization and classification in flowering
plants cont’d.:• Complete flower - has
calyx, corolla, stamens and pistil
• Incomplete flower - corolla or other flower parts missing
• Perfect flower - both stamens and pistil present
Male flower
Female flower with inferior ovary
Phylum Magnoliophyta Trends of specialization and classification in flowering
plants cont’d.:• 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
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 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 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 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 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
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
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
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 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