Flowers and Their Evolution
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Transcript of Flowers and Their Evolution
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Flowers and Their Evolution
Spring 2014
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Flower = a short, determinate shoot bearing highlymodified leaves, some of which are fertile (i.e.,bearing either microsporangia or megasporangia),with the microsporangia in stamens and the megasporangia in carpels.
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Flower• REPRODUCTIVE STRUCTURE – Evolutionary requirement to
reproduce by sexual means. Pollen transfer and seed dispersal needed.
• MODIFIED FOLIAR APPENDAGES – all function together to form the reproductive organ known as the FLOWER.
• MODIFICATIONS OF LEAVES – All floral organs are modified
LEAVES. Four terminal WHORLS of modified leaves:
- Two outermost whorls (sepals, petals) are sterile (non- meiotic tissues)- Two innermost whorls (sporophylls) are “fertile” with
tissues capable of undergoing meiosis
• SPOROPHYLLS – those modified leaves with meiotic capacity.- Microsporophylls – stamens – produce pollen in
anthers- Megasporophylls – carpels – produce eggs in ovules
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Fig. 6.2 from Simpson
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Floral Whorls• Attached to RECEPTACLE • Sepals (collectively the Calyx) • Petals (collectively the Corolla)• Stamens (anthers + filaments)
collectively the Androecium (andros = male; -oecium = house)
• “Pistil” – carpel(s) [fused or not] collectively the Gynoecium (gynos = female; -oecium = house)
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Floral Parts: Major whorlspistil (1-many carpels)- gynoecium
stamens - androecium
petals - corolla
sepals - calyx
receptacle
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young leaves
XS of flowerbud
Sepals and petals arerelatively leaf-like.
sepal
petal
stamen
pistil
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Fig. 6.5 from Simpson
“ABC”Model ofFloraldevelopment
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Floral Anatomy
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Evolution of the Androecium• DERIVED FROM MODIFIED LEAVES
- Microsporangia (meiosis microspores pollen grains) on lamina originally
• INCREASING LEVELS OF REDUCTION- Lamina becomes filament- Sporangial tissue becomes anther wall - Provides for release of pollen
• CAN BE IN A SINGLE WHORL OR MULTIPLE WHORLS- Tremendous variation in flowering plants.- Often associated with specific type of
pollinator.
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laminar stamens
Stamen evolution
microsporangia
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Fig. 9.26
Fig. 9.25
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Floral Anatomy
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Evolution of the Carpel• MODIFICATION OF MEGASPOROPHYLL
- Evolution of megasporophyll structure traced back to seed ferns – 200 to 300 mybp
• LEAF WITH MARGINAL MEIOTIC ZONES FOLDS- Ovules located at margins of sporophylls- Lamina curves inward (toward the floral axis - adaxially) - Carpel is formed by folding – conduplicate - Margins fuse, enclosing ovules- Carpel(s) = gynoecium
• FUSION OF CARPELS - Unfused (separate) carpels - apocarpous- Fused (united) carpels - syncarpous
• POSITION OF THE GYNOECIUM relative to other floral whorls is important in describing floral structures.
• PLACEMENT OF OVULES (placentation) within the gynoecium is also important; shows evolutionary origins of the carpel.
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The Ovule = integumented megasporangium
integuments
femalegametophyte(derived from a singlespore)
sporangium
micropyle
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Carpel evolution
(Ovules)
(megasporophyll)
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Folding of megasporophyll to form simple carpel
S = suture; formed by fusion of leaf margins; receptive to pollen
Folding of one megasporophyll
receptacle
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Carpel evolution
stigmaticcrest
stigma
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Fig. 9.30 from Simpson
3 pistils 1 pistil
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Simple Carpel – One Pistil
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Apocarpy – Separate Carpels = 5 pistils in this example
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Apocarpous gynoecium – Ranunculus sp. with many pistils
elongated receptacle
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MagnoliaLiriodendron
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Fig. 9.31 from Simpson
Simple vs. compound ovary
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Syncarpous gynoecium – One pistil, 3 carpels
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Various gynoecia – Apocarpous vs. Syncarpous(Hint: stigma number usually = carpel number)
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Syncarpy – How many carpels? Locules?
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Adnation: Fusion of different whorls
Stamens (filaments) adnate to petals, petals adnate to sepals
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Connation: Fusion of parts from the same whorl
Fusion of filaments into a staminal tube
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Fig. 9.32 from Simpson
Ovary position relative to other parts
The hypanthium (floral cup) requiresboth connation and adnation.
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Ovarysuperior
Citrus sp.
Partshypogynous
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Ovary superior, parts perigynous(floral cup or tube = hypanthium present)
Rosa sp.
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Ovary inferior,parts epiperigynous(hypanthium present)
Fuchsia sp.
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Ovaryinferior,partsepigynous
Vaccinium sp.
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Ovules and Placentation• OVULES CONTAIN THE MEGAGAMETOPHYTE
- Provides for fertilization of egg cell in megagametophyte and protection during development.
- Ovule matures into the SEED. • ATTACHMENT OF THE OVULES VIA FUNICULUS
- Analogous to the mammalian “umbilical cord” - Point of attachment on inner ovary wall is the
PLACENTA - Can vary depending on type of flower.
• PLACENTATION IS OFTEN DIAGNOSTIC- Plant families typically have one placentation type.- Often best seen with cross section through ovary.
• PLACENTATION REFLECTS EVOLUTIONARY DEVELOPMENT- Fusion of carpels, presence of vascular bundles, etc.
can support hypotheses about evolution of particular flower
structures.
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Fig. 9.33fromSimpson
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Fig. 9.33,Part A only
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Axile Parietal
Placentation
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Floral Symmetry
Radial Bilateral
ZygomorphicActinomorphic
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Merosity = basic number of parts in each whorl
-3 sepals, 3 petals, 6 stamens, 3 carpels = 3-merous (or trimerous)
-4 sepals, 4 petals, 6 or 8 stamens, 2 or 4 carpels = 4-merous (or tetramerous)
-5 sepals, 5 petals, 5 or 10 stamens, 3 or 5 carpels = 5-merous (or pentamerous)
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Interpretation of Floral Structures
• OBSERVE STRUCTURES IN EACH WHORL- How many whorls are there?
- How many parts are present in the calyx? Corolla? - Describe the androecium, then the gynoecium.
• DETERMINE POSITION OF THE FLOWER PARTS RELATIVE TO THE OVARY
- Hypogynous? Perigynous? Epigynous? Epiperigynous?
• GYNOECIUM- Apocarpous? Syncarpous? If so, how many carpels?- Position? Superior or inferior or half-inferior?- Placentation?
• ADNATION or CONNATION?- Fusion of floral parts can sometimes be diagnostic.
• UNUSUAL OR REMARKABLE FLORAL STRUCTURES?- Specializations for pollination?