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Flowers and Their Evolution. Spring 2013. Flower = a short, determinate shoot bearing highly modified leaves, some of which are fertile (i.e., bearing either microsporangia or megasporangia ), with the microsporangia in stamens and the megasporangia in carpels . Flower. - PowerPoint PPT Presentation
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Flowers and Their Evolution
Spring 2013
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.
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
Fig. 6.2 from Simpson
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)
Floral Parts: Major whorlspistil (1-many carpels)- gynoecium
stamens - androecium
petals - corolla
sepals - calyx
receptacle
young leaves
XS of flowerbud
Sepals and petals arerelatively leaf-like.
sepal
petal
stamen
pistil
Fig. 6.5 from Simpson
“ABC”Model ofFloraldevelopment
Floral Anatomy
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.
laminar stamens
Stamen evolution
microsporangia
Fig. 9.26
Fig. 9.25
Floral Anatomy
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.
The Ovule = integumented megasporangium
integuments
femalegametophyte(derived from a singlespore)
sporangium
micropyle
Carpel evolution
(Ovules)
(megasporophyll)
Folding of megasporophyll to form simple carpel
S = suture; formed by fusion of leaf margins; receptive to pollen
Folding of one megasporophyll
receptacle
Carpel evolution
stigmaticcrest
stigma
Fig. 9.30 from Simpson
3 pistils 1 pistil
Simple Carpel – One Pistil
Apocarpy – Separate Carpels = 5 pistils in this example
Apocarpous gynoecium – Ranunculus sp. with many pistils
elongated receptacle
MagnoliaLiriodendron
Fig. 9.31 from Simpson
Simple vs. compound ovary
Syncarpous gynoecium – One pistil, 3 carpels
Various gynoecia – Apocarpous vs. Syncarpous(Hint: stigma number usually = carpel number)
Syncarpy – How many carpels? Locules?
Adnation: Fusion of different whorls
Stamens (filaments) adnate to petals, petals adnate to sepals
Connation: Fusion of parts from the same whorl
Fusion of filaments into a staminal tube
Fig. 9.32 from Simpson
Ovary position relative to other parts
The hypanthium (floral cup) requiresboth connation and adnation.
Ovarysuperior
Citrus sp.
Partshypogynous
Ovary superior, parts perigynous(floral cup or tube = hypanthium present)
Rosa sp.
Ovary inferior,parts epiperigynous(hypanthium present)
Fuchsia sp.
Ovaryinferior,partsepigynous
Vaccinium sp.
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.
Fig. 9.33fromSimpson
Fig. 9.33,Part A only
Axile Parietal
Placentation
Floral Symmetry
Radial Bilateral
ZygomorphicActinomorphic
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)
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?