Flowers and Their Evolution
Spring 2010
Flower = a short, determinate shoot bearing highlymodified leaves, some of which are fertile (i.e.,bearing either microsporangia or megasporangia),with 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 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. 4.16 from the textFig. 4.16 from the text
Floral Whorls
• Attached to RECEPTACLE
• Sepals - Calyx
• Petals - Corolla
• Stamens (anthers + filaments) Androecium (andros = male; -oecium = house)
• “Pistil” – carpel(s) [fused or not] Gynoecium (gynos = female; -oecium =
house)
Floral Parts: Major whorls
pistil - gynoecium
stamens - androecium
petals - corolla
sepals - calyx
receptacle
young young leavesleaves
flowerflowerbudbud
Sepals and petals arerelatively leaf-like.
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 A SINGLE WHORL OR MULTIPLE WHORLS- Tremendous variation in flowering plants.- Often associated with specific type of
pollinator.
laminar stamen
Stamen evolution
Stamen diversity
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
femalefemalegametophytegametophyte(derived (derived from a singlefrom a singlespore)spore)
sporangiumsporangium
Carpel evolution
Folding of megasporophyll to form simple carpel
S = suture; formed by fusion of leaf margins; receptive to pollen
(Receptacle located(Receptacle located below)below)
Folding of one megasporophyll
Simple Carpel – One Megasporophyll
Apocarpy – Separate Carpels
E.g., Ranunculus, Magnolia, LiriodendronE.g., Ranunculus, Magnolia, Liriodendron
Fig. 4.19 from the textFig. 4.19 from the text
Syncarpous gynoecia
E.g., EuphorbiaE.g., Euphorbia
Syncarpy – Fused Carpels
Fig. 4.21 from the textFig. 4.21 from the text
Superior
Citrus sp.
Perigynous—floral cup or hypanthium
RosaRosa
Inferior
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 attachement 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. 4.22 from the textFig. 4.22 from the text
Floral Symmetry
Radial Bilateral
ZygomorphicActinomorphic
Floral Symmetry
Radial Bilateral
ZygomorphicActinomorphic
Adnation: Fusion of different parts
Stamens adnate to petals
Stamens adnate to gynoecium
Connation: Fusion of similar parts
Fusion of filaments into a staminal tube
Interpretation of Floral Structures
• OBSERVE STRUCTURES IN EACH WHORL- How many parts are present in the calyx? Corolla?
- Describe the androecium, then the gynoecium.
• DETERMINE CONDITION OF THE FLOWER- Hypogynous? Perigynous? Epigynous?
• GYNOECIUM- Apocarpous? Syncarpous? If so, how many carpels?- Placentation? Position of stigma relative to other parts.
• ADNATION or CONNATION?- Fusion of floral parts can sometimes be diagnostic.
• UNUSUAL OR REMARKABLE FLORAL STRUCTURES?- Specializations for pollination?
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