flowers and their evolution spring 2011. flower = a short, determinate shoot bearing highly modified...
TRANSCRIPT
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
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
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.
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
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
Apocarpy – Separate Carpels
E.g., Ranunculus, Magnolia, LiriodendronE.g., Ranunculus, Magnolia, Liriodendron
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.
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?