Presentation on theme: "Flowers and Their Evolution Spring 2014. Flower = a short, determinate shoot bearing highly modified leaves, some of which are fertile (i.e., bearing."— Presentation transcript:
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 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
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.
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 single spore) sporangium micropyle
Folding of megasporophyll to form simple carpel S = suture; formed by fusion of leaf by fusion of leaf margins; receptive margins; receptive to pollen to pollen Folding of one megasporophyll megasporophyll receptacle
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.
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?