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:
Flowers and Their Evolution Spring 2014
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
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)
young leaves XS of flower bud Sepals and petals are relatively leaf-like. sepal petal stamen pistil
Fig. 6.5 from Simpson ABC Model of Floral development
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
Carpel evolution (Ovules) (megasporophyll)
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
Carpel evolution stigmatic crest stigma
Fig from Simpson 3 pistils1 pistil
Simple Carpel – One Pistil
Apocarpy – Separate Carpels = 5 pistils in this example
Apocarpous gynoecium – Ranunculus sp. with many pistils elongated receptacle
Fig 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 from Simpson Ovary position relative to other parts The hypanthium (floral cup) requires both connation and adnation.
Ovary superior Citrus sp. Parts hypogynous
Ovary superior, parts perigynous (floral cup or tube = hypanthium present) Rosa sp.
Ovary inferior, parts epiperigynous (hypanthium present) Fuchsia sp.
Ovary inferior, parts epigynous 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.
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?