Flowers and Their Evolution Spring 2012 Flower = a short, determinate shoot bearing - PowerPoint Presentation

Flowers and Their Evolution Spring 2012

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) Androecium ( andros = male; - oecium = house) “Pistil” – carpel(s) [fused or not] Gynoecium ( gynos = female; - oecium = house)

Floral Parts: Major whorls pistil (1-many carpels) - gynoecium stamens - androecium petals - corolla sepals - calyx receptacle

young leaves flower bud Sepals and petals are relatively leaf-like. sepal petal stamen pistil

Fig. 6.5 from Simpson “ABC” Model of Floral development

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

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) = gynoeciumFUSION 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 female gametophyte (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 margins; receptive to pollen (Receptacle located below) Folding of one megasporophyll

Carpel evolution stigmatic crest stigma

Fig. 9.30 from Simpson

Simple Carpel – One Pistil

Apocarpy – Separate Carpels = 5 pistils in this example

Apocarpous gynoecium – Ranunculus sp . with many pistils elongated receptacle

Magnolia Liriodendron

Fig. 9.31 from Simpson

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 parts Stamens adnate to petals

Connation: Fusion of similar parts Fusion of filaments into a staminal tube

Fig. 9.32 from Simpson Ovary position relative to other parts The hypanthium requires both c onnation and adnation .

Ovary s uperior Citrus sp. Parts hypogynous

Ovary superior, parts perigynous (floral cup or tube = hypanthium present) Rosa sp.

Ovary inferior, p arts epiperigynous (hypanthium present ) Fuchsia sp.

Ovary i nferior, p arts 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 .

Fig. 9.33 f rom Simpson

Fig. 9.33, Part A only

Axile Parietal Placentation

Floral Symmetry Radial Bilateral Zygomorphic Actinomorphic

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?