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?