Flowers and Their Evolution Spring 2010. Flower = a short, determinate shoot bearing highly modified leaves, some of which are fertile (i.e., bearing.

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


Syncarpous gynoecia

E.g., EuphorbiaE.g., Euphorbia

Syncarpy – Fused Carpels


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.


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?

Flowers and Their Evolution Spring 2010. Flower = a short, determinate shoot bearing highly modified leaves, some of which are fertile (i.e., bearing.

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