Flowers and Their Evolution

Spring 2013

Flower = a short, determinate shoot bearing highlymodified 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)

Floral Parts: Major whorlspistil (1-many carpels)- gynoecium

stamens - androecium

petals - corolla

sepals - calyx

receptacle

young leaves

XS of flowerbud

Sepals and petals arerelatively leaf-like.

sepal

petal

stamen

pistil

Fig. 6.5 from Simpson

“ABC”Model ofFloraldevelopment

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

microsporangia

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) = 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 singlespore)

sporangium

micropyle

Carpel evolution

(Ovules)

(megasporophyll)

Folding of megasporophyll to form simple carpel

S = suture; formed by fusion of leaf margins; receptive to pollen

Folding of one megasporophyll

receptacle

Carpel evolution

stigmaticcrest

stigma

Fig. 9.30 from Simpson

3 pistils 1 pistil

Simple Carpel – One Pistil

Apocarpy – Separate Carpels = 5 pistils in this example

Apocarpous gynoecium – Ranunculus sp. with many pistils

elongated receptacle

MagnoliaLiriodendron

Fig. 9.31 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. 9.32 from Simpson

Ovary position relative to other parts

The hypanthium (floral cup) requiresboth connation and adnation.

Ovarysuperior

Citrus sp.

Partshypogynous

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

Rosa sp.

Ovary inferior,parts epiperigynous(hypanthium present)

Fuchsia sp.

Ovaryinferior,partsepigynous

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. 9.33fromSimpson

Fig. 9.33,Part A only

Axile Parietal

Placentation

Floral Symmetry

Radial Bilateral

ZygomorphicActinomorphic

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?