Flowers and their evolution
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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.

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Flowers and Their Evolution

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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

megasporangiain 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


stamen

pistil

Sepals and petals are

relatively leaf-like.

sepal

young

leaves

flower

bud

petal


“ABC”

Model of

Floral

development

Fig. 6.5 from Simpson


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.


Stamen evolution

laminar stamens


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

sporangium

female

gametophyte

(derived

from a single

spore)

integuments

micropyle


Carpel evolution

(Ovules)

(megasporophyll)


Folding of megasporophyll to form simple carpel

Folding of one

megasporophyll

S = suture; formed

by fusion of leaf

margins; receptive

to pollen

(Receptacle located

below)


Carpel evolution

stigma

stigmatic

crest


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


Liriodendron

Magnolia


Fig. 9.31 from Simpson


Syncarpousgynoecium – One pistil, 3 carpels


Various gynoecia – Apocarpousvs.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


Ovary position relative to other parts

The hypanthium requires both

connation and adnation.

Fig. 9.32

from Simpson


Ovary

superior

Parts

hypogynous

Citrus sp.


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.


Fig. 9.33

from

Simpson


Fig. 9.33,

Part A only


Placentation

Axile

Parietal


Floral Symmetry

Radial

Bilateral

Actinomorphic

Zygomorphic


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?


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