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Flowers and Their Evolution
Spring 2014
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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REPRODUCTIVE STRUCTURE – Evolutionary requirement to
reproduce by sexual means. Pollen transfer and seed dispersal
needed.
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MODIFIED FOLIAR APPENDAGES – all function together to form
the reproductive organ known as the FLOWER.
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MODIFICATIONS OF LEAVES – All floral organs are modified
LEAVES.
Four terminal WHORLS of modified leaves:
- Two outermost whorls (sepals, petals) are sterile (nonmeiotic tissues)
- Two innermost whorls (sporophylls) are “fertile” with
tissues capable of undergoing meiosis
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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 whorls
pistil (1-many carpels)
- gynoecium
stamens - androecium
petals - corolla
sepals - calyx
receptacle
stamen
pistil
Sepals and petals are
relatively leaf-like.
young
leaves
XS of
flower
bud
sepal
petal
“ABC”
Model of
Floral
development
Fig. 6.5 from Simpson
Floral Anatomy
Evolution of the Androecium
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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
microsporangia
laminar stamens
Fig. 9.26
Fig. 9.25
Floral Anatomy
Evolution of the Carpel
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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
Carpel evolution
stigma
stigmatic
crest
3 pistils
Fig. 9.30 from Simpson
1 pistil
Simple Carpel – One Pistil
Apocarpy – Separate Carpels = 5
pistils in this example
Apocarpous gynoecium – Ranunculus sp.
with many pistils
elongated receptacle
Liriodendron
Magnolia
Simple vs. compound ovary
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 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
Ovary position relative to other parts
Fig. 9.32
from Simpson
The hypanthium (floral cup) requires
both connation and adnation.
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
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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
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
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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?