1. Flowers and Their Evolution
Spring 2012

2. 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.

3. 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

4. Fig. 6.2 from Simpson

5. 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)

6. Floral Parts: Major whorls
pistil (1-many carpels)
- gynoecium
stamens - androecium
petals - corolla
sepals - calyx
receptacle

7. Sepals and petals are relatively leaf-like. sepal young leaves flower
stamen
pistil
Sepals and petals are
relatively leaf-like.
sepal
young
leaves
flower
bud
petal

8. “ABC”
Model of
Floral
development
Fig. 6.5 from Simpson

9. Floral Anatomy

10. 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.

11. Stamen evolution
laminar stamens

12. Fig. 9.26
Fig. 9.25

13. Floral Anatomy

14. 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.

15. integumented megasporangium
The Ovule =
integumented megasporangium
sporangium
female
gametophyte
(derived
from a single
spore)
integuments
micropyle

16. Carpel evolution
(Ovules)
(megasporophyll)

17. Folding of one megasporophyll S = suture; formed by fusion of leaf
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)

18. Carpel evolution
stigma
stigmatic
crest

19. Fig from Simpson

20. Simple Carpel – One Pistil

21. Apocarpy – Separate Carpels = 5 pistils in this example

22. Apocarpous gynoecium – Ranunculus sp.
with many pistils
elongated receptacle

23. Liriodendron
Magnolia

24. Fig from Simpson

25. Syncarpous gynoecium – One pistil, 3 carpels

26. Various gynoecia – Apocarpous vs. Syncarpous
(Hint: stigma number usually = carpel number)

27. Syncarpy – How many carpels? Locules?

28. Adnation: Fusion of different parts
Stamens adnate to petals

29. Connation: Fusion of similar parts
Fusion of filaments into a staminal tube

30. Ovary position relative to other parts
The hypanthium requires both
connation and adnation.
Fig. 9.32
from Simpson

31. Ovary
superior
Parts
hypogynous
Citrus sp.

32. Ovary superior, parts perigynous
(floral cup or tube = hypanthium present)
Rosa sp.

33. Ovary inferior,
parts epiperigynous
(hypanthium present)
Fuchsia sp.

34. Ovary
inferior,
parts
epigynous
Vaccinium sp.

35. 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.

36. Fig. 9.33
from
Simpson

37. Fig. 9.33,
Part A only

38. Placentation
Axile
Parietal

39. Floral Symmetry
Radial
Bilateral
Actinomorphic
Zygomorphic

40. 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?