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
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
Four terminal WHORLS of modified leaves:
- Two outermost whorls (sepals, petals) are sterile (non-
- Two innermost whorls (sporophylls) are “fertile” with
tissues capable of undergoing meiosis
- Microsporophylls – stamens – produce pollen in anthers
- Megasporophylls – carpels – produce eggs in ovules
Fig. 6.2 from Simpson
Gynoecium (gynos = female; -oecium =
Floral Parts: Major whorls
pistil (1-many carpels)
stamens - androecium
petals - corolla
sepals - calyx
Sepals and petals are
Fig. 6.5 from Simpson
- Microsporangia (meiosis microspores
pollen grains) on lamina originally
- Lamina becomes filament
- Sporangial tissue becomes anther wall
- Provides for release of pollen
- Tremendous variation in flowering plants.
- Often associated with specific type of pollinator.
- Evolution of megasporophyll structure traced back to seed ferns – 200 to 300 mybp
- 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
- Unfused (separate) carpels - apocarpous
- Fused (united) carpels - syncarpous
The Ovule =
from a single
Folding of megasporophyll to form simple carpel
Folding of one
S = suture; formed
by fusion of leaf
Fig. 9.30 from Simpson
Apocarpous gynoecium – Ranunculus sp.
with many pistils
Fig. 9.31 from Simpson
Syncarpousgynoecium – One pistil, 3 carpels
Various gynoecia – Apocarpousvs.Syncarpous
(Hint: stigma number usually = carpel number)
Stamens adnate to petals
Fusion of filaments into a staminal tube
Ovary position relative to other parts
The hypanthium requires both
connation and adnation.
Ovary superior, parts perigynous
(floral cup or tube = hypanthium present)
- Provides for fertilization of egg cell in megagametophyte and protection during development.
- Ovule matures into the SEED.
- Analogous to the mammalian “umbilical cord”
- Point of attachment on inner ovary wall is the PLACENTA
- Can vary depending on type of flower.
- Plant families typically have one placentation type.
- Often best seen with cross section through ovary.
- Fusion of carpels, presence of vascular bundles, etc. can support hypotheses about evolution of particular flower
Part A only
- How many parts are present in the calyx? Corolla? - Describe the androecium, then the gynoecium.
- Hypogynous? Perigynous? Epigynous?
- Apocarpous? Syncarpous? If so, how many carpels?
- Placentation? Position of stigma relative to other parts.
- Fusion of floral parts can sometimes be diagnostic.
- Specializations for pollination?