FLOWERS
Notes for Biology 2410* at Utah State University
*Plants and fungi: ecosystem essentials
Flowers: A Marvelous
Innovation
Flowering plants first
appeared around 140 million
years ago (Upper Jurassic).
Oldest flower fossil is 125
million years old.
The dominant forms of plant
life were gymnosperms,
cycads, and ferns.
Today
Flowering plants are now the dominant form of plant
life over most of the earth’s land surface.
Ferns are rarely dominant and gymnosperms are
dominant only in cold, or seasonally cold locations.
Cycads are hanging on by a thread.
Why have flowering plants been so successful?
Flowers
• Double fertilization: saves energy
• Ovaries protect ovules and developing seeds;
mature into fruits that promote seed dispersal
• Floral structure encourages pollinator fidelity;
nectar and pollen to reward pollinators
• Fast reproductive cycle compared to gymnosperms
• Shorter haploid phase (a genetically risky phase)
than all other plants
Origin of benefits
• Some benefits derive from structure of
flower
• Some benefits derive from short
reproductive cycle
• Some benefits from double fertilization
• Many benefits derive from evolutionary
modifications within the flowering plant
lineage
Structure of flowers
Flowers are composed of
four whorls. From the
outside in, they are:
• Calyx (sepals)
• Corolla (petals)
• Androecium (stamens)
• Gynoecium (pistils)
Calyx
• Outermost whorl
• Usually green
• Protects developing flower
– Physically
– Chemically
• Made up of SEPALS
• Sepals free or not
• Calyx radially or
bilaterally symmetric
Corolla
• Is usually colorful and
showy;
• Attracts pollinators
• Guides pollinators;
• Is composed of petals
• Petals may be united or
separate;
• Corolla may be radially or
bilaterally symmetric.
Androecium
• Is composed of
stamens
• Stamens have
filaments and anthers
• Pollen is produced in
anthers
• Stamens can be free or
united
Gynoecium
• Is composed of pistils
• A pistil is composed
of an ovary, style, and
stigma.
• Styles may be
separate, branched, or
united
• Pistils have a slide of
their own – just wait
Hypanthium
If the corolla and calyx are
attached to a cup or tube
that is then attached to the
receptacle, the cup or tube
is called a hypanthium.
If a hypanthium is present,
the perianth may be
– hypogynous
– perigynous
– epigynous
Flower structure (2)
• Perianth refers to the
combination of the
calyx and corolla.
• Used
– when the two are very
similar (tulips)
– to refer to both
structures at the same
time
Receptacle
• The top floral whorls
are attached to the
receptacle – the tissue
where the stalk to the
flower changes to
being part of the
flower
Pedicels
• Flowers are borne on
pedicels
• Think of a pedicel as
the stalk to a flower
Pistils
• Composed of
– Stigma – landing
platform for the pollen
grains, first selection
point
– Style – helps select
among pollen tubes;
may be branched
– Ovary – contains
ovules; matures into
fruit
Ovaries (and fruits)
• Ovaries protect the
ovules and developing
seed
• Become fruits,
modified for seed
dispersal.
• Fruits may have other
parts of flower
attached.
Identifying Flowering Plants
• To survive, species must reproduce
• Pollination is the first major step in the
reproduction of seed plants like
gymnosperms and flowering plants
• Flowers persuade animals to serve as
pollinators, preferably faithful pollinators
• To get the right pollen, flowers need to be
memorable so …
Start thinking like a pollinator
• Is there a reward?
• How can I find another like this one?
– Symmetry – radial or bilateral
– Color pattern
– Odor
• Will it be safe for me?
– Corolla – united or separate petals
– Corolla more important than calyx – why?
• Can I reach the reward and is it adequate?
– nectaries and androecium
Next, start counting …
• sepals (or calyx lobes)
• petals (or corolla
lobes)
• stamens
• pistils
• style branches
Ovary and 2-branched style
http://botany.cs.tamu.edu/FLORA/dcs420/cr/hdw15039981b.jpg
Then look at the ovary position
• Above the calyx and
corolla (ovary
superior; perianth
hypogynous)
• Below the calyx and
corolla (ovary
inferior; perianth
epigynous)
• One more possibility
exists …
And that possibility is …
• Ovary partially Ovary partially inferior
inferior; perianth
perigynous
Perianth
perigynous
And finally, the glories of the
gynoecium …
• How many pistils are
there?
• How many styles or
style branches?
• How many chambers
are there in the ovary
(need a cross-section)
• Where are the seeds
attached?
Still part of the ‘little more’
• Each theoretical leaf unit is
termed a carpel
• A pea pod is a mature pistil
made up of 1 carpel
• Pistils with 2 style branches
and/or chambers in the ovary
are said to have 2 carpels
• Pistils with 3 style branches
and/or chambers are said to
have 3 carpels etc. etc.
And that is enough to start …
Assignment:
Pick 4 non-cultivated plants and carefully
examine their flowers. Show you understand
its structure by completing the handouts.
Why a non-cultivated plant?
• Many cultivated plants have been selected
for their abnormalities, which often makes it
difficult for beginners (and even old-timers)
to identify them.
• Most identification keys include a very few
purely cultivated plants so the chances are
good that a cultivated plant is not in the key.