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 inferior; perianth perigynous

Ovary partially inferior

Perianthperigynous

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.