Flower structure


Angiosperm life cycle







A. Animal/insect pollinators

Plants have formed associations with animals over a million years ago to ensure pollination. To facilitate this process, they provide the pollinator with the appropriate clues and positive reinforcements:

... vision, shape, and scent signals

... nutrition in the form of nectar, waxes or the pollen itself,

... sometimes narcotic drugs which modify the insect's behavior

... structures which may simulate or teach mating behavior,

... products which the pollinator may use for pheromones or defense.

How flowers attract pollinators:

Visual Cues:

Showy petals or sepals with obvious shape, size, and color for the vector's vision are critical and specific signals for the appropriate pollinator .

Butterflies and birds are attracted to red and yellow colors, and these contrasting colors in the same flower are readily seen by a number of pollinators.

Bees have vision that is shifted toward the blue end of our visible spectrum. They do not see red colors but do see into the near-ultraviolet. Thus when a bee visits a brightly colored flower ( which may attract other pollinators) they are cued most likely by a UV pattern we can not detect without a UV detector.

For animals such as moths and bats that fly at night a white or very pale color is certainly more observable.

Nectar guides and other color patterns may help either make the plant more distinguishable or orient the oncoming "bomber" to make the right flight pattern landing. .





Olfactory cues

Some pollinators have limited vision but the critical ability to find a flower by its fragrance. Plants can produce volatile chemicals that diffuse through the environment. A pollinator that can recognize an odor and fly up the concentration gradient of this fragrance, can easily find the next flower of a particular species. Some plants have specialized by producing odors which suggest they are something that in fact they are not... flies are attracted to the aroma of rotting meat or fecal matter - thus some flowers have evolved these odor mimics. Other produce compounds that are similar to the pheromones that insects use to attract each other. Many incredible examples exist without us having a clue of this floral forgery.


Read this ENN article on the economics of odor...


"The neighborhood flower shop can give you a bouquet of huge, vibrant flowers that will brighten a room for days at a time. Unfortunately, the selective breeding process that led to such splendid flowers left floral scent behind.

"Selective breeding has reduced flower scent to almost nothing," said Natalia Dudareva, a Purdue University assistant professor in the Department of Horticulture and Landscape Architecture.

"Flowers are bred for color, size and shelf life without any attention to scent. Floral scent disappeared, and nobody knows why." Dudareva is working on the answer to that question. To date she has isolated three genes for enzymes that form volatile compounds. Volatile compounds are essential oils that evaporate in warm weather and combine in various proportions to produce distinctive odors that are identical among plants of the same variety.

An orchid could produce about 100 different volatile compounds, while a snapdragon produces about seven to 10. While much has been done to analyze the composition of scent, we're just learning about the genes that produce the compounds and how they are given off. Plants use volatile compounds to repel and kill pests, to attract pollinators and to communicate between cells and plants.

"There are known examples of plants that have been infected with a virus and have released a volatile compound that signals other plants to set up defenses against the virus," Dudareva says. Once she understands how plants produce volatile compounds and what genes are involved, Dudareva said she hopes to be able to manipulate the genes to enhance the beneficial aspects of scent. "Of course, we'll also be able to produce lots of nice smelling flowers," she said.

She's also intrigued by the field of aromatherapy and the attempts to link health benefits with exposure to certain smells. If links can be established between scent and health effects, such as stress reduction, Dudareva knows that interest in her work will blossom. "What if you could make yourself feel better simply by putting a vase of enhanced flowers on your desk?" Dudareva asks. "That would be better than taking a pill!

Shape considerations

The flower has to be designed to accommodate the vector and prevent pollen and nectar robbers from stealing the rewards. The shape can also make a flower more attractive.

Flowers pollinated by hovering vectors generally hang down and have long styles and filaments.

Flowers pollinated by hungry beetles need to provide lots of structural food (and yet protect the ovules!). A beetle-pollinated flower needs to have easy entrance as beetles are clumsy in flight.

Non-hovering insects and birds need perches or landing platforms as part of the flower. The sizes and shapes of the flower parts and their alignments are critical to assure pollination when the vector does visit. A long nectar spur protects the reward from robbers and yet allows reward for the actual vector. Moreover, the body alignment while the proboscis is in the nectar spur is critical to the pollination event.


Getting the pollinator to land isn't enough to insure pollination. The vector is usually' "intelligent enough" to avoid the energy waste of behaviors that do not result in some kind of reward. Thus plants have devised suitable givings that help recruit pollinators and help them keep faithful to the specific species.

Food and other products as attractants:


Nectar is produced by the nectaries. Although primarily sugar water, it does contain limited amounts of amino acids, vitamins and minerals. For species dependent on a high energy diet, such as bees, butterflies and hummingbirds, the amounts produced must be copious to sustain their life styles.

In the tropics, large flowering plants such as Heloconias produce a flower depth which matches the beak length of a specific hummingbirds to ensure fidelity of the pollinating species

Different flowering species "take turns" flowering over the year to 'insure' that their pollinators always have something to feed upon.Work out how this phenology could have evolved?


Pollen grains hold amino acids, starch, oil, and other nutrients. As you see in the photo below, a bee enters the snapdragon. As he searches for more pollen, he brushes against the stigma placed overhead, transferring pollen to this new flower.

Flowers which depend on insects, especially bees for pollination, have honey guides ( spots that lead the insect in the right direction; these may include pigments which fluoresce under UV or show up with visible light. They are also often zygomorphic, that is having mirror images ( can only be cut in one direction to obtain 2 equal halves).

Other tempting offerings..

A number of orchids and other advanced angiosperms produce chemicals or heat sources which attract insects. Some of the compounds are narcotics, and insects are lured and drugged till pollination is complete. Skunk cabbage, which blooms early in the season offers heat as an attractant along with its fetid odor.

An Ophrys orchid produces sex pheromones used by the female wasps whose mate's virgin attempts at copulation are with the orchid flower that makes the pheromone scent.

Some flowers also produce other products such as waxes, pheromones, chemicals which insects store in their bodies to make them repellent to other predators, and hormones. We will view some of these offerings in a video in class.

Getting more for the effort!

As noted under the section on flower basics, the least advanced plants were rather large and single. A lot of effort is put into producing just one such flower to support just a few ovaries and their seeds. In the more advanced plants, flower size per ovary is minimized, but to remain attractive ( in the sense of a pollinator finding you), they have bundled up a number of these smaller offerings in clumps rendering them visible from afar.

Several different patterns exist to exploit this strategy...

B. Wind pollination

  • As we saw earlier with the grasses, wind pollinated plant are mostly green flowers without sepals or petals
  • plants need to produce increased amounts of pollen to survive; may be aided by feathery stigmas to carry pollen great distances
  • dense populations form i.e. grass, oaks, conifers


C. Pollination strategies:

Cross pollination

positive cross pollination causes a new combination of genes which produce offspring that are better adapted to their environment than their parents

negative cross pollination causes a combination of genes which produce offspring that are less adapted to their environment than their parents


  • maturation of stamen and style; flowering must be synchronized
  • proteins on stigma and style need to be compatible
  • proteins on pollen need to be compatible