Introduction to Pollination

There are two critical stages in the life cycle of a flowering plant:

1) the transfer of pollen from anther to stigma. As we will saw in the lecture on "Breeding Systems", most flowering plants have different types of mechanisms to promote the transfer of pollen from an anther in one flower to a stigma in a different flower, and hopefully this flower will be on a different plant.
2) the dispersal of seeds away from the parent plant.

Plants often solicit the aid of animals, as well as abiotic forces such as wind, to accomplish both of these.


"Sexual reproduction is just as important for plants as it is for animals when it comes to generating genetic variation, but plants have a singular disadvantage compared to animals when it comes to sex: they can't just get up and find themselves a mate. " — May R. Berenbaum, Bugs in the System (1995).

Much of the flower diversity that you have observed thus far this semester is because of adaptations for pollination by different mechanisms. In this lecture, we will discuss the main types of pollination mechanisms. However, please keep in mind that there are always exceptions; plants and animals that visit flowers have minds of their own!

The vast majority of flowering plant species are pollinated by insects; in fact, it seems that flowering plants and many major groups of insects co-evolved together. Animals other than insects can also be important pollinators: bats, birds (especially hummingbirds), and even a few mammals.

In order to attract biological pollinators, flowers must:

1. Advertise

2. Offer reward

3. Provide access to flower

4. Have way to transfer pollen from anther to critter and then to stigma


Beetle Pollination
The least specialized flowers are members of subclass Magnoliidae. These are often pollinated by beetles, although beetles also visit a wide variety of flat flowers (or flat inflorescences that function as single flowers). Some general features of beetle flowers are:

Time: Either day or night
Structure: Actinomorphic, numerous floral parts; large bowl shaped, ovules protected
Color: Dull, white, few visual attractions, no nectar guides
Scent: Strong, fruity or aminoid; no nectar, food primarily pollen or food bodies

Carrion & Dung Beetles & Flies
Time: Either day or night
Structure: Actinomorphic, deep corolla tube with appendages forming traps
Color: Purple, brown (like meat), no nectar guides or nectar
Scent: Strong, like rotting meat or feces, really awful to humans

Fly-gnats
Time: Stay open for several days
Structure: Urn or kettle-shaped traps, structures that mimic fungus gills or pores, close to ground level
Color: Dark purple or brown
Scent: Musky like a fungus; intense transpiration during flowering period for humidity

Flies & Bee Flies
Time: Mostly day
Structure: Actinomorphic, little depth
Color: Variable, but often dull or light, nectar guides present
Scent: Little to (to much) odor, nectar present or absent, accessible, food often pollen

Bees
Time: Day
Structure: Often zygomorphic, shape variable, little depth to tubular
Color: Yellow, blue, or white, usually not red, often with nectar guides
Scent: Sweet odor, nectar usually present, often hidden

Butterflies (Lepidoptera)
Time: Day
Structure: Actinomorphic, erect anthers on narrow tubular corolla
Color: Yellow, blue, pink and red, often with nectar guides
Scent: Strong or weak, nectar present in corolla tubes or spurs

Hawkmoths
Time: Night or dusk
Structure: Actinomorphic, narrow tubular corolla, anthers often, versatile, flowers horizontal or hanging down
Color: White or some-times pale green to yellow, no nectar guides
Scent: Heavy, sweet odor at night, abundant nectar

Birds (especially hummingbirds)
Hummingbirds are found only in the Americas, and only one species, the ruby-throated hummingbird, breeds east of the Rocky Mountains. A number of different plants from many plant families are adapted for hummingbird pollination.

Time: Day
Structure: Actinomorphic, stiff, wide tubular with hanging stamens
Color: Often bright red, no nectar guides
Scent: No odor, abundant nectar; ovary protected

Bats
Time: Night
Structure: Large, sturdy, wide, somewhat zygomorphic, accessible
Color: White, cream, or drab, no nectar guides
Scent: Strong at night, often smelling like fermenting yeast, abundant nectar, food bodies, and/or pollen

Mammals (other than bats) Mice, Mouse-Lemurs, Black & White Ruffed Lemur [see Judd et al.] & Small Marsupials (S. Africa, Madagascar, Australia)
Time: Night
Structure: Sturdy, often "snout" shaped corolla, many stamens or dense heads of flowers, accessible, near the ground
Color: White, cream, dull red, or drab, no nectar guides
Scent: Yeasty odor, abundant nectar and/or pollen

Wind
Time: Day
Structure: Small, imperfect, produced before leaves unfold or outside crown of plant, inflorescences often hanging
Color: Little color
Scent: none

Water
As seen in the video "Sexual Encounters of the Floral Kind", some flowers use water as the vector for pollination.