What is pollination? Birds and Bees
What is Pollination?
Pollination is the act of transferring pollen grains from the male anther of a flower to the female stigma. The goal of every living organism, including plants, is to create offspring for the next generation. One of the ways that plants can produce offspring is by making seeds. Seeds contain the genetic information to produce a new plant.
Flowers are the tools that plants use to make their seeds. The basic parts of the flower are shown in the diagram below.
Seeds can only be produced when pollen is transferred between flowers of the same species. A species is defined a population of individuals capable of interbreeding freely with one another but because of geographic, reproductive, or other barriers, they do not interbreed with members of other species.
Parts of a flower.
This wasp is a specialist pollinator of Penstemon, which is the flower it is visiting, or rather sleeping in here. Photo by Dr. Jim Cane, USDA ARS Bee Biology and Systematics Laboratory, Logan, Utah.
How does pollen get from one flower get from one flower to another? Flowers must rely on vectors to move pollen. These vectors can include wind, water, birds, insects, butterflies, bats, and other animals that visit flowers. We call animals or insects that transfer pollen from plant to plant “pollinators”.
Pollination is usually the unintended consequence of an animal’s activity on a flower. The pollinator is often eating or collecting pollen for its protein and other nutritional characteristics or it is sipping nectar from the flower when pollen grains attach themselves to the animal’s body. When the animal visits another flower for the same reason, pollen can fall off onto the flower’s stigma and may result in successful reproduction of the flower.
Referring to the animated image, pollen from the anthers of Flower 1 is deposited on the stigma of Flower 2. 0nce on the stigma, pollen may “germinate,” which means that a “pollen tube” forms on the sticky surface of the stigma and grows down into the ovule of the plant.
This growth can result in:
- Successful fertilization of the flower and the growth of seeds and fruit; or,
- A plant can be only partially fertilized, in which the fruit and/or seeds do not fully develop; or,
- The plant can completely fail to be pollinated, and may not reproduce at all.
Plants can be:
- Self-pollinating - the plant can fertilize itself; or,
- Cross-pollinating - the plant needs a vector (a pollinator or the wind) to get the pollen to another flower of the same species.
Read about “The Birds and the Bees”…
The Birds and the Bees
Seed production for angiosperms begins with the flowering parts. Flowers must be pollinated in order to produce seeds and fruit.
Self-Pollination
Self-pollinating species can reproduce even if animal pollinators are not present. However, reproduction through self-pollination reduces genetic diversity.
The anther opens and the pollen lands on the stigma of the same flower.
To promote cross-pollination and increase genetic diversity, plants have evolved a wide variety of sexual strategies to attract pollinators and spread pollen from one flower to another of the same species.
Self-pollinationCross-Pollination
Anthers open on one flower and a vector (insects, wind, or animals) moves pollen to the stigma of another flower. Pollinators may visit several flowers on one plant or may visit several flowers of the same species on a few different plants
Some plants have evolved to have self-incompatibility mechanisms to avoid self-pollination. A physiological barrier makes it difficult or impossible for a flower to fertilize itself even though it may have been abundantly pollinated with its own pollen. To learn more about plant breeding mechanisms visit Self-Incompatibility: How Plants Avoid Inbreeding on the Kimball's Biology Pages website.
Cross-pollinationWhy do pollinators visit flowers?
Insect and other animal pollinators obtain food in the form of energy-rich nectar and/or protein-rich pollen, from the flowers they visit and in return, the flowers receive the services of pollinators carrying pollen from one flower to another.
While food is often a sufficient lure for pollinators, flowering plants also attract pollinators using a combination of petal shapes, scents, and colors. “Pollination syndromes” have been described to depict the attraction of certain types, shapes, colors, and fragrances of flowers to a range of pollinators.
Specialized Pollination
There are many examples of specialized plant and pollinator relationships in which flowers and insects have adapted to one another to accomplish mutually beneficial goals. These are a solid example of co-evolution. To learn more about these important interrelationships visit the CoEvolution Institute website.
Some orchids have evolved ingenious ways to lure pollinators by developing flowers that appear to be female insects. Using sight and/or scent, these flowers resemble female insects so convincingly that males of the same species will attempt to copulate with the flower, inadvertently picking up pollen before visiting another female-mimicking flower.
Notice all of the pollen carried by this bee. Digger bee (Andrenid). Photo by Beatriz Moisset.
Red spotted purple, Limenitis arthemis,on butterflybush. Photo by Beatriz Moisset.
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