Life Science: Session 4
Flowers and Pollination
What is pollination?
One of the most crucial events in ensuring the completion of a flowering plant’s life cycle is pollination. With very few exceptions, this involves cross-pollination, where two different parent plants are involved. Successful cross-pollination results in pollen being delivered from the male part, or stamen, of one flower to the female part, or pistil, of another (see picture below). After this occurs, sperm are produced within the pollen grain and a pollen tube carrying the sperm grows toward the ovary at the base of the flower. Fertilization, which is another crucial event, occurs when the pollen tube grows into the ovary and sperm are united with egg.
What can flowers reveal about their pollinators?
The means by which pollination occurs varies from plant to plant, with most plants having specific pollinators. Pollinators include wind, water (rarely), and animals like bees, beetles, flies, hummingbirds, and even bats. Flowers have evolved to use their pollinators in ways that maximize efficiency and effectiveness.
The position of the stamens, for example, tends to maximize the delivery of pollen to the right pollinator. For example, stamens extending far away from the flower can be an indication that wind is the pollen vector. Stamens that lay close to the flower can indicate a pollen vector that must land and walk inside — like a fly. The same is true of the pistils. Most flowers that are pollinated by insects (including the one pictured above) have pistils that extend beyond the stamens. This increases the chances that a pollinator delivers pollen from another flower on the way in and picks up new pollen on the way out. This also decreases the chances of pollen from one flower falling onto the pistil of the same flower.
One of the most amazing examples of adaptation is seen in the intricate relationships between flowers and their animal pollinators. Many flowers have ways of attracting pollinators from a distance. Their shapes, background colors, and fragrances bring the pollinator to the flower. Sweet fragrances, for example, attract nectar feeders, like butterflies, flies, and bees. Non-red flowers attract bees, which can’t see red. Once at a flower, intricate color patterns — such as “nectar guides” that point the way to nectar — act to ensure that pollen is both delivered and picked up by the right pollinator. The shapes of many orchids actually mimic those of insects and fool them into “mating” with the flower. Even the placement of nectar — such as deep within a flower — is an adaptation to increase the chance of effective pollination.
Here are some other adaptations that match flowers to pollinators:
- Flowers with “landing pads” at the bottom that allow insects, like bees, to walk in
- Flowers shaped as deep, narrow tubes that are pollinated by insects with long “tongues,” like butterflies
- Flowers that smell like rotting meat, which attract flies (see picture below)
- Tubular red or yellow flowers that attract hoverers, like hummingbirds
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