Pollination

Pollination is the process by which mature pollen grains are transferred from the anthers to the mature stigmas of flowers. There are two main types of pollination:

1. Self-pollination
2. Cross-pollination

Cross-Pollination

Cross-pollination occurs when pollen grains from one flower are transferred to the stigma of another flower on a different plant of the same species. This method is often risky and inefficient, as many pollen grains fail to reach the target stigma. However, cross-pollination promotes genetic diversity among offspring, which tends to make them healthier and better adapted to their environments.

Characteristics of Cross-Pollination

1. Male and female reproductive parts are located in separate flowers.
2. In dioecious plants, male and female flowers occur on different plants of the same species.
3. In monoecious plants, where male and female flowers are on the same plant, self-pollination is prevented through mechanisms such as:
   • Stamens maturing before the stigmas (protandry).
   • Stigmas maturing before the stamens (protogyny).
4. Some flowers exhibit self-sterility, where pollen from the same plant cannot fertilize its own ovary due to slow or no pollen growth on the stigma. Examples include leguminous plants and Ixora.
5. In some upright flowers, the stamens are positioned below the stigmas to ensure insects touch the stigmas before the stamens, preventing self-pollination.

Self-Pollination

Self-pollination occurs when pollen from the anther is transferred to the stigma of the same flower or another flower on the same plant. This process is common in short-lived annual plants and has a high success rate. However, it results in offspring with little genetic variation.

Characteristics of Self-Pollination

1. In composite plants like sunflowers, cross-pollination is initially attempted (protandry). If it fails, self-pollination occurs when the stigmas curl back to collect the remaining pollen.
2. Some flowers do not open, ensuring self-pollination (e.g., cleistogamous flowers).
3. Certain flower structures, like those of caladium (ornamental cocoyam), trap insects to facilitate self-pollination.

Insect and Wind Pollination

Flowers are typically pollinated by insects or wind. The structural adaptations of flowers are influenced by the type of pollination they use.

Agents of Pollination

• Wind (Anemophily): Pollen grains are lightweight and adapted for dispersal by air currents. Wind-pollinated plants produce large quantities of small, light pollen grains. Examples include grasses, pine, oak, wheat, and corn.
• Water (Hydrophily): In aquatic plants, pollen grains are released into water and carried to stigmas. This method is rare and typically seen in submerged or floating plants.
• Insects (Entomophily): Bright colors, patterns, and scents attract insects like bees, butterflies, and beetles. These insects transfer pollen as they feed on nectar.
• Other Animals: Birds like hummingbirds and sunbirds, as well as some mammals, also aid in pollination. They are often attracted by specific flower traits such as bright colors or unique structures.

Insect-Pollinated Flowers

1. Have brightly colored parts to attract insects.
2. Produce rough pollen grains that stick easily to insects.
3. Emit a strong fragrance to lure pollinators.
4. Feature broad or sticky stigmas to ensure efficient pollen transfer.
5. Produce nectar, a sugary liquid that serves as food for pollinators.
6. Contain stamens located inside the flower.

Wind-Pollinated Flowers

1. Are often unisexual.
2. Produce abundant pollen grains.
3. Have light, smooth pollen grains that easily float in the air.
4. Feature large, branched, and leathery stigmas to capture airborne pollen.
5. Lack nectar and scent.