How Does the Structure of the Stigma Aid in Pollination

Pollination is the process of moving pollen grains from the anther of one flower to the stigma of the same or different flower during sexual reproduction of plants. It initiates the fertilization process by allowing the pollen grains to germinate on the stigma. The anther belongs to the male parts of the flower that produces pollen grains. Stigma belongs to the female parts of the flower and receives pollen grains during pollination. Both anther and stigma are composed of adaptations for the betterment of their function in flowers. Therefore, the stigma itself has adaptations to facilitate pollination. These adaptations are described in this article.

Key Areas Covered

1. What are the Reproductive Organs in a Flower
     – Definition, Stamen, Pistil
2. What are the Types of Pollination
     – Self-Pollination, Cross Pollination
3. What are Pollination Methods
      – Pollination Methods
4. How Does the Structure of the Stigma Aid in Pollination
     – Adaptations of the Stigma

Key Terms: Anther, Cross Pollination, Pistil, Pollination, Self Pollination, Sculptures, Stamen, Stigma

What are the Reproductive Organs in a Flower

The reproductive organs of the flower are organs that aid in the sexual reproduction of angiosperms by producing gametes. Stamen and pistil are the male and female reproductive organs of the flower respectively.

Stamen

Anther and filament are the male parts of the flower and are collectively known as the stamen. The main function of the stamen is producing pollen grains in the anther and facilitating the dispersal of pollen grains with the help of the filaments. The structures of a typical flower are shown in figure 1.

Figure 1: Structure of a Flower

Pistil

Stigma, style, and ovary are the female parts of the flower, collectively known as the pistil. Stigma is the structure to which the pollen grains are stuck during pollination. It is held by the style.  The ovary is present at the base of the style. The eggs are produced inside the ovary. The seeds are also produced inside the ovary after the fertilization of the eggs.

What are the Types of Pollination

Two types of pollination can be identified among plants. They are self-pollination and cross-pollination.

Self-pollination – Pollen grains are transferred from an anther of a flower either into the stigma of the same flower or different flower in the same plant.

Cross-pollination – Pollen grains are transferred from the anther of a flower into a different flower of a different plant in the same species

What are Pollination Methods

Different plant species use different pollination methods. Some of them are listed below.

1. Anemophily – Anemophily is the pollination by wind. Grasses, many conifers, and some dicots use anemophily.
2. Hydrophily – Hydrophily is the pollination by water. Most of the water plants use hydrophily.
3. Pollination by insects – Insects such as bees and butterflies are involved in the pollination.
4. Pollination by bats – In the deserts, most plants are pollinated by bats.
5. Pollination by birds
6. Pollination by deception – Both food deception and sexual deception are used by many orchids to attract insects for pollination.
7. Pollination by animals

How Does the Structure of the Stigma Aid in Pollination

Pollen grains are microscopic structures that are sticky. They have to be deposited on the stigma to initiate the fertilization process by germinating on the stigma. Therefore, for successful fertilization, stigma should have special characteristics to gather pollen grains. These adaptations are listed below.

1. Elevated stigma – The elevated stigma facilitates the reach of animals/insects to the stigma. Elevated stigma can be mainly found in plants that use cross-pollination. The plants that use self-pollination have short styles to facilitate self-pollination.
2. Sticky surfaces – The sticky surface on the stigma allows the pollen grains to stick onto the stigma easily. 

Figure 2: Sticky Stigma

3. Three-dimensional sculptures and hair – The sculptures and hair trap pollen grains on the surface of the stigma.
4. Watery surfaces – The watery nature of the stigma facilitate the germination of the pollen grain.

Other than these basic features, stigma shows special characteristics based on the method of pollination as well.

• The flowers that are pollinated by insects have their stigma inside the flower, and these stigmas are sticky.
• The flowers that are pollinated by the wind have their stigma outside the flower. These stigmas have feathery structures to capture and trap the drifting pollen grains.

Conclusion

Pollination is the deposition of a pollen grain on the stigma. It initiates the fertilization process, allowing the germination of the pollen grain on the stigma. Hence, the structure of the stigma is important in the capturing pollen grains. Adaptations such as sticky and watery surface, elevated stigma, and three-dimensional sculptures are the special characteristics of the stigma that help to trap pollen grains.

Image Courtesy:

1. “Flower Anatomy” by ProFlowers (CC BY 2.0) via Flickr
2. “Black Yellow and Red Flower Stigma in Tilt Shift Lens” (CC0) via PEXELS