Stigma (botany)

Stigma PSFen

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Closeup of Stamen and stigma of Lilium 'Stargazer' (the 'Stargazer lily')

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Stigma (botany) is a crucial part of a flower's reproductive system, playing a vital role in pollination. It is the terminal portion of the pistil, the female reproductive organ of flowering plants. The stigma is designed to receive pollen, which contains the male gametes necessary for fertilization to occur. This article delves into the structure, function, and significance of the stigma in the plant kingdom.

Structure

The stigma is typically located at the top of the pistil, which also includes the style and the ovary. Stigmas can vary widely in shape and size among different species, reflecting the diversity of pollination mechanisms in the plant world. Some are sticky or feathery, adaptations that enhance their ability to capture pollen grains. The surface of the stigma is often covered in a sticky substance that helps to trap pollen, facilitating its germination and the growth of the pollen tube down the style towards the ovary.

Function

The primary function of the stigma is to receive pollen grains during pollination. Once a pollen grain lands on the stigma, it needs to be compatible for successful fertilization to occur. If the pollen is from the same species and is recognized as compatible, the stigma facilitates the germination of the pollen grain. This process involves the growth of a pollen tube through the style towards the ovary, where fertilization of the ovule takes place. The successful union of the male and female gametes results in the formation of seeds, which can grow into new plants.

Significance

The design and efficiency of the stigma are critical for the reproductive success of flowering plants. It not only ensures that pollen is effectively captured but also plays a role in selective breeding. By discriminating against pollen from different species or genetically incompatible individuals, the stigma helps maintain the genetic integrity of the plant species. Furthermore, the diversity in stigma forms and functions reflects the wide range of pollination strategies in the plant kingdom, from wind and water pollination to the involvement of insects, birds, and other animals as pollinators.

Pollination Strategies

The structure of the stigma is closely related to the plant's pollination strategy. For example, plants that rely on wind pollination often have feathery stigmas that increase the surface area for catching pollen grains that are carried by the air. In contrast, plants that are pollinated by animals typically have sticky stigmas that can effectively trap pollen transferred by their pollinators.

Evolutionary Aspects

The evolution of the stigma has been a key factor in the diversification of flowering plants. The development of specialized stigmas that efficiently interact with specific pollinators has allowed flowering plants to exploit various ecological niches and has led to the co-evolution of plants and their pollinators. This mutualistic relationship has been fundamental in the success and widespread distribution of angiosperms (flowering plants) across the globe.

Conclusion

The stigma plays an indispensable role in the reproduction of flowering plants. Its ability to receive, recognize, and facilitate the germination of pollen is crucial for the fertilization process, ultimately leading to the production of seeds and the continuation of plant species. The diversity in stigma structures across the plant kingdom underscores the complexity and adaptability of plant reproductive systems, highlighting the intricate relationships between plants and their pollinators.