Stipule

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Rosa canina blatt 2005.05.26 11.50.13

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Stipules are an often overlooked, yet integral part of the anatomy of many plant species. These small, leaf-like appendages are found at the base of the leaf stalk (petiole) or at the point where the leaf attaches to the stem. Stipules can vary greatly in shape, size, and function, depending on the plant species. They play a crucial role in the protection of the developing leaf or bud and can also have other specialized functions.

Definition

A stipule is defined as a small, leafy outgrowth found at the base of the petiole in many dicotyledons. Stipules are usually paired but can be found singly or in multiples. In some cases, they may be fused to the petiole or to each other, forming a structure known as an interpetiolar stipule or ochreate stipule, respectively.

Function

The primary function of stipules is to protect the developing leaf or bud from herbivores and environmental damage. In some plants, stipules fall off once the leaf or bud has fully developed; in others, they remain and may perform photosynthesis, similar to leaves. Some stipules have evolved into thorns or spines, serving as a defense mechanism against herbivory.

Types of Stipules

Stipules can be classified based on their persistence, position, and whether they are free or adnate (fused to the petiole or stem): - Caducous stipules: These stipules fall off early in the leaf's development. - Persistent stipules: These stipules remain attached to the plant for a significant period. - Free stipules: Stipules that are not attached to the petiole or stem. - Adnate stipules: Stipules that are fused to the petiole or stem. - Interpetiolar stipules: Stipules that are found between the petioles of opposite leaves. - Intrapetiolar stipules: Stipules that are found within the angle between the petiole and the stem.

Morphology

The morphology of stipules can be highly variable among plant species. They can be leaf-like, scaled, spiny, or even modified into glands. This diversity reflects the wide range of functions that stipules can perform, from protection to photosynthesis and even water storage in some xerophytic plants.

Ecological and Evolutionary Significance

Stipules play a significant role in the ecology and evolution of plants. Their protective function can influence the survival and fitness of young leaves and buds, impacting the overall success of the plant. The variation in stipule form and function across different environments and plant lineages also provides insights into the evolutionary pressures and adaptations that have shaped plant diversity.

Examples

Some well-known plants with stipules include: - Roses (Rosa spp.), which have leafy stipules that are often serrated. - Peas (Pisum sativum), with large, conspicuous stipules that are sometimes mistaken for leaves. - Figs (Ficus spp.), which have small, scale-like stipules that cover the developing bud.

Conclusion

Stipules are a fascinating and diverse component of plant anatomy, with a wide range of forms and functions. Their study can provide valuable insights into plant biology, ecology, and evolution.