Sepals are specialized leaf-like structures that form the outermost whorl of a flower. These structures are typically green and are located at the base of the flower, directly beneath the petals. Collectively, all the sepals of a flower are known as the calyx. Their arrangement provides a foundational layer for the developing floral parts above them.
Protecting the Bud
The primary function of sepals is to encase and protect the delicate floral bud before it opens. As the flower develops, the sepals form a protective casing, shielding the internal reproductive organs and petals from potential harm. This layer guards against physical damage.
Sepals also prevent desiccation, or drying out, of the tender tissues within the bud. Their outer covering helps to retain moisture, which is particularly important for the sensitive developing petals and reproductive structures. Furthermore, the sepals offer a defense against fluctuating temperatures, insulating the bud.
The tough texture and overlapping arrangement of sepals provide a barrier against herbivory, protecting the nascent flower. For instance, the sepals of a rose bud are tightly closed and somewhat leathery, forming a robust shield around the unfurling petals. This comprehensive protection ensures the flower can mature undisturbed until it is ready to bloom.
Supporting the Bloom
Once the flower begins to open, the sepals transition to providing structural support for the expanding bloom. They help to hold the petals in place, contributing to the flower’s overall shape and stability. In many species, the sepals reflex downwards or remain appressed against the stem, forming a stable base for the showier petals and reproductive structures.
For green sepals, an additional function can be photosynthesis. These green structures contain chlorophyll and can capture sunlight, much like leaves. In some plant species, sepals are modified to be colorful or petal-like, such as in bougainvillea, where the showy “flowers” are actually modified bracts and sepals that serve to attract pollinators. This adaptation highlights the diverse evolutionary paths flowers have taken.
Sepals After Flowering
The fate of sepals after flowering varies significantly among plant species. In some cases, sepals are “caducous,” meaning they fall off shortly after the flower opens or after fertilization has occurred. For example, the sepals of poppy flowers (Papaver species) typically detach and drop away as soon as the petals unfurl.
Conversely, many plant species have “persistent” sepals, which remain attached to the developing fruit. These sepals can often be seen at the end of fruits like apples, where they form the small, dry calyx at the bottom. The persistent sepals on a strawberry (Fragaria × ananassa) remain green and cup-like beneath the developing fruit, continuing to enclose and support it. This persistence can offer ongoing protection to the young fruit or even aid in its dispersal.