A sepal is one of the outer parts of a flower that typically form a ring beneath the petals and reproductive organs. Sepals are usually green and leaf-like, and together they make up the calyx. They protect the developing flower bud and, in many plants, remain attached to the fruit. For a general discussion of flowers see flower, and for the more conspicuous inner organs see petals.
Structure and variation
Sepals vary widely in shape, size and color. In many species they are small and green, resembling leaves. In some plants the sepals are large and brightly colored, taking on the role of attracting pollinators when petals are reduced or absent. When sepals and petals are indistinguishable in appearance they are referred to collectively as tepals. Sepals may be free from one another (polysepalous) or fused into a tube or cup (gamosepalous).
Development and botanical origin
Botanically, sepals are considered modified leaves that develop at the base of a flower. Their development is controlled by the plant's floral organ identity program; in model species these genetic pathways determine whether a developing floral primordium becomes a sepal, petal, stamen or carpel. Evolutionarily, changes in sepal form and number have accompanied diversification of flower shapes and pollination strategies across plant lineages.
Functions and ecological roles
- Protection: Sepals enclose and shield the enclosed floral parts while the bud is forming.
- Support: After opening, a persistent calyx may support or enclose the developing fruit.
- Photosynthesis: Green sepals can carry out photosynthesis and contribute to the plant's energy budget.
- Attraction and deception: In some species sepals become colorful or conspicuous and help attract pollinators; in others they mimic petals or act as visual cues.
Terminology and distinctions
The collective term for all sepals is the calyx; together with the petals (the corolla) they form the perianth. Distinguishing sepals from other leaf-like structures is important: bracts are modified leaves associated with an inflorescence or single flower but lie outside the calyx, while tepals describe undifferentiated tepaloid perianth segments. Botanists also use terms such as persistent (remaining attached) or deciduous (falling away at anthesis) to describe how sepals behave during and after flowering.
Examples and notable cases
Common examples illustrate sepal diversity: roses have small green sepals beneath the showy petals; tulips and lilies typically display tepals that look petal-like; and plants such as Physalis (the Chinese lantern) develop an enlarged, papery calyx that encloses the fruit. Monocot flowers often have parts in threes or multiples of three, while many dicots show fours or fives in sepal and petal counts, reflecting broader patterns in floral organization.
Understanding sepals helps in identifying species, interpreting flower development, and appreciating how small modifications in these outer organs can influence pollination, fruit protection and the overall reproductive success of plants.