Protector and Supporter of Flowers - Sepals
Sepal (sepalum in modern Latin, a word of Greek origin meaning covering) in simple terms is the 'handle' by which you normally would hold a flower. The green covering underneath that connects and supports the petals is called sepals. Before flowering, protection of the bud is done by this part. In collective terminology, sepals are called calyx and petals as corollas (see diagram). The sepal is an important part of angiosperm flowers. The androecium and gynoecium are involved with reproduction germination of pollen grains, by the creation of pollen grains and later fertilization. The sepals and petals help during the process i.e. petals attract pollinators with their attractive colors and scents and help in pollination. Sepals, greener in color, are involved with providing protection during the delicate budding conditions of the flower. This is the distinction between petals and sepals. The calyx, however, isn't helpful after flowering and begins to wither.
Calyx (again, a collective term of sepals) becomes reduced and appears as flakes or ridges in few crops until the fruits become mature. This becomes the protective coating for fruits and seeds. Few species in cases are Acaena, Solanaceae and Trapanatans. In plants without a prominent calyx, a urinary bladder-like structure begins to grow, enclosing the fruit. This enclosure acts as an efficient protective cover that protects the fruits from insects and birds. Hibiscus trionum and Cape gooseberry are few examples.
On other hand, the petals are a significant structure of the flowers. They are considered into be modified leaves which surround the reproductive units: androecium and gynaeceum of the flower. The petals as a whole are referred to as corolla. Sepals are present just beneath the corolla. They are easily distinguishable since the corolla or petals are brightly colored whereas the sepals are not. Petaloids are constructions where undifferentiated tepals resemble petals i. (e.g. alstroemeria). According to Charles Darwin, the origin of your corolla is an elongated tube. Darwin has, as part of the evolution theory, explained that source of petals may be an elongated tube that was converted into corolla during the process.
The number of petals in monocotyledons and dicots differ. Partially fused petals in corolla are known as gamopetalous and fusion of tepals is referred to as synsepalous.
Renu Joshi, Ph.D.