The calyx is the outermost whorl of a flower composed of sepals. The sepals are usually green in colour, but sometimes, become brightly coloured then, said to be petaloid as in Caesalpinia pulcherrima. in Musseanda frondosa the sepals are transformed into large, yellow or white and leafy structure.
The primary function of the calyx is protective. It protects the inner parts of the flower from mechanical injury, rain and excessive sun shine, and from drying out in the bud condition. Green in colour, it can also do the phosynthetic function. When petaloid, it performs the function of attracting insects for pollination. When spiny, its function is defensive and as pappus, it helps in the dispersal of fruit.
The calyx may be regular or irregular. The sepals are free from one another and is said to be ploysepalous, when united united,gamosepalous.
The calyx may sometimes be absent or modified into scaly structure as in Sunflower.
In some cases, it is modified into a bunch of hair - like structures called pappus eg. Vernonia.
After the opening of the flower, the calyx usually falls off but it may persist in some cases.
According to its duration, it may be described as follows:
1. Caducous or Fugacious:Sometimes the calyx falls off, even before flowers are opened and such a calyx is said to be caducous.eg.Papaver, magnolia etc.
2. Deciduous:When it falls off after the opening of the flower, it is said to be deciduous. (eg) Nelumbo
3. Persistent: In somw other cases, when the calyx persists (unwithered) even after fruit formation, it is said to be persistent. eg. Brinjal,
4. Accresent: Calyx not only persistent but also grows along with development of the fruit. eg. Physalis.