Calyx is the outer-most whorl and is made up of sepals which are usually green but sometimes coloured (petaloid) as in garden nasturtium. In dicotyledons usually the number of sepals is five and in monocotyledons usually three.
All the sepals of a whorl may be free (polysepalous condition) or they may be fused (gamosepalous condition). The sepals are superior in epigynous flowers or inferior in hypogynous flowers. On the basis of time for which sepals remain attached upon the flower they may be caducous (if they fall just at the time of opening of flower from bud as in poppy), deciduous (if they are attached on the flower until the flower weathers) e.g., Mustard or Persistant (remains attached to fruit e.g., Tomato, brinjal). Sometimes sepals are modified as follows.
1. Pappus : In some plants e.g., Sunflower the sepals are modified into hairy structure called pappus and help in dispersal of fruits.
2. Leafy : In Mussaenda, one of the sepals is modified into a large leaf like coloured structure.
3. Spinous : In Trapa the calyx is persistant and modified into two spines.
4. Spurred : In Larkspur, from the base of one of the sepals arises a tubular outgrowth called spur.
5. Hooded : In Aconitum one of the sepals is modified into a hood thus covering the whole flower.