The calyx and the corolla are the accessory organs of a flower, while the androecium and the gynoecium are the reproductive organs. When the calyx is free, it is called polysepalous and when united, it is called gamosepalous. When the corolla is united, it is called gamopetalous and when free, it is called polypetalous. The positional arrangement of the sepals and petals with respect to other members of the same whorl within a flower bud is called aestivation.
In valvate aestivation, the sepals or petals within a whorl touch each other at the margin, while in twisted aestivation, one sepal or petal overlaps that of the next one. In imbricate aestivation, the margins of the sepals or petals overlap one another, but in no particular direction and in vexillary aestivation, each sepal or petal covers the sepal or petal that is within it.
The stamens are epipetalous or epiphyllous. Stamens are polyandrous in lily or united into one bundle called monoadelphous, or united into two bundles called diadelphous, or united into more than two bundles called polyadelphous. The sterile stamen is called a staminode.
The carpels can be syncarpous or apocarpous. An ovary bears one or more ovules on a soft tissue called placenta. The arrangement of ovules within an ovary is called placentation. In axial placentation, the carpels are folded inward and the ovules are attached to the central axis of the ovary. In free or central placentation, the ovules develop on the central axis. In parietal placentation, the ovules develop on the peripheral wall of the ovary. In basal placentation, a single ovule is attached to the base of the ovary. In marginal placentation, ovules are attached along the ventral suture of the ovary.