whorl [ hwawrl, hwurl ] noun: a circle of plant parts; a circular arrangement of three or more similar anatomical parts such as leaves, petals or other plant parts arising from a common point or node at the same level on a stem or other axis; a verticil
While the word whorl has other meanings outside of botany, along with one of the patterns of phyllotaxis, the arrangement of leaves on the same stem as described in the definition above, it is also the term used to describe the four main groups of flower parts: sepals, petals, stamens, and pistils.
The word whorl has been in use since the 15th century and comes from Middle English wharle, probably related to whirl. Some plant parts may be described as whorled due to appearance rather than structure. For example, the leaves of Coreopsis verticillata (whorled tickseed), which according to Flora of Virginia are sometimes three-parted to the base therefore appearing whorled, have given rise to its epithet and one of its common names. The inflorescence of Monarda punctata (spotted beebalm) may be characterized as whorled while it is actually arranged in verticillasters or false whorls.