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
Left to right: Trillium grandiflorum, Veronicastrum virginicum, Lilium canadense, Hemerocallis ‘Paper Butterfly,’ Stellaria pubera, Lilium superbum.
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: petals (corolla), sepals (calyx), stamens (androecium), and pistils (gynoecium). The corolla and calyx form the non-reproductive part of the flower called a perianth. The corolla is the inner whorl of the separate or fused petals of the perianth. The calyx is the sometimes green outer whorl of the perianth consisting of separate or fused sepals. When there is no differentiation between the petals and the sepals in a perianth (as in the flowers of Lilium superbum and Uvularia grandiflora), then each segment of the perianth is called a tepal.
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.
Left to right: Coreopsis verticillata (leaves and specimens ), Monarda punctata (flowers and specimens).
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