Single flowers consist of two whorls (layers) of perianth segments: the sepals and the petals. A double flower has more than one petal whorl (hose-in-hose double), or a stamen whorl that contains petaloid (petal-like) stamens (peony type double). In the peony type, carpels may also be petaloid. Individual flowers may be a combination of both double types.
Outgrowths of extra tissue from the midrib or elsewhere on a petal are not considered to constitute a double. Extra segments in the normal two perianth whorls classify a flower as polymerous rather than double.
See also: floral whorl, hose-in-hose double, peony type double, petaloid, stamens, carpel, polymerous.
AMETHYST ART (Kropf 1988) hose-in-hose, photo by Terrie Mann. Used with permission.
WEE WILLIE WONKA (Stamile-G.2007) petaloid stamen double. Photo by Patrick Stamile. Used with permission.
PURSUIT OF PLEASURE (Kirchhoff '08) combination of hose-in-hose and petaloid stamen types, Photo by David Kirchhoff. Used with permission.