Biology

The great majority of orchid buds are positioned with the labella uppermost and the gynostemia below them. However, flowers are borne with gynostemia above the labella which are lowermost. This reversal of positions occurs as a result of a process called resupination which takes place as buds open. In most species the buds turn only to the extent necessary to place the labellum lowermost which is usually 180, but depending on the position of the inflorescence the turning can be more or less than that. Some species do not resupinate at all and their flowers are often described as being borne upside down. And, the buds of a few species turn 360 ending up as they started, with the labella uppermost.

A drawing by Marcello Malpighi (1628-1694) of an orchid he refers to as Palma Christi shows spiral grooves which are indicative of resupination on the ovary.  What may well be the first illustrations of resupination by American orchids were prepared ca. 1760 by the artists of an expedition to New Grenada led by Jose Celestino Mutis (1732-1808).

Resupination usually occurs just prior to or shortly after anthesis. Once flowers are fully open, they can no longer resupinate. However, the flowers of somespecies deresupinate following pollination. Another interesting characteristic of resupination is that in some species and hybrids the buds alternate in resupinating clock (CL)- and counter clock (CO)- wise. In other orchids the flowers may resupinate in one direction (CL or CO) only.

Darwin suggested that resupination facilitates pollination because the labellum assumes the position of a lower petal, so that insects can easily visit the flower. Perhaps, but according to the literature Euglossa cordata pollinates both resupinate and non resupinate flowers. Another possibility is that resupination positions flowers in a manner which exposes them to light in a way that emphasizes pat-terns and nectar guides. Sunlight can also raise the temperature of flowers and volatilize scents. Resupination may also provide flowers with space to open because in many orchids the buds are arranged tightly on the developing inflorescence.

Finally it is interesting to note that even if not limited to orchids, resupination is viewed by some as a trademark characteristic of the Orchidaceae. Regardless of whether it is or is not a specific characteristic of orchids, resupination does provide orchids with a survival advantage.