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|Vol. 6(12), pp. 3||The McAllen International Orchid Society Journal||December 2005|
The basic orchid perianth is made up of six perianth parts in two whorls; an outer whorl of three sepaloid parts known as the sepals, and an inner whorl of three petaloid parts including the petals and lip. Because an orchid has a bilaterally symmetrical flower, in each of these whorls two of the segments are matching mirror-images of one another and one is unmatched and symmetrical around its own longitudinal axis. In almost all fields of the orchid fancy, the term "dorsal sepal" is widely used to describe the unmatched sepal of the outer whorl. It is an apparently sensible term, primarily because in the majority of orchids, certainly those we cultivate most often, this particular floral part is presented in a dorsal (= of the back) position. The reason this is so, is because--in most orchids--there is rotation through 180° of the flower from the position of its formation to the position of its final display. This is known as resupination.
From this, it can be readily realised that this sepal's characteristic of being "dorsal" is but a secondary phenomenon; in early bud it is "ventral". Additionally, in some genera, such as Polystachya and Satyrium, no resupination takes place, and in other genera, such as Plectrelminthus and some species of Angraecum, the twisting takes place through 360° (what I call "hyper-resupination"), in both these latter cases this means that the "dorsal sepal" is offered in a ventral (= of the belly) position. So ultimately even if the lateral (= of the side) sepals are always presented laterally, the "dorsal" sepal starts life in a ventral position, and later either remains ventral, becomes dorsal, or passes from ventral to dorsal and back again. "Dorsal" sepal must therefore be seen as a misnomer!!
There is a way out of this semantic quagmire. As the disputed sepal is unmatched and always lies in the middle, be that dorsally or ventrally, the term median sepal can be used. As an alternative, from a developmental point of view, the half of the flower made up of the lip and lateral sepals starts life facing inwards, that is towards the axis, and this half may be called the adaxial (= toward the axis) part of the flower. The other half therefore becomes the abaxial (= away from the axis) part. Based upon this, abaxial sepal becomes an appropriate term. However, exactly how I am going to get 28 years of using "dorsal sepal" out of my system and replace it with "median sepal" or "abaxial sepal" should make for an interesting exercise.