Floral structure and development of Apostasia and Neuwiedia (Apostasioideae) and their relationships to other Orchidaceae

Floral development, structure, and pollination biology of species of Apostasia and Neuwiedia constituting the basalmost orchid subfamily Apostasioideae were studied. The perianth organs arise from a ring primordium. The two adaxial sepals appear first, followed by the petal primordia. This contrasts with monandrous orchids, in which the median petal primordium is the first organ of the inner perianth whorl to develop. Stamens are only formed on the abaxial side of the flower. However, in Neuwiedia veratrifolia and Apostasia nuda, vestiges of stamen primordia are present on the adaxial side of the flower. In Apostasia only, the lateral stamens are fully developed, whereas the median organ is a staminode; in A. nuda the staminode is suppressed. At anthesis the anthers are free in Neuwiedia, but they are postgenitally connected in Apostasia. In both genera, stamens and style are congenitally fused at the base. The inferior ovary is at first unilocular and then becomes trilocular by postgenital fusion of the ovary septa and the development of a short synascidiate zone. The three carpels of both genera are equal from the beginning, but in A. nuda only, the median carpel dominates during early development. The stigma is convex and papillate. Conspicuous raphide idioblast accumulations occur in staminode and style and in the tips of anthers and filaments. In contrast to all other orchids, both genera offer pollen as a reward. Neuwiedia has lip flowers, which are pollinated by Trigona bees; Apostasia has Solanum-type flowers with recurved perianth and united anthers. Our study supports the view that Apostasia and Neuwiedia are closely related and that Apostasioideae are at the base of the orchid clade, sharing characters with orchids and lower Asparagales.