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

Floral development, structure, and pollination biology of species of Aposta sia and Neuwiedia constituting the basalmost orchid subfamily Apostasioidea e were studied. The perianth organs arise from a ring primordium. The two a daxial 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 t he abaxial side of the flower. However, in Neuwiedia veratrifolia and Apost asia nuda, vestiges of stamen primordia are present on the adaxial side of the flower. In Apostasia only, the lateral stamens are fully developed, whe reas the median organ is a staminode; in A. nuda the staminode is suppresse d. At anthesis the anthers are free in Neuwiedia, but they are postgenitall y connected in Apostasia. In both genera, stamens and style are congenitall y fused at the base. The inferior ovary is at first unilocular and then bec omes trilocular by postgenital fusion of the ovary septa and the developmen t 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 an thers and filaments. In contrast to all other orchids, both genera offer po llen as a reward. Neuwiedia has lip flowers, which are pollinated by Trigon a bees; Apostasia has "Solanum-type" flowers with recurved perianth and uni ted anthers. Our study supports the view that Apostasia and Neuwiedia are c losely related and that Apostasioideae are at the base of the orchid clade, sharing characters with orchids and lower Asparagales.