POLLINIUM DEVELOPMENT AND NUMBER IN THE ORCHIDACEAE

Development of anthers in three subfamilies of Orchidaceae was studied anatomically to examine homology hypotheses for pollinium number char acters and to produce a model of pollinium development for the family. Serial sections of plastic-embedded embryonic inflorescences revealed that anther primordia were either flattened or ovoid; subsequent expa nsion of thecae and their inward (adaxial) reorientation (''rotation'' ), achieved by differential cell division and elongation in the connec tive, result in a mature anther with strongly introrse morphology and pollinia oriented side by side (juxtaposed). Strongly introrse anthers occur in at least some members of all subfamilies and are probably th e basal state for the family. All anthers examined (from Orchidoideae, Spiranthoideae, and Epidendroideae) showed a single meristematic regi on, which would later give rise to pollen, per theca at earliest stage s; septation of each of these regions resulted in four or eight pollin ia per anther, while lack of septation in some members of the Epidendr oideae gave two pollinia. In contrast, the two bipartite pollinia foun d in many Spiranthoideae and Orchidoideae were produced by adherence o f the contents of two locules at a late ontogenetic stage, and should be recognized as a distinct character state. Eight pollinia result fro m partitioning by two longitudinal septa or a longitudinal and a trans verse septum; these two morphologies may also represent separate chara cter states.