Pollinium development and number in the Orchidaceae

Abstract

Development of anthers in three subfamilies of Orchidaceae was studied anatomically to examine homology hypotheses for pollinium number characters 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 expansion of thecae and their inward (adaxial) reorientation (“rotation”), achieved by differential cell division and elongation in the connective, 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 the basal state for the family. All anthers examined (from Orchidoideae, Spiranthoideae, and Epidendroideae) showed a single meristematic region, which would later give rise to pollen, per theca at earliest stages; septation of each of these regions resulted in four or eight pollinia per anther, while lack of septation in some members of the Epidendroideae gave two pollinia. In contrast, the two bipartite pollinia found in many Spiranthoideae and Orchidoideae were produced by adherence of the contents of two locules at a late ontogenetic stage, and should be recognized as a distinct character state. Eight pollinia result from partitioning by two longitudinal septa or a longitudinal and a transverse septum; these two morphologies may also represent separate character states.