Post‐anthesis changes in the labellum of Cypripedium macranthos var. rebunense (Orchidaceae) and a speculation on functional design of the labellum

In bumblebee‐pollinated Cypripedium macranthos var. rebunense, some plants have small labella that do not allow pollinating queens to enter, but they occasionally produce capsules for reasons that remain unclear. We found that after anthesis, the labellum gradually droops, and its width and length (but not height) distinctly increase, eventually producing an elongate and dorsoventrally compressed shape. The rate of increase in labellum size was significantly greater in flowers that initially had a smaller labellum, so that the post‐anthesis changes actually enabled large‐bodied bumblebees to enter those labella. Therefore, we conclude that C. macranthos var. rebunense relied on only bumblebee queens for pollination, irrespective of labellum size at the time of anthesis. Observations of in‐labellum bumblebee behavior suggested that the elongate and dorsoventrally compressed shape and the inclination seemed to result in more predictable and desirable bumblebee movements into the labellum.