Glass-like flowers in the rain

In June 2022 during the Tsuyu, a month-long season of persistent light rain unique to East Asia, I observed Diphylleia grayi flowers taking on a fantastic glass-like appearance in Nagano, Japan. Endemic to Japan and Sakhalin but distributed mostly in central Japan, the species is known informally as the “skeleton flower” outside of Japan. In dry weather, the petals of the skeleton flower appear white because light is diffusely reflected by numerous air-filled gaps in their cellular structure. When these gaps become filled with rain, however, the petals become transparent – a phenomenon that has attracted the attention of materials scientists (ACS Appl Mater Interfaces 2018; doi.org/10.1021/acsami.8b12490). Notably, the petals do not become transparent immediately after rain begins to fall; rather, light rain must fall continuously for about one day. Also, even after the weather clears, the petals remain temporarily transparent, until they dry.

Three Diphylleia species are known. In addition to D grayi, the familiar skeleton flower, Diphylleia sinensis occurs in central China, and Diphylleia cymosa is found in the southern Appalachian Mountains of the southeastern US (J Arnold Arbor 1984; doi.org/10.5962/p.36691). However, it is not known whether the flower petals of these two species also become transparent during rainy weather. If transparent petals are unique to D grayi, they may be an adaptation to the Tsuyu. What ecological function might the transparent petals have? Do flowers with transparent petals provide signals for pollinating insects? Is it possible to discern whether insect pollinators are more or less likely to visit flowers with transparent petals versus those with white petals, despite the potentially confounding presence of rain?