Mechanical flexibility of fertile frond stipes in the rheophytic fern Osmunda lancea.

Riparian environments are unique habitats highly susceptible to sudden flooding following heavy rainfall. In such conditions, the sterile in dimorphic fronds that remain aboveground for several months have adapted to thrive by developing linear and lanceolate laminae supported by robust yet flexible petioles. However, it remains unclear whether fertile fronds, which develop only for a short period of several days to weeks, exhibit similar adaptations. To address this question, we conducted morphological, anatomical, and mechanical analyses of the stipes of Osmunda lancea (Osmundaceae), which inhabits riversides, and its inland sister species O. japonica. Our results revealed that the stipes of O. lancea were more flexible than those of O. japonica, accompanied by a reduction in cell length within the sterome. Furthermore, the sterome of the stipe was found to be composed of thin-walled cells, enabling the formation of the stipe at a low carbon cost within a short period and facilitating spore dispersal.