Movement without muscles: the independent innovation of hygromorphy in a widespread fern

The sensitive fern, Onoclea sensibilis, is a widespread species in eastern North America and has an atypical timing of spore dispersal among temperate ferns. During early summer, this dimorphic species produces heavily modified spore-bearing fronds with leaflets tightly enveloping their sporangia and spores. These fronds senesce and persist above ground as dead mature structures until the following early spring when the leaflets finally open and spores are dispersed. While this timing of spore dispersal has been observed for over 120 years, the structural mechanisms underpinning this phenology have remained elusive. Jacob found that fertile leaflets of the sensitive fern move in direct response to changes in humidity, exhibiting structural and functional parallels with structures in seed plants, like pine cones. In this week’s presentation he will discuss how the sensitive fern achieves this movement, why, and how it has evolved.