Leaflets are fully closed initially in high relative humidity, and then fully open at low relative humidity. Video recording of the underside of the leaflets opening as they dry out and closing as they gain moisture. With his doctoral work on hold, Suissa determined to find out how this odd fern achieves this unique phenology. 'I started reading everything,' Suissa said, 'I read papers and books from 1960, then 1950, and all the way back to 19.' He realized that scientists had long known about the timing of spore dispersal in the sensitive fern, but he could find nothing on what mechanism drove this phenology. Wanting to learn more, Suissa grabbed a handful of fronds off of the plant and returned home. It was on one of his walks, in late winter with the ground frozen, that he noticed a green plant from the previous summer was now a dead leaf sticking up out of the snow. When COVID-19 began Suissa could not access the labs to do his work so he took daily walks through Harvard's Arnold Arboretum. candidate in The Department of Organismic and Evolutionary Biology and Fellow of the Arnold Arboretum at Harvard University, reveals that the unique timing of spore dispersal in the sensitive fern, known as Onoclea sensibilis, is determined by a structural mechanism of humidity-driven movement in spore bearing leaves. In a new study published in the Annals of Botany Jacob Suissa, Ph.D. While the timing of spore dispersal has been observed for over 120 years, the structural mechanisms driving this phenology have remained elusive.

In the early spring their leaflets open to disperse spores. These fronds deteriorate with age, yet continue to persist above ground during the winter as dead mature structures. In the early summer they produce heavily modified spore-bearing fronds (or leaves) with leaflets tightly enveloping their sporangia and spores. While most fern species in temperate regions produce and disperse their spores in the summer, the sensitive fern has an atypical timing of spore dispersal. It is a dimorphic plant because it has two distinct types of leaves-one for photosynthesis and one for reproduction. The sensitive fern-named due to its sensitivity to drought and frost-is a widespread species found throughout eastern North America and eastern Asia. The current year's developing fertile leaf will eventually senesce and look like the adjacent leaf from last year. Senesced fertile leaf from last year, alongside a new developing fertile leaf from the current year.

North spore