Humidity changes in dead fern fronds drives unique timing of spore dispersal in a widespread fern species

The sensitive fern—named due to its sensitivity to drought and frost—is a widespread species found throughout eastern North America and eastern Asia. It is a dimorphic plant because it has two distinct types of leaves—one for photosynthesis and one for reproduction. 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. In the early summer they produce heavily modified spore-bearing fronds (or leaves) with leaflets tightly enveloping their sporangia and spores. These fronds deteriorate with age, yet continue to persist above ground during the winter as dead mature structures. In the early spring their leaflets open to disperse spores. While the timing of spore dispersal has been observed for over 120 years, the structural mechanisms driving this phenology have remained elusive.

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