Sphagnum Moss Vortex Rings: Cap or No Cap?

On warm summer days the capsules Sphagnum moss dehydrate and shrink, causing the gas contained within to increase in pressure. The strains within the collapsing capsule and the pressure of the gas within cause force the lid of the capsule to break free releasing the gas and spores within. The escaping gas forms a vortex ring, which carries the dust-like spores to a height of over 10 cm where they can be dispersed by wind. This dispersal mechanism, which is not observed in any other bryophytes, might explain how Sphagnum has become so prolific in its short 20 Ma history.



Here we present a finite element analysis of the explosions of Sphagnum capsules. High speed videos of explosions show that the capsule dehiscence and cap trajectory vary widely across explosions, yet we see high consistency in the trajectory of the vortex rings. Consequently, we present two computationally manageable 2D axisymmetric models to assess the vortex rings from peat moss. Models both with and without a lid create vortex rings with trajectories that are compared with video data from specimens collected in the field. The results from these two models are used to determine the initial pressure of the capsules before explosion and to assess the optimality of the vortex rings created by peat moss.