A close-up view of Cardmine hirsuta’s flight jacket

Being rooted, plants have very little opportunity to travel. They can as pollen to find a mate, and also as seeds. In the case of Cardamine hirsuta, that travel is explosive. The plant stores energy in the pericarp of the fruit. When fruit pod fractures, all that energy is released in one moment, flinging the seeds up to five metres away. While there has been research on the mechanism for explosive seed dispersal, there hasn’t been so much work on how the seeds are adapted. Until now.

Ulla Neumann and Angela Hay have examined the seeds of Cardamine hirsuta microscopically, to see how the seed surface interacts with the fruit and the air in flight. Normally when botanists want to examine something, they reach for a well-studied model species, but Neumman and Hay noted a difficulty with this approach, “A key problem is that traits such as explosive seed dispersal are not found in a model species like Arabidopsis thaliana where the experimental tools for functional studies exist. To address this problem, genetic tools have been developed in the related species C. hirsuta to study the origin of traits, such as explosive seed dispersal, that are not present in A. thaliana.“

The key for successful dispersal of the seeds was in the development of the cell wall. “Form and function of the C. hirsuta seed coat depend on localized deposition of specialized cell walls prior to death of the seed coat cells. Differentiation of the two outermost seed coat layers involves polar deposition of distinct types of pectin produced by the Golgi apparatus.” Neumann and Hay write.

The end result is a ridged coat for the seed. Neumann and Hay say more work is needed on how these ridges influence flight. “[I]t is interesting to speculate how a ridged seed surface could influence explosive seed dispersal in Cardamine species. At very low Reynolds numbers these ridges are unlikely to act as turbulators, like dimples on a golf ball, to reduce aerodynamic drag. However, a ridged seed surface may influence seed launch or flight in other ways that affect dispersal. For example, the explosively dispersed seeds of Ruellia ciliatiflora are launched with stabilizing backspin in an orientation that minimizes drag, thereby increasing dispersal range.”Yet, if the seed coat does aid flight, it didn’t evolve that way because of flight, the authors say. “Importantly, we found that other Brassicaceae species with non-explosive seed dispersal had diverse seed coat morphologies, contributed by both epidermal and sub-epidermal seed coat layers. So, while it is possible that the surface structure of Cardamine seeds may aid seed launch or ballistic flight, this characteristic of the seed coat did not coevolve with explosive seed dispersal.”