Marchantia polymorpha gemmae interact as capillary multipoles on fluid interfaces

Marchantia polymorpha, or common liverworts, reproduce asexually by growing cup-like structures on their leafy thalli which contain multicellular propagules called gemmae. When rain fills the gemmae cups, mature gemmae are released, adsorb to the air-water interface, and subsequently are splashed out to grow into new plants. Gemmae have long been assumed to be passive agents, but their capillary interactions while attached to the air-water interface suggest they play a much more active role in liverwort reproduction. To investigate this, we directly measure capillary interactions of gemmae on both flat and curved fluid interfaces using brightfield and phase contrast microscopy as well as optical profilometry, and find that gemmae adsorbed to the water surface interact as capillary multipoles with a dominant quadrupolar character. Intriguingly, we observe different self-assembly behavior of multiple gemmae on flat fluid interfaces versus in their natural environment. This suggests that the gemmae cups may have evolved to shape the water surface to arrange the gemmae for better splash dispersal.