Liverwort Gemmae Interact as Capillary Multipoles

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 passengers in this process, but their long-range capillary interactions while attached to the air-water interface suggest they play a much more active role in liverwort reproduction. To investigate this, we develop a novel imaging technique that provides simultaneous information on fluid surface profiles and the dynamic movement of gemmae. From single gemma and pairwise gemmae interactions, we find that gemmae adsorbed to the water surface interact and self-assemble as capillary multipoles, with dominantly quadrupolar character. Additionally, stable configurations found when ~10 gemmae are allowed to interact with each other suggest a different mechanism may be at play for many-gemmae interactions.