Dandelion diaspore dispersal: frictional anisotropy of cypselae of Taraxacum officinale enhances their interlocking with the soil

Abstract

We analysed the surface anisotropy of Taraxacum officinale cypselae, without their pappus, microscopically and biomechanically. A scanning electron microscope was used for visual analyses. We then measured the angle, at which cypselae start to slide from tilted substrates of different roughness and compared cypselae sliding along their structure with those sliding against their structure. Finally, forces during pushing cypselae into the soil and pulling them out were measured with a force transducer moved by a micromanipulator. The cypselae had a ribbed surface pattern with apically pointing spines, plenty of scale-like structures and small protuberances. In general, the angle at which cypselae started to slide-off from an inclined surface increased with increasing roughness of the inclined surface, but this did not hold for cypselae sliding along their structure and for the glass surface. The angles at which the cypselae started to slide were on all but the glass surface significantly higher for cypselae sliding against the structure compared to cypselae sliding along the structure. Pull-out forces were higher when pulling cypselae out of the soil against their structure compared to pulling along their structure. Anisotropic structures hinder cypselae from being drawn from soil by wind.