Mechanical limits shape the eccentric form of rose prickles

Abstract

Rose prickles are vegetal weapons familiar to most green-thumbed individuals. Their distinctive feature is an oval cross-section, which contrasts with the mostly circular shapes of natural and man-made stingers. However, the reason for their eccentricity remains poorly understood. We explore the hypothesis that the morphology of prickles is physically limited by trade-offs between bending, buckling and cutting. In our experiments, polydimethylsiloxane stingers with varying cross-sectional shapes were used to lacerate a gelatin skin analogue. Circular stingers bend along the direction of travel and create superficial scratches. By contrast, prickles with a moderate aspect ratio have enough mechanical stability to cut (or provide grip). However, when they become too narrow, the blades weaken and buckle. The least unfavourable aspect ratio falls between 2 and 4, not inconsistent with the available biological data (2.7±0.7). This novel mechanical trade-off suggests that cutting mechanics are a potential driver of the rose prickle morphology.