Why rose prickles are shaped like blades rather than needles

The flattened shape of a rose prickle is often overlooked as a simple biological detail, but physical modeling reveals it as a precise engineering solution. Researchers from Aarhus University have demonstrated that this specific geometry is not designed for a simple puncture, but as a mechanism for gripping and slicing, representing a calculated evolutionary compromise between cutting efficiency and structural rigidity. Unlike the needles of cacti or the thorns of many other plants, which possess a nearly circular cross-section, rose prickles are significantly flattened, functioning more like short, curved blades. A circular profile is ideal for direct penetration and resists bending equally from all directions. However, the flattened geometry is optimized for interaction with soft surfaces during lateral movement, allowing stems to anchor themselves to neighboring branches and support the vertical growth of long stalks. To quantify this mechanical advantage, the team analyzed 113 prickles across 19 rose varieties. They found a consistent pattern: a typical prickle is nearly three times wider than it is thick. To test the impact of this ratio, the researchers moved from biological observation to physical simulation, using a robot equipped with silicone models of varying cross-sections—ranging from perfectly circular to highly elongated—which were dragged through gelatin to mimic animal tissue or environmental supports.