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.

The results highlight a tension between penetration and stability. While a circular tip withstands direct pressure well, it bends easily during lateral movement, resulting in superficial scratches rather than deep engagement. The flattened models showed superior penetration and grip. Yet, this advantage reaches a breaking point; when the width-to-thickness ratio becomes too high, the structure loses stability and collapses under the load.

The data indicates that the optimal balance—where the prickle cuts effectively without buckling—exists within a width-to-thickness ratio of two to four. This precise range aligns with the physical parameters of most natural rose varieties. This suggests that the shape is not random but is tuned to specific mechanical tasks: gripping and slicing.

While the study clarifies the function of the rose prickle, the use of silicone and gelatin remains a simplification of complex biological materials. Further validation requires high-precision pressure sensors on living tissue. The findings are published in the Journal of the Royal Society Interface.

Physicist Dmitriy Lapshin noted that the study's conclusions are based on these physical models and simulated environments.