Humidity‐driven shape morphing enhances fog harvesting in porous cactus spines

Summary Cacti develop spines instead of conventional leaves, which often serve as mechanical defence against herbivores. However, some cactus species grow porous and flexible spines, suggesting fundamentally different functions. Here we demonstrate that the porous spines of Turbinicarpus alonsoi , a cactus native to central Mexico, function as hygro‐morphing fog harvesters. Using experiments, we show that the spines are highly hygroscopic and straighten when exposed to fog, leading to increased fog‐water collection rates. Numerical simulations confirm that straightening is driven by swelling‐induced pressure in the cell walls of the spines. Swelling results from capillary imbibition of fog water and predominantly generates expansion in the transverse plane, which causes the pre‐curved spines to straighten. Despite their porosity and hygroscopicity, the spines prevent direct absorption of fog water into the living cortex due to the presence of a suberin‐rich tissue layer that instead promotes surface runoff towards the roots. Our work suggests that hygro‐morphing emerges from distinct structural, biochemical and geometric adaptations of cactus spines and enables a fine modulation of the flow dynamics on the spine surface. The increasing plant water supply from fog by shape morphing provides a potential advantage for survival in a hot, semi‐arid region with frequent fog formation.