Painful Stowaways

November 08, 2014


Photographer: Rob Sheridan
Summary Authors: Rob Sheridan; Jackie Phillips

Plants don’t move around a great deal, so all successful flowering plants have evolved a strategy for seed dispersal. The most common strategies are airborne dispersal, such as the spiraling pods of maple tree seeds, or ingestion and dispersal by birds or animals with sweet fruits. Some plants have evolved a less pleasant strategy. Tribulus terrestris, more often called Devil’s Thorn, Goathead Weed or Tack Weed produces a pleasant small yellow flower that matures into a nut with very sharp 0.2 - 0.4 in (5 - 10 mm) spikes. As the nuts dry and fall from the plant their geometry has evolved such that at least one spike is directed up, awaiting the unsuspecting paw pad, foot or bicycle tire. The nuts are very hard to see and very painful to step on! The wounded animal or bicycle tire transports the seed some distance before the spike breaks off and drops the nut, a method known as epizoochory. This hardy ground-hugging plant is very drought tolerant and invades many dry soils throughout North America. Photo taken on October 21, 2014, in Utah.

Photo Details: Camera: iPhone 5s; Focal Length: 4.12mm (35mm equivalent: 63mm); Digital Zoom: 1.573x; Aperture: f/2.2; Exposure Time: 0.025 s (1/40); ISO equiv: 8000; Software: Microsoft Windows Photo Viewer 6.3.9600.16384.