Carol Walker / Wild horses / Mustangs, close up of mane tangled up with burrs and grass seeds, South Dakota, USA

Gary K. Smith / English / Pendunculate Oak (Quercus robur) trunk showing swollen burr growths. Norfolk, England, April.

Gary K. Smith / English / Pendunculate Oak (Quercus robur) trunk showing swollen burr growths. Norfolk, England, April.

Jeff Vanuga / Circle Cliffs along the Burr Trail, Grand Staircase-Escalante National Monument, Utah, USA

David Welling / Burr Trail from Strike Valley Overlook with storm arising, Waterpocket Fold, Utah, USA

David Welling / Burr trail canyon and Henry mountains. Long Canyon, Utah, USA