Morphokinematics of the Hygroactuation of Feather Grass Awns

Certain awns utilize actuating mechanisms that harness energy from variations in aerial humidity to self-burrow their seeds into the soil. Here the morphokinematics of such hygroresponse from the awn of the feather grass Stipa epilosa is described. The elongated body of the awn is typically doubly bent, and has three segments with different functionalities: the long and stiff proximal segment twists reversibly to generate thrust for burrowing the seed, the plumose and flexible distal segment contributes to aerial dispersion, and the short and stiff middle segment provides a bent geometry for effective burial. Periodic variation in humidity results in expansion and twisting of individual cells, and the collective expansion generates a torque that drives reversible twisting of the proximal and middle segments.