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(American Journal of Botany. 2005;92:1820-1826.)
© 2005 Botanical Society of America, Inc.

Physiology and Biochemistry

The importance of frictional interactions in maintaining the stability of the twining habit1

Wendy K. Silk2,4 and N. Michele Holbrook3

2Department of Land, Air, and Water Resources, University of California, Davis, California 95616-8627 USA; 3Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138 USA

ABSTRACT

The stability of twining vines under gravitational loads suggests an important role for friction. The coefficient of friction, µ, between vine stems and wood is high, often five times greater than between leather and wood, as determined by slip tests on an inclined plane. Stem trichomes function like ratchets to facilitate climbing upward (or to facilitate slipping if the stem is inverted). A mathematical model predicts large masses (kg) must be applied to the base of a twining vine to cause slipping. Vines slip as predicted when µ is low and arc length on the pole is short, and they break before slipping when µ is large or arc length is long. In contrast, twining vines are unstable in compression, collapsing when small masses (<10 g) are hung from the top of the vine. However, if the loads are applied below the uppermost gyre, the stabilizing tensional effect dominates. Therefore, in nature vines twining on a cylindrical support are stable under gravitational loads, unless these loads occur near the apex. A corollary is that a short apical coil can hold up large masses of maturing shoot.

Key Words: biomechanics • curvature • friction • helix • Ipomoea purpurea • structural stability • twining • vines






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