A mechanical perspective on foliage leaf form and function

The mechanical behaviour of large foliage leaves in response to static and dynamic mechanical forces is reviewed in the context of a few basic enginee ring principles and illustrated in terms of species drawn from a variety of vascular plant lineages. When loaded under their own weight or subjected t o externally applied forces, petioles simultaneously bend and twist, and th us mechanically operate as cantilevered beams. The stresses that develop in petioles reach their maximum intensities either at their surface or very n ear their centroid axes, where they are accommodated either by living and h ydrostatic tissues (parenchyma and collenchyma) or dead and stiff tissues ( sclerenchyma and vascular fibres) depending on the size of the leaf and the species from which it is drawn. Allometric analyses of diverse species ind icate size-dependent variations in petiole length, transverse shape, geomet ry and stiffness that accord well with those required to maintain a uniform tip-deflection for leaves with laminae differing in mass. When dynamically loaded, the laminae of many broad-leaved species fold and curl into stream lined objects, thereby reducing the drag forces that they experience and tr ansmit to their subtending petioles and stems. From a mechanical perspectiv e, the laminae of these species operate as stress-skin panels that distribu te point loads more or less equally over their entire surface. Although com paratively little is known about the mechanical structure and behaviour of foliage leaves, new advances in engineering theory and computer analyses re veal these organs to be far more complex than previously thought. For examp le, finite-element analyses of the base of palm leaves reveal that stresses are decreased when these structures are composed of anisotropic as opposed to isotropic materials (tissues).