The functional morphology of the petioles of the banana, Musa textilis

Bananas are among the largest herbs in the world and their lightweight peti oles hold up huge leaves, This study examined how the petioles manage to ac hieve adequate rigidity to do this, while allowing extensive and reversible reconfiguration in high winds. Morphological and anatomical examination of the petioles and leaves of Mesa textilis suggested how these two apparentl y incompatible abilities are achieved. The hollow U-shaped section of the p etiole and the longitudinal strengthening elements in its outer skin give i t adequate rigidity, while its ventral curvature help support the leaf with out the need for thick lateral veins. These features, however, also allow t he petiole to reconfigure by twisting away from the wind, while the leaf ca n fold away. In addition, two sets of internal structures, longitudinal par titions and transverse stellate parenchyma plates, help prevent dorsoventra l flattening, allowing the petiole to flex further away from the wind witho ut buckling. These ideas were tested and verified by a range of mechanical tests. Simple four-point-bending and torsion tests showed that the petioles are indeed far more compliant in torsion than in bending. Axial bending te sts and crushing tests showed that petioles could be flexed twice as far an d were four times as resistant to dorsoventral flattening when intact than when the internal tissue is removed, The banana petiole, therefore, seems t o be an excellent example of natural integrated mechanical design.