Structure-function relationships of different vascular bundle types in the stem of the Mexican fanpalm (Washingtonia robusta).

Structure and mechanics of fibre caps of different types of vascular bundles were studied at a certain height in the trunk of the palm Washingtonia robusta. By correlating these features with the distribution of the different bundle types across the trunk, possible strategies to cope with mechanical loads were elucidated. Micromechanical properties, cell parameters, microfibril orientation and lignification were studied using micromechanical testing, image analysis, synchrotron X-ray diffraction and UV-microspectrophotometry. The adjustment of stiffness followed a common principle in the fibre caps of all bundle types. Gradients in stiffness appeared across the caps in the centre of the trunk whereas stiffness remained high across the caps in the periphery of the trunk. The difference in stiffness profiles was interpreted as an adaptation to different mechanical constraints arising across the trunk. The gradual transition in stiffness prevents high local stress discontinuities between cap fibres and parenchyma and might be beneficial for trunk damping. At the periphery, the fibre caps without stiffness gradients contribute to a high flexural stiffness of the trunk as this has to be exclusively maintained by the material properties because geometric adjustments through increasing trunk diameter hardly occur in monocotyledonous palms.