On the Function of Wall Sculpturing in Xylem Conduits

The water-conducting network of capillaries in vascular plants has evolved over hundreds of millions of years in order to be able to cope with bubble clogging, a problem which also affects modern microfluidic devices. Decades of anatomical studies have revealed that plants growing in habitats in which the formation of bubbles, or emboli, is likely to be a frequent occurrence often have various forms of geometrical sculpturing on the internal surfaces of the xylem conduits. The possible function of such wall sculpturing has long been the subject of speculation. We have investigated the hypothesis that wall sculpturing is a functional adaptation designed to increase the wettability of the walls of xylem conduits, an effect which could be described as the inverse of the well-known lotus-effect. Our results show that wall sculpturing does enhance wettability. Importantly, theoretical calculations reveal that the geometric parameters of various types of wall sculpturing are such that the resulting surfaces are sufficiently rough to enhance wettability, but not significantly rougher. The results provide an appealing answer to the long-standing debate on the function of wall sculpturing in xylem conduits, and may provide biomimetic clues for new approaches to the removal of bubbles in microfluidic channels.