Leaf surface structures enable the endemic Namib desert grass Stipagrostis sabulicola to irrigate itself with fog water A. Roth-Nebelsick 1, *, M. Ebner 2 , T. Miranda 2 , V. Gottschalk 4 , D. Voigt 3 , S. Gorb 3 , T. Stegmaier 4 , J. Sarsour 4 , M. Linke 4 and W. Konrad 2 1 State Museum of Natural History, Rosenstein 1, 70191 Stuttgart, Germany 2 Department of Geosciences, University of Tu ¨bingen, Sigwartstrasse 10, 72076 Tu ¨bingen, Germany 3 Zoological Institute, University of Kiel, Am Botanischen Garten 1-9, 24098 Kiel, Germany 4 Institut fu ¨r Textil- und Verfahrenstechnik Denkendorf, Ko ¨rschtalstraße 26, 73770 Denkendorf, Germany The Namib grass Stipagrostis sabulicola relies, to a large degree, upon fog for its water supply and is able to guide collected water towards the plant base. This directed irrigation of the plant base allows an efficient and rapid uptake of the fog water by the shallow roots. In this contribution, the mechanisms for this directed water flow are analysed. Stipagrostis sabulicola has a highly irregular surface. Advancing contact angle is 988+ 58 and the reced- ing angle is 568+ 98, with a mean of both values of approximately 778. The surface is thus not hydrophobic, shows a substantial contact angle hysteresis and therefore, allows the develop- ment of pinned drops of a substantial size. The key factor for the water conduction is the presence of grooves within the leaf surface that run parallel to the long axis of the plant. These grooves provide a guided downslide of drops that have exceeded the maximum size for attachment. It also leads to a minimum of inefficient drop scattering around the plant. The combination of these surface traits together with the tall and upright stature of S. sabulicola contributes to a highly efficient natural fog-collecting system that enables this species to thrive in a hyperarid environment. Keywords: contact angle; surface grooves; directional water flow; surface roughness; fog collection; Stipagrostis sabulicola 1. INTRODUCTION Wetting properties are important for many biological processes as well as for technical applications. A wide variation of wetting effects can be obtained by combin- ing surface chemistry and surface structures [1]. Plant surfaces represent a group of biological surfaces that recently have generated much interest the Lotus effect leading to highly water repellent leaves as an example [2,3]. The underlying benefits of wetting effects on plant surfaces are manifold ranging from preventing pathogens from settling (as proposed for many water- repellent leaves), to ensuring the floating ability of aquatic plants [4] or to facilitating the catching of prey by carnivorous plants [5]. Wetting properties are also involved in the inter- actions between plants and dew or fog [6]. In many regions of the world, fog and dew represent regularly occurring phenomena, and the impact of these events on hydrology and ecology of the local vegetation is often substantial [7 – 9]. For example, fog drip (the shed- ding of droplets from leaves to the ground) and stem flow (the running-off of water from plant stems) may alter the local hydrological conditions [10 – 12]. In var- ious arid regions, fog represents a valuable water source for the plants. This applies particularly for deserts with regular fog events, such as the hyperarid Namib Desert of South Western Africa [13]. Here, many organisms are adapted to use fog [14]. As fog col- lection has attained increasing interest during the last years as a sustainable water source in arid environ- ments, a better understanding of the strategies that are employed by fog-harvesting organisms is expected to contribute to further improve the already existing technical fog collectors and fog-collecting strategies. There is evidence that Stipagrostis sabulicola, a grass species endemic to the sand dunes of the Namib, *Author for correspondence (anita.rothnebelsick@smns-bw.de). Electronic supplementary material is available at http://dx.doi.org/ 10.1098/rsif.2011.0847 or via http://rsif.royalsocietypublishing.org. J. R. Soc. Interface (2012) 9, 1965–1974 doi:10.1098/rsif.2011.0847 Published online 22 February 2012 Received 3 December 2011 Accepted 31 January 2012 1965 This journal is q 2012 The Royal Society Downloaded from https://royalsocietypublishing.org/ on 17 February 2022