Seed dispersal is a central stage in the life cycle of higher plants. It is important for the growth and collection of agricultural products, and in understanding the spread of invasive weeds. Plants develop specific structures that help their seeds to move from the mother plant to a safe germination spot, such as hairs, wings, thorns, edible parts, and more. Many grasses have awns, which are long, narrow appendages to the dispersal unit. The awns can passively affect the falling course of seeds from the mother plant. In addition they may actively push the seeds along and into the soil, using a hygroscopic mechanism. Recently we detected such a mechanism in wheat awns (1). The awns bend at their base when dry and straighten when damp, and so a long lever-arm is created. This movement is translated to locomotion of the dispersal unit when the surface of the awns interacts with the ground surface.
Using electron microscopy I identified a continuous silica layer deposited at the epidermis, covering hairs and papillas. The silicified surface creates low friction when the awns slide in the direction of the seeds, and high friction when they slide in the opposite direction. Under the silicified epidermis, the active cell wall shrinks upon drying. In this part the cellulose fibril layers are arranged in a plywood-like structure. Scanning acoustic micrographs showed micrometer sized patches of uniformly aligned plywood (2). Using Raman microscopy is was possible to detect high levels of aromatic compounds. This architecture may explain the increase in the speed of water diffusion within the active part, as detected by thermal gravimetric analysis. The faster reaction to changes in the ambient humidity probably allows the awns to bend and straighten even at moderate conditions. Adjacent to the active part, the cellulose fibrils are densely packed along the awns, forming a stiff structure. This is necessary for pushing the seeds against the soil without buckling. Altogether, the structure of awns is optimized for assisting the seed in moving away from its mother plant, along and into the soil, increasing its chances to successfully sow.

(1)Elbaum R., Zaltzman L., Burgert I., and Fratzl P. 2007. Sceince 316 884-886.
(2)Elbaum R., Gorb S., and Fratzl P. 2008. J. Struct. Biol. 164 101-107.