The Role of Wheat Awns in the Seed Dispersal Unit

Abstract

The dispersal unit of wild wheat bears two pronounced awns that balance the unit as it falls. We discovered that the awns are also able to propel the seeds on and into the ground. The arrangement of cellulose fibrils causes bending of the awns with changes in humidity. Silicified hairs that cover the awns allow propulsion of the unit only in the direction of the seeds. This suggests that the dead tissue is analogous to a motor. Fueled by the daily humidity cycle, the awns induce the motility required for seed dispersal.

Finite Element Modeling of the Cyclic Wetting Mechanism in the Active Part of Wheat Awns

“…1a) measured experimentally under varying humidity conditions. The geometry of a representative cross-section of the awn for the finite element model came from scanning acoustical microscopy (SAM) images [4]. Based on the gray level, three distinct regions were manually identified: the active part, the intermediate gap, and the resistance part (Fig.…”

“…1b). The elastic moduli were taken from a previous study on the wheat awn, where Elbaum et al determined E Ã a ¼ 10:0 AE 2:8 kN mm À2 for the active part and E Ã r ¼ 20:5 AE 2:6 kN mm À2 for the resistance part by nanoindentation [4]. The elastic modulus for the intermediate gap could not be measured directly, thus it was obtained by interpolation between the data of the resistance part and active part based on gray scale intensity of the SAM images.…”

“…However, the ensemble properties are more or less isotropic for this very special arrangement. In the resistance part, on the other hand, all these fibrils are uniformly oriented in axial direction [4], thus for this part a special case of orthotropytransversally isotropic material properties [20,21]-were chosen.…”

“…soils containing pores and aggregates of different sizes [3]. Seeds with hygroscopically active awns are able to propel themselves below the ground by coiling and uncoiling of their awns [4]. The mechanism of hygroscopic movement is a consequence of wetting and drying of tissue, which results in different anisotropic swelling in different regions of the tissue.…”

“…The active part (responsible for the awn bending with changing air humidity [4,10]) is built of cylindrical cells containing layers with parallel cellulose fibrils embedded in a soft hygroscopic matrix (Fig. 2).…”