Yahya S. Masrahi, Najla A. Al Shaye
Microstructure of hygroscopic awns in diaspores of Aristida funiculata, Danthoniopsis barbata, and Dichanthium foveolatum (Poaceae) was examined. Hygroscopically active parts of the awns are twisted and sensitive to ambient humidity. These features reflect the fine structure of the cell wall in two distinctive layers of fiber cells, which lead to twisting and untwisting of hygroscopically active parts when they absorb or lose water. Periodic changes in ambient humidity initiate hygroscopic action that causes the diaspore to move across soil surface and drill into soil. The fine barbs on the surface of twisted part prevent awns from pushing themselves out of soil. The geometrically ideal conical shape of the barbs and different wettability between barbs and hygroscopic surface of twisted fibers generate gradients in Laplace pressure and surface energy, respectively, that derive water droplets to the hygroscopic part, then enhance "moisture collection" in conditions of dew formation. These characteristics enhance establishment of the diaspores in suitable soil microsite for germination.
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