Recently, an interesting discussion was active in GrainGenes mailgroup, describing about an unusual behavior of wheat awn asking for further explanation on the phenomenon. The story went like this:
During a recent trip Mr Norman Rossen and colleagues met a lady who was a worker at a US Department of Agriculture facility where she did chemical research on wheat components. Mr Rossen happened to relate an experience about wheat to this lady which she never had heard and did not believe. When he was a young boy, while traveling with his dad along rural Pennsylvania they happened to stop along a wheat field. His father broke off an awn of wheat, put it on his bare, but hairy, forearm and told him to watch what happened. The head of wheat very slowly moved up his fathers arm. He recalls that the same movement repeated when they put the head of wheat on his not-quite-so-hairy arm too.
I believed this is a common phenomenon with dry awns of many grass species, especially the ones which make longer awns. I have seen kids playing with some grass species whose awns are long and 'L' shaped which spin around when they wet them. Such grass species are common in tropical South India, which is not a wheat growing region. Nevertheless the phenomenon was very similar to what Mr Norman experienced with the wheat awn.
The reason for the movement is nothing but moisture. The dry awn must be having its cells under tension because of the loss of moisture and when it absorbs water it probably must be expanding causing the movement. For the grass awns, wetting is must for the rotation. So the movement Mr Nornam observed on his dad's and his arms could have been caused by the perspiration which the wheat awns absorbed causing the movement.
Dr Maarten van Ginkel of the Plant Genetics and Genomic Division of the Primary Industries Research Victoria (PIRVic) says he has regularly demonstrated this phenomenon to interested parties using wild oats. Hold the separated seed-filled floret by the tip of the L shaped twisted awn, wet the twisted region at the base of the awn with some spittle and the floret will make a 360-degree turn in the air and sometimes more as the awn unwinds. The assumption is that lying on the ground the L-shaped tip is lodged in between some rocks or soil clods, and when the first rains set in or even excessive dew, the seed drills itself into the soil by the spring-loaded action of the trapped awn unwinding. The tips of wild oat seeds are heavily bristled forming an arrow-shaped projectile of the glumes closely adhering to the seed. Thus a handful of oat seeds (rather seeds plus fused-glumes) on the open soil one day the next morning can have fully drilled themselves into the soil, leaving no trace.
In a recently published article in Science Elbaum et al., (2007) explains the phenomenon of movement of the awns. The dispersal unit of wild wheat bears two pronounced awns that balance the unit as it falls. They 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.
Elbaum,R., Zaltzman,L., Burgert,I., Fratzl,P. (2007) The Role of Wheat Awns in the Seed Dispersal Unit. Science 316: 884 - 886
Peart, M.H. 1979. Experiments on the biological significance of the morphology of seed dispersal units in grasses. J. Ecol. 67: 843-163.
Peart, M.H. 1981. Further experiments on the biological significance of the morphology of seed dispersal units in grasses. J. Ecol. 69:425-436.
Peart, M.H. 1984. The effects of morphology, orientation, and position of grass diaspores on seedling survival. J. Ecol. 69:425-436.