Studies on the inflorescence of wild oats (Avena fatua). Morphology and anatomy of the awn in relation to its movement

The mature awn of wild oats (Avena fatua L.) is geniculate consisting of a proximal twisted portion and a distal subulate awl; the awl is placed at right angles to the proximal part. The proximal part of the awn can untwist upon wetting with water and twist upon drying. The awl by itself cannot twist or untwist, but it can rotate along with the proximal part of the awn. It can, however, act as a brace causing the wild oat lemma-covered caryopsis to turn in the opposite direction dislodging it from the spikelet. The proximal part untwists on wetting to become straight when the two ridges of the awn, wide adaxial and narrow abaxial, become obvious and remain superimposed alternating with adjacent furrows. The two ridges internally are joined by a vertically extending thin sheet, three to four cells thick, called the connective. The ridges and the connective in a mature dry awn contain unlignified, thick-walled fiber cells which are individually capable of twisting upon drying and untwisting upon wetting. The short fiber cells of the connective, especially the ones centrally situated, undergo longitudinal compression with little or no twisting. The more peripherally placed fiber cells are long and tend to twist extensively and (or) form helices. The extent of longitudinal compression owing to the loss of water decreases in cells from the center to the periphery, but the twisting of cells increases. These differences in the pattern of shrinkage between central and peripheral cells of the awn result in a change of form of the awn from a straight to a geniculate condition.