Generation of Torque by the Column of Stylidium

Abstract

The gynostemium or column of the flower of Stylidium, when stimulated mechanically, flips through an angle of about 4 rad in 15-20 ms and then resets to its original position in 300-600 s. The torque causing this movement is produced in a specialized motor tissue when the angular displacement, 8, of the column from its set position exceeds a threshold value of about 4 x lo-' rad. An experimental apparatus has been developed to change 0 in a controlled manner and measure the resultant torque, 7, exerted by the column. For subthreshold angular displacement, the column opposes the applied torque and the stiffness of the column, defined as d~/dOi,s 1.5 x 10' pN m rad-'. At threshold displacement, 7 reverses in direction and the stiffness decreases to about 0.5 % of this value in about 2-4 ms. The stiffness remains low for the remainder of the fast movement and slow recovery, but returns to the original high value after the column has been set for about 20-30 min. Most of the stiffness of the set colunln seems to reside in very turgid cells within the concave or anterior side of the motor tissue. The cells when turgid could provide a holding torque, maintaining the column in the set position against an equal and opposite torque in the posterior layer. On stimulation, the anterior cells apparently cease to exert torque, with subsequent movement of the column caused by a torque within cells in the opposite, posterior, layer. The mechanism by which the column loses stiffness in the short time of 2-4 ms is not clearly understood; two possibilities are considered.