Circumnutations: from Darwin to space flights.

More than a century ago, plant physiologists were aware that plant organs-roots, hypocotyls, shoots, branches, flower stalks-rarely grow in just one direction. The mean growth direction may be maintained for long intervals, but the organ's instantaneous growth direction usually oscillates slowly about that mean. The plant organ tip, as seen from a dista1 viewpoint, describes an ellipse. The axes of the ellipse can vary; at one extreme the ellipse approximates a line and at the other a circle. As the organ grows, its tip advances and (in three dimensions) traces a somewhat irregular helix. That oscillating growth pattern was well known to nineteenth century plant scientists as "revolving nutation" until Darwin (Darwin and Darwin, 1880) introduced the terms "circumnutate" and "circumnutation," which are used today. Thus, circumnutational oscillations are manifestations of the radially asymmetric growth rate typical of elongating plant organs.