THE DISPERSAL EFFECTIVENESS OF THE ACHENE–PAPPUS UNITS OF SELECTED COMPOSITAE IN STEADY WINDS WITH CONVECTION

Abstract

SUMMARY: The role of the involucral bracts of the Compositae capitulum in fruit dispersal, the behaviour of the pappus of composite fruits to changes in humidity, and the presentation of the achene‐pappus units for wind dispersal, are briefly reviewed. Arguments put forward in the literature for and against the effectiveness of the pappus in facilitating wind dispersal are presented. A method is described for comparing the dispersal efficiency of achene‐pappus units of selected composites. The computed trajectories for fruits of selected species under the independent effects of wind speed and plant height, wind speed and boundary layer, and the combination of these with convection velocity are given. It is shown that the efficiency of dispersal is determined more by the fine details of the pappus geometry, which directly affects its aerodynamic properties, than by the size ratio of pappus to achene. Under steady horizontal winds, increased height of fruit release increases dispersal distance. Reported patterns of wind dispersal are discussed in terms of air movement, flight path interference by neighbouring plants and, for some species, the fruit dissemination mechanism. While increased wind velocities increase the trajectory distance, dispersal of many composites is hampered by the pappus response to humidity. Increased dispersal distances are given by steady convection currents, whereas turbulence may either curtail or prolong transportation. The apparent inefficiency of wind dispersal reported under field conditions is explained by the combined effects of these various environmental factors upon the dispersal units during the period of dissemination.