Fruit morphology and terminal velocity in Tragopogon dubius

In a manipulative experiment the following predictions about the relationship between fruit mass, pappus area, and terminal velocity in the fruits of Tragopogon dubious (L.) were tested: (1) terminal velocity should be proportional to the square root of disk loading (disk loading = mass/area), (2) when pappus area is held constant, terminal velocity should be proportional to the square root of the mass and (3) when fruit mass is held constant, terminal velocity should be inversely proportional to the square root of pappus area. Although qualitative support was found for each prediction, terminal velocity declined more slowly than predicted with increasing pappus area and increased more quickly than expected when mass was increased. When disk loading was held constant, fruits with larger pappuses fell more quickly than fruits with smaller pappuses. These results suggest that the area of the pappus is not a good estimator of its ability to produce drag (its effective area) for two reasons: (1) because the radial morphology of the pappus causes the density of drag producing cross hairs to decrease along the length of the pappus ray and (2) because the projected area of the pappus may change if the pappus is bent upwards by the drag force acting on it. Thus, increasing parental investment in pappuses does not decrease terminal velocity as quickly as expected. Key-words: Disk loading, morphological constraints, pappus morphology, seed dispersal, terminal velocity, Tragopogon dubious