| Summary: | Bidens pilosa L.. (Compositae) one among the World's worst weeds, produces open globose heads of desmochorous achenes disposed radially around the receptacle. These achenes are more numerous (about 60%) in the inferior hemisphere and their length decreases, while their width increases, along an apex — basis gradient. The achenes nearer to the basis (about 20%) are much shorter and differ from the others also in shape and in ornamentations. These brachycamous achenes remain connected to the receptacle longer than the other ones and showed a very low percentage of germination, in common laboratory conditions. B. pilosa, therefore, shows a certain degree of heterocarpy, perhaps associated with the process of dispersal and germina-tion of its achenes.
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