Myxodiaspory in Adenostemma brasilianum (Asteraceae): morphological and histochemical strategies for diaspore dispersion

Myxodiaspory, the extrusion of sticky substances by the diaspores (seeds, fruits, anthocarps, and parts of infrutescences), has several selective advantages, one of which is aiding the adherence of diaspores between feathers/fur of animals that transport them over short or long distances. The diaspores of Asteraceae have three structures that can contain sticky substances: exocarp epidermal cells, exocarp trichomes, and viscid pappus. The South American species Adenostemma brasilianum (Asteraceae) has all three features. We examined the anatomy, secreted substances, and mode of adhesion of the diaspores of A. brasilianum to understand its strategies for dispersion. The fruit comprises an exocarp with glandular trichomes arranged spirally and scarce nonglandular trichomes, two layers of mesocarp separated by phytomelanin, and an endocarp. The pappus has a basal ring, a stalk, and a head with glandular trichomes. The mucilage, secreted by the glandular trichomes of fruit and pappus, reacted positively to tests for pectins, essential oils, lipophilic substances, and gums. The test for tannins gave a homogeneous positive reaction in the phytomelanin area, and as droplets, in the outer mesocarp layer and in the exocarp. The whole capitulum of A. brasilianum is adapted for successful transportation of its diaspores via diaspore release, secretion, and positioning. We discuss the idea that the dispersal adaptive traits found here evolved as phylogenetic parallelisms in the family.