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Anatomical Aspects of Pollen Release from Staminate Flowers of Ambrosia trifida (Asteraceae)

Ambrosia trifida flowering was observed in the field and laboratory; individual staminate and pistillate flowers were collected and processed for light and scanning electron microscopy. The inverted staminate flower consists of a corolla with five connate petals, five stamens with connate anthers, and a central pistillodium (transformed gynoecium). Each anther is terminated by a "claw," a thin, flexible appendage that curves back toward the pistillodium. Abaxial epidermal cells of the claw resemble cells of a fern sporangium annulus, with thick inner walls and thin outer walls. This differential wall thickening causes the claw to straighten partially under dry conditions, thereby rupturing the anther and releasing pollen. Just before the pollen is released, however, the pistillodium elongates rapidly, tripling its length in less than an hour. As it grows past the anthers, it further straightens the anther claws and pushes a wad of pollen beyond the corolla. While still within the corolla, trichomes attached in a ring around the pistillodium apex extend beyond its tip parallel to its axis. After the pistillodium grows out of the corolla, however, these trichomes bend outward 90⚬. The pistillodium stalk later shrinks, which pulls the now brushy pistillodium tip back to block partially the open distal end of the flower. This seems to allow continued slow release of pollen. The pistillodium bears little resemblance to the pistillate flower gynoecium since it lacks an ovary, ovules, and a bifid stigma. It is interpreted as a gynoecium transformed into a completely different structure with new functions.