THE CONNECTIVE BASE OF CIRSIUM HORRIDULUM (ASTERACEAE): DESCRIPTION AND COMPARISON WITH THE VISCOELASTIC FILAMENT

Structural, autofluorescent, and mechanical characteristics show that the so-called “collar” of the stamen is an elongated basal portion of the connective in Cirsium horridulum. The connective base/filament boundary is precisely demarcated by epidermal cells with thick, autofluorescent walls. In the region of the connective base near the anther lobes, the autofluorescence is less intense and the cells are longer, but wall thickness does not decrease. The autofluorescence can be assayed in fresh, frozen, or rehydrated materials, and is resistant to boiling in water and to acetone. Additional features delineate the connective base from the filament. The abaxial and lateral portions of the connective base surface are relatively flat due to the underlying thick secondary walls. Ultrathin sections taken at various points along the length of the connective base and the filament show that cuticle thickness gradually increases from approximately 30 nm in the connective base to 450 nm in the filament. The connective base/filament junction appears to be a weak mechanical link within the stamen. These data establish several new connective base characteristics in Cirsium and define the connective base/filament boundary.