Regulated pollen issue in Isotoma, Campanulaceae, and evolution of secondary pollen presentation

The floral characteristics (white, salverform, nocturnal sweet fragrance emission) of Isotoma petraea F. Muell, are indicative for pollination by night flying Lepidoptera. The pollen issue is regulated by a valve which has to be operated by the visitor. The valve consists of two connected, dead papillae. The distal parts of these papillae form the trigger hair. A tuft of smaller, also dead papillae under the valve acts as a spring and produces the counter force that repositions the valve after operation. The required force on the trigger hair is 9.5 Dyne. At anthesis the pollen grains are produced inside a closed anther box. Inside this box the piston shaped style gives pressure onto the mass of pollen. On the opening of the valve, this pressure causes the pollen grains to flow out of the box. A radiant ring of papillae around the style tip acts as a safety valve, that allows the pollen grains to pass if the pressure becomes too high. This happens when the valve is not operated, and the style continues to increase the push forward. In the genus Isotoma, this mechanism of regulated secondary pollen presentation seems to have evolved by differentiation of brush papillae on the anther tips. The evolution in Asteraceae and in Campanulaceae seems to have developed along convergent lines. Firstly secondary pollen presentation produced spatial exactness. Secondarily, in at least three independent lines, a temporal exactness evolved, presumably under a selection pressure that acted towards a prolonged pollen protection.