Viscin threads on pollen of Circaea × intermedia Ehrh. (Circaeeae: Onagraceae)

Viscin threads are here described for the exine of Circaea × intermedia Ehrh. ( C . alpina L. × C . lutetiana L.). Often, the threads bifurcate, taper distally and have branched terminal ends. The smooth surface has lightly sculptured bands and randomly occurring and positioned sporopollenin globules. At the point of attachment with the exine the threads show either a natural transition with exine surface elements or have bulb­like enlargements. Circaea × intermedia viscin threads present a remarkable contrast to the generally poorly defined viscin threads on Circaea pollen.