POLLEN DEVELOPMENT IN OENOTHERA BIENNIS (ONAGRACEAE)

Development of the exine and viscin threads in Oenothera was studied by a combination of transmission electron microscopy and field emission scanning electron microscopy. Exine formation is initiated in the early tetrad stage by plate-like structures of preexine formed evenly around the microspore within a callosic wall. In the late tetrad stage, an endexine accumulates on white-lined lamellae underneath the preexine. After dissolution of the callosic wall, the preexine develops into beaded ektexine which is differentiated into an undulate tectum-like layer and columellae-like components. Interwoven fibrous strings connect the developing ektexine and the surface of the tapetal cells, and later develop into the viscin threads. These developmental processes imply that the columellae-like components are different in structure from the columellae of other angiosperms and that the formation of viscin threads is associated with the tapetal cells.