On the ontogeny of the oncus in the pollen grain of Parietaria officinalis ssp. judaica (Urticaceae)
During the pollen ontogeny of Parietaria officinalis L. ssp. judaica Bég. the apertural zone and operculum are differentiated relatively early and they are well apparent when the microspores are liberated into the thecal cavity. At the late stage of the microspore the endexine becomes ample and granulose in the apertural zone, and a cavity is conspicuous between the operculum and the endexine. On the other hand a structured periplasmic space between the endexine and the plasmalemma is formed. Subsequently the structured periplasmic space becomes thin and a new material, apparently different from the structured one, is produced from the plasmalemma under the apertural zone, as a result of cytoplasmic activity. Finally, the structured periplasmic space is no longer apparent except for a very narrow portion beneath the aperture (Zwischenkörper). Beneath the Zwischenkörper the secondary material, produced by the cytoplasmic activity, becomes globular and constitutes a spherical body that is easily discerned under the transmitted light microscope and which is called the oncus in the classical optical microscopic observation of the fresh pollen grain. By histochemical reactions with periodic acid – Schiff s reagent, Sudan black, and Coomassie brillant blue we have found abundant glucosidic, lipoidal, and proteinaceous material in this globular body.