The Existence of a Special Exine Coating in Geranium robertianum Pollen

The outer pollen wall of Geranium robertianum L. (Geraniaceae) is exclusively formed by the ectexine, characterized by a perforated footlayer, unbranched columellae, a tectum that forms a reticulum, and supratectate pila. An endexine is missing. The intine protrudes through the channels of the footlayer to the exine cavities. The intine material in the exinic interstices is immediately covered by tapetally synthesized vesicles, called pollen-coating vesicles (PCV), which form a compact layer within the exine cavities. The PCV layer is interpreted as a protecting barrier. During final pollen maturation the PCV layer is superimposed with lipidic pollenkitt. Endoplasmic reticulum and elaioplasts participate in the pollenkitt formation. The electron-translucent zones at the bottom of the exine cavities may represent degeneration products of the primexine matrix.