Pollen morphology and ultrastructure of several Gnetum species: an electron microscopic study

Pollen grains of five Gnetum species have been studied in scanning and transmission electron microscopy: G. africanum, G. funiculare, G. indicum, G. leptostachyum, and G. macrostachyum. The exine ultrastructure was described for the first time for G. funiculare, G. leptostachyum, and G. macrostachyum. The pollen grains are small, inaperturate, and microechinate. The sporoderm includes a rather thin tectum, granular infratectum, and lamellate endexine. The foot layer is indistinct or absent in G. africanum, G. funiculare, and G. macrostachyum and thin in G. indicum and G. leptostachyum. Gnetum africanum differs from other studied species of the genus in having smaller supratectal microechini. They occur on considerably raised exine regions (islands) that are interpreted as an equivalent to the plicae in Ephedra and Welwitschia. In Asian species of Gnetum, a microechinus and area around it are interpreted as equivalent to the islands of G. africanum and the plicae in Ephedra and Welwitschia. The infratectum in G. africanum consists of few, widely spaced large granules in contrast to small densely packed granules of other studied Gnetum species. A comparison of the published and original data on extant pollen of Gnetales and fossil ephedroid pollen shows a great similarity in the sporoderm ultrastructure. Absence of Gnetum-like pollen in the fossil record may be due to their thin ectexine, possible separation of the ect- and endexine or misinterpretation.