Pollen ultrastructure of Panax (the ginseng genus, araliaceae), an eastern Asian and eastern North American disjunct genus

Pollen of ten species of Panax and six species of Aralia was examined in light microscopy and scanning and transmission electron microscopy. Grains of both genera have similar complex apertures, short columellae, and overlapping rectal sculptures, suggesting a close relationship. Most species of Panax have pollen characterized by striato-reticulate tecta, short columellae, thick foot layers, costa ectocolpi, and lalongate endoapertures. The eastern North American P. trifolius, commonly known as the dwarf ginseng, has a distinctive pollen morphology and exine structure, supporting the hypothesis of its phylogenetically isolated position. Pollen of the eastern Asian P. ginseng (ginseng) can be distinguished from the eastern North American P. quinquefolius (American ginseng) by differences in ultrastructure. The monophyly of the three medicinally important species, P. ginseng, P. notoginseng, and P. quinquefolius, suggested by triterpenoid data, is not supported by pollen data. The results of the pollen study are generally congruent with those from the sequences of nuclear ribosomal DNA.