The gametophyte-sporophyte junction in Apotreubia hortonae (Treubiaceae, Hepaticophytina): Structure and systematic implications

The gametophyte-sporophyte junction of Apotreubia hortonae consists of a bu lbous foot with an indistinct collar, embedded in the parental gametophyte tissue. Both generations are separated by a placental space filled with col lapsed cells of gametophyte origin. The ultrastructural organization of the outermost cell layers of the sporophyte foot and the surrounding parental gametophyte tissue, the placenta, is unique in archegoniate land plants. On the sporophyte side of the placenta there are clusters of 1-2 layered cell s with coarse wall ingrowths (transfer cells) alternating with clusters of cells without wall ingrowths. Gametophyte placental cells lack wall ingrowt hs. The unique structure indicates an isolated position of Treubiopsida wit hin Hepaticophytina (liverworts), but with a Jungermanniopsidalean affinity .