The gametophyte-sporophyte junction in Isoëtes boliviensis Weber (Isoëtales, Lycopodiophyta)

The gametophyte-sporophyte junction in Isoëtes boliviensis Weber consists of a sporophytic conical foot embedded in the maternal gametophytic tissue. Both generations are separated by a narrow placental space filled with thin-walled collapsed cells of gametophytic origin. Gametophytic and sporophytic placental cells lack wall ingrowths, or transfer cells. Interdigitation or intermingling of placental cells is not observed. Cell wall ornamentation is restricted to a nacreous thickening of the outermost cell wall of the sporophyte foot. This structure of the gametophyte-sporophyte junction in Isoëtes boliviensis is different from that of all other pteridophyte groups for which micrographic details are available. This may be interpreted either as a mere reduction as in some Metzgerialean liverworts, or as an adaptation to the mostly aquatic life history, or as a plesiomorphic condition preserved in the extant relict genus Isoëtes since the divergence of the Isoëtes-clade as a distinct evolutionary line in the Upper Devonian.