The similarities between gametophyte and sporophyte in early fossil
vascular plants contrast strongly with the marked dissimilarities typical
of living land plants (b in figure). The phylogenetic position of fossils
suggests that, after the development of a simple, unbranched, ‘parasitic’
sporophyteamong early land colonizers at the bryophyte grade (such as
mosses) there was elaboration of both gametophyte and sporophyte in
vascular plants. The implications for interpreting life cycles in living
vascular plants18,26 are shown. The small, simple, often subterranean
and saprophytic gametophytes of living clubmosses (such as Lycopodiaceae)
and ferns (such as Psiloataceae, Stromatopteridaceae,
Ophioglossaceae) result from morphological loss. Phylogenetic evidence
indicates that gametophyte reduction was independent in clubmosses
and the fern–seed plant lineage. These data provide a new
interpretation of the gametophyte morphology of living clubmosses
(Lycopodiaceae)18.