to the upper Silurian (Feist and Grambast-Fessard, 1991 ? ; Gensel and Edwards, 1993 ? ). Reproduction is oogamous, and sperm morphology is complex (Garbary et al., 1993 ? ). THE ORIGIN OF LAND PLANTS A clade of their own Standard textbooks on botany and phycology acknowledge a world view in which the green algae as a whole are no longer monophyletic to the exclusion of land plants (embryophytes; Raven et al., 1999 ? ; Graham and Wilcox, 2000a ? ; Graham et al., 2002 ? ; Judd et al., 2002 ? ). The molecular data on this point support the already strong case from ultrastructural data and are overwhelming, coming from the nuclear ribosomal repeat unit, mainly the small (18S) subunit, but including 5S and large subunit (26S) rDNA sequences as well (reviews in Chapman and Buchheim, 1991 ? ; McCourt, 1995 ? ; Melkonian and Surek, 1995 ? ; Chapman et al., 1998 ? ). Several plastid genes sampled broadly enough to address this question yield similar results: rbcL (Manhart, 1994 ? ; McCourt et al., 1996 ? , 2000 ? ; Delwiche et al., 2002 ? ) and small and large subunit rDNA (Turmel et al., 2002a ? ). Other molecular synapomorphies supporting the charophyte clade (though not uniformly present or sampled in all groups) are genomic features not found in the chlorophyte clade: transfer of the tufA gene from the plastid to the nucleus in the common ancestor of some charophyte algae (Zygnematales, Coleochaetales, Charales) and