The number of putative xylem orthologs in different plant groups. Loblolly pine xylem unigenes (18,320) were blasted using tblastx against the unigenes of white spruce and sitka spruce, and the gene models of poplar, Arabidopsis, rice, Selaginella and moss. The number of putative xylem orthologs common to different plant groups is presented at two E-value cut-offs (1e-50 and 1e-5). Conifers 1: loblolly pine plus white spruce; Conifers 2: conifers 1 plus sitka spruce; Woody plants: conifers 2 plus poplar; Seed plants 1: woody plants plus Arabidopsis; Seed plants 2: seed plants 1 plus rice; Vascular plants: seed plants 2 plus Selaginella; Land plants: vascular plants plus moss. The xylem orthologs shared in conifers 2, woody plants, seed plant 1, seed plants 2 and vascular plants represent genes specific to conifers, woody plants, seed plants, lower seed plants (lacking secondary xylem) and vascular plants, respectively. The high proportion of xylem orthologs homologous to moss suggests that the ancestral xylem genome was largely present in non-vascular plants.