[Henderson's] n., n.plu. the plant kingdom, which contains multicellular eukaryotic photosynthetic organisms and, in some classifications, the algae (unicellular and multicellular). It includes the bryophytes (mosses and liverworts), seedless vascular plants (ferns, club mosses, horsetails) and the seed plants (gymnosperms and angiosperms). The algae are sometimes placed in the Protoctista. see Appendix 1.