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An organismal concept for the Late Devonian/Mississippian hydrasperman seed fern Tetrastichia bupatides is developed from specimens collected at Oxroad Bay, East Lothian Scotland and Ballyheigue, County Kerry, Ireland. Specimens include interconnected fragments of stems, frond rachides, pinnae, pinnules, roots, pollen organs with enclosed pre-pollen, and cupules, as well as dispersed ovules. Both morphological and anatomical features are documented. The plant produces an unbranched, upright stem with a branched taproot, and small adventitious roots at the base of the stem. Stems have a mesarch actinostele with sympodial protoxylem architecture. Phyllotaxis ranges from helical to opposite/decussate, with planar fronds that typically fork twice at the base, and then produce pinnules of the Rhodea -type. Compact aggregate pollen organs are attached distally on secondary rachides and are constructed of cruciately forking axes that terminate in inverted, round, simple synangia of six elongated microsporangia attached to a basal pad of tissue. Ovulate cupules of the Calathospermum fimbriatum type are attached at the base of the frond. Ovules possibly could be Salpingostoma dasu or Eospermum oxroadense. Tetrastichia bupatides is now one of the most completely reconstructed of all Devonian-Mississippian hydrasperman seed ferns, and the most ancient gymnosperm for which the pattern of rooting has been established. The occurrence of a taproot at the base of the stem suggests that the plant may exhibit bipolar growth derived from a cotyledonary embryo. • An organismal concept for the seed fern Tetrastichia bupatides is developed. • Structure of fronds, pollen organs, and the rooting system are described. • Interconnections of organs are documented. • Relationships among the most ancient hydrasperman seed ferns are evaluated. [ABSTRACT FROM AUTHOR]