Martin's (1946) seed classification system has 10 types based on embryo and endosperm characteristics and two additional types based on seed size: dwarf (0.3–2.0 mm) and micro ( ≤ 0.2 mm). He listed 17 families and 12 genera (in five other families) as having dwarf seeds. Our recent discovery of morphophysiological dormancy in dwarf seeds of several taxa of Campanulaceae and one of Gentianaceae prompted an evaluation of dwarf seeds. Martin's paper contains 37 families with one to several small (0.3–2.0 mm) seeded species that he did not list as being dwarf. Comparison of Martin's dwarf families and the 37 small-seeded non-dwarf families revealed no consistent differences between the two groups in endosperm texture, seed-coat anatomy, embryo morphology, class of seed dormancy or phylogenetic position. Also, Martin's dwarf seeds include a variety of embryo morphologies. Consequently, we have revised Martin's key to seed types. The dwarf category has been removed and the micro category replaced by ‘undifferentiated’ to reflect the state of the embryo in fresh seeds. Further, the key now includes linear fully developed, linear underdeveloped, spatulate fully developed and spatulate underdeveloped seed types, which Martin illustrated but did not include in his key. In the revised key, all seeds are distinguished on the basis of embryo and endosperm characteristics.