Some readers do seem to care strongly about the plural spelling of
stemma. Dictionaries such as the OED offer three standard meanings (the simple eye of certain insects / the record of an ancient Roman genealogy / the tree diagram showing the relationships among manuscripts of a literary work) and the standard plural spelling "stemmata." On the Macro-Typography website,
stemma is being used in a fourth, novel meaning, as a term for any graphical representation of any branching inter-relationships. It is a sense that Karl-August Wirth used in an article in German reviewing four typical medieval graphic forms used in educational texts: the table, stemma, arbor and rota. This usage of
stemma is preferable to "tree." The objections to tree are that the word is misleading, especially because it is loaded in favour of twig-like graphic patterns, but also because it has generated a long history of confusion about that figure's "correct" orientation. A broader term would be useful. But is "stemma" in this sense ready for prime time? With a plural that is drawn from Greek, stemma is bound to remain a term of scientific and scholarly jargon. Diagram, also of Greek origin, would not be a term in international standard English if its plural were still "diagramata". We need to anglicize stemma with a regular English plural, "stemmas", if we hope to bring the term into wider currency. The spelling "stemmata" will remain in place on the Macro-Typography website, because the audience there is mainly of historians, but expect the anglicized plural "stemmas" for wider audiences interested only in graphic design.