Hermann Theodor Geyler (15 January 1835 at Schwarzbach – 22 March 1889 at Frankfurt-am-Main) was a German botanist, specializing in paleobotany.
Geyler was the son of Hermann Gustav and d’Adelgunde Schiller (née von Schillershausen).
[1] From 1864 to 1867, he worked in Basel, Switzerland with Swiss botanist Carl Eduard Cramer, a specialist of fossil woods.
[2] During this period, Geyler greatly expanded their collection of plants to include more than 4,000 varieties.
[1] Among the species he described is Onychiopsis elongata, a fossil fern of the family Dicksoniaceae, from the Jurassic period.