I.  Hagemann, ?On subspecific taxa of Hypericum empatrifolium (Hypericaceae) from Crete.?, Pl. Syst. Evol., vol. 155, pp. 165-187 (1987) - illus., map., 1987.

I.  Hagemann, ?Wuchsformenuntersuchungen an balkanischen Hypericum-Sippen. (Investigation of the growth form of Hypericum taxa from the Balkan Peninsula.)?, Bot. Jahrb., vol. 102, pp. 239 - 254 (1981) - illus., 1981.

I.  Hagemann, ?Wuchsformenuntersuchungen an zentraleuropaischen  Hypericum- Arten. (Analysis of growth forms of central European  Hypericum species.)?, Flora (Germany), vol. 173, pp. 97-142 (1983)- illus., 1983.

I.  Hagemann and Meusel, H., ?Hypericum triquetrifolium Turra, ein Wurzelspross-Geophyt Wuchsform und Verbreitung. (Hypericum triquetrifolium Turra, a perennial with shoot bearing roots, growth form and distribution.)?, Flora (Germany), vol. 175, pp. 385-405 (1984) - illus., maps., 1984.