1. Ahmadjian, V.: The Lichen Symbiosis. Waltham-Toronto-London: Blaisdell 1967.
2. Ahmadjian, V., Heikkelä, H.: The culture and synthesis of Endocarpon pusillum and Stanrothele clopima. Lichenologist 4, 259?267 (1970).
3. Fogg, G. E.: Extracellular products. In: Physiology and biochemistry of algae, p. 475?489 (Lewin, R. A., ed.). New York-London: Academic Press 1962.
4. Green, T. G. A.: The biology of lichen symbionts. Oxford University: D. Phil. Thesis 1970.
5. Heikkilä, H., Kallio, P.: On the problem of subarctic basidolichens I. Ann. Univ. Turku. A. II. 36, 48?74 (1966).