1. Ahmadjian V and Jacobs J B 1983 Algal-fungal relationships in lichens: recognition, synthesis, and development. In Algal Symbiosis. Ed. L J Goff. Cambridge University Press, New York, pp 147–172.
2. Alexander V 1981 Nitrogen-fixing lichens in tundra and teiga ecosystems. In Current Perspectives in Nitrogen Fixation. Eds. A H Gibson and W E Newton. Australian Academy of Science Canberra, p. 257.
3. Allen E K and Allen O N 1965 Nonleguminous Plant Symbiosis. In Microbiology and Soil Fertility (Proc. 25th Annual Biology Colloquium). Eds C M Gilmour and O N Allen. Oregon State University Press, Corvallis. pp 7–106.
4. Ashton P J 1974 The effect of some environmental factors on the growth of Azolla filiculoides Lam. In The Orange River Progress Report. Ed. E M Zinderen-Bakker. Bloemfontein, South Africa, pp 123–138.
5. Ashton P J and Walmsley R D 1976 The aquatic fern Azolla and its Anabaena symbiont. Endeavour 35, 39–43.