1. Allen, E. K., Allen, O. N.: Biological aspects of symbiotic nitrogen fixation. In: Encyclopedia of plant physiology, Vol. 8 (W. Ruhland, ed.), pp. 48?118. Berlin-Göttingen-Heidelberg: Springer 1958
2. Allen, E. K., Allen, O. N.: Non-leguminous plant symbiosis. In: Microbiology and soil fertility, pp. 77?106. Proc. 25th Ann. Biol. Colloq. Oregon State Univ. (C. M. Gilmour, O. N. Allen, eds.), 164 p. Corvallis: Oregon State University Press 1965
3. Allen, E. K., Gregory, K. F., Allen, O. N.: Morphological development of nodules on Caragana arborescens Lam. Canad. J. Bot. 33, 139?148 (1955)
4. Becking, J. H.: Interactions nutritionelles plantes-actinomycètes. Ann. Inst. Pasteur (Paris), Rapp. Gén. (Suppl.) 111, 211?246 (1966)
5. Becking, J. H.: Nitrogen fixation by non-leguminous plants. Stikstof 12, 47?74 (1968)