Abstract
Tumor-like structures appeared on the roots of Medicago sativa, Alysicarpus vaginalis, and Trifolium pratense inoculated with a non-nodulating strain of Rhizobium trifolii or with irradiated cultures of either of two nodulating Rhizobium strains. The structures were composed of disorganized plant tissues which, on the basis of microscopic examination, were devoid of bacterial cells. Rhizobia which could nodulate legumes of one cross-inoculation group and which were able to induce formation of such tumor-like structures on plants of a second cross-inoculation group were isolated from extracts of these root growths. The apparent tumorogenic activity of some of the rhizobia, but not their nodulating capacity, was lost when the bacteria were transferred in laboratory media.