A long controversy has been held as to whether the nodule organisms found in different host-plants all belong to one species, or whether there are a number of separate species, each capable of infecting a small group of host-plants. As the term “species” has at present no exact meaning when applied to bacteria, the discussion in this form is unlikely to reach a conclusion. The evidence seems to show that the nodule organisms form a group that is in a state of divergent specialisation to life in different host-plants, and that this specialisation has reached different degrees with different hosts. Thus the organisms from the nodule of the pea (_Pisum sativum_) will also produce nodules on vicia, Lathyrus, and Lens, but seem to have lost the ability normally to infect other legumes. On the other hand, the bacteria from the nodules of the Soy Bean (_Glycine hispida_) have become so specialised that they do not infect any other genus of host-plant, and soy beans are resistant to infection by other varieties of the nodule organism. Burrill and Hansen,[6] after an extensive study, divided the nodule bacteria into eleven groups, within each of which the host-plants are interchangeable. The existence of different groups of nodule organisms has been confirmed by the separate evidence of serological tests (Zipfel, Klimmer, and Kruger).[40] The results of cross-inoculation tests have sometimes been conflicting. It seems, indeed, that the host-plant has a variable power of resisting infection, so that when its resistance is lowered it may be capable of infection by a strange variety of the nodule organism. The question that has thus arisen of the ability of the legume to resist infection is of fundamental importance, and its elucidation should throw light on the relation of plants to bacterial infection as a whole. The stage of the organism that infects the plant is not at present known. It may be supposed that it is the motile “swarmer.” The entry is normally effected through the root-hairs. The hair is attacked close to the tip, and an enzyme is apparently produced which causes the tip to bend over in a characteristic manner. The organisms multiply within the root hair and pass down it, producing a characteristic gelatinous thread filled with bacteria, in the rod form. This “infection thread” passes down into the cells of the root tissue, where it branches profusely. In young stages of nodule formation the branches can be seen penetrating cells in the pericycle layer. Rapid cell division of these root cells is induced. In the course of this cell division abnormal mitotic figures are sometimes found, such as occur in pathological growths. The cells push outward the root cortical layer, and so form a nodule.