In the present study a verification is undertaken of the research published so far on the formation of root nodules in alder plants (Alnus glutinosa (L.) Vill.) as a result of a symbiosis between the host plant and an endophyte, probably an actinomycete. After a brief discussion of the growth of the endophyte within the cortex of the host root and of the morphogenesis of root nodules, the hypothesis to the effect that a root nodule is a modified lateral root was tested. This possibility had to be rejected and the conclusion was drawn that root nodules appear adventitiously in-between the normal points of lateral root production. As, from an anatomical point of view, a nodule shows a great deal of resemblance to a lateral root, the hypothesis was formulated that root nodules are adventitious lateral roots – induced by an endophyte – which develop into nodules following the penetration of their cortex by the endophyte. In order to imitate and study the reactions in the host plant set in motion by the endophyte, phytohormones were administered to whole plants as well as to explants of roots. Use was made of kinetin and the following auxins: indole-3-acetic acid (IAA), indole-butyric acid (1BA), naphtalene-acetic acid (NAA), and 2,4-dichlorophenoxy-acetic acid (2,4-D). The auxins and the mixture of 2,4-D and kinetin produced an induction of primordia; by means of IBA, moreover, a slight increase in the number of nodules in the inoculation experiments was obtained. Finally, some experiments with tetrazolium salts were undertaken in order to ascertain whether the reducing conditions which are essential for the reduction of atmospheric nitrogen are already present in young nodules. Seventeen days after the inoculation a reduction of tetrazolium could be observed on the vesicles.