Symbiotic Nitrogen Fixation in Non-Leguminous Plants. II. The influence of the inoculation density and external factors on the nodulation of Alnus glutinosa and its importance to our understanding of the mechanism of the infection

1. The relation between inoculation density and the number of nodules in watercultures of Alnus glutinosa is in accordance with the hypothesis that the infection occurs through a chance contact of the endophyte with a limited number of preformed foci. 2. The influence of some external factors on the formation of nodules is principally the same as has been observed in Leguminous plants. Nitrates and ammonium salts inhibit nodule formation while fosfates and good light conditions are necessary. 3. The formation of nodules is inhibited by the presence of sugars in the nutrient solution. 4. A further analysis of the mutual effects of inoculation density and light intensity on the formation of nodules suggests that the light intensity influences the number of foci.