Symbiotic Nitrogen Fixation in Non-Leguminous Plants. IV. The Influence of some environmental Conditions on different Phases of the Nodulation Process in Alnus glutinosa

1. The influence of external circumstances on nodulation in alder is studied by restricting the comparison of different environmental circumstances to certain periods of the nodulation process. 2. The number of nodules depends on the light conditions and the presence of nitrates during or shortly before the inoculation period, but even after this period these environmental factors may inhibit the formation of nodules. 3. The growth of the nodules especially depends on the environmental conditions (light, nitrate) after the infection has occurred. 4. The absence of KH2P04 in the nutrient solution has no effect during or before inoculation, but inhibits nodule formation during the week immediately following the inoculation period. The absence of phosphate during the later weeks does not influence either the number of nodules or their growth. 5. It is suggested that the number of nodules not only depends on the infection of preformed foci but that during a subsequent phase of nodule initiation it is decided which of the infected foci will develop into nodules.