The effect of oxygen on the symbiotic fixation of nitrogen proceeding in root nodules has received little attention. One reason for this is that the existence of organisms fixing nitrogen anaerobically encourages the belief that oxygen can have little direct significance for fixation in general. Another reason is that until recently the effect of oxygen tension on symbiotic fixation could only be investigated by means of arduous and prolonged experiments involving the culture of nodulated plants under conditions of controlled access to oxygen, the interpretation of such experiments being complicated by the difficulty of distinguishing between functional and formative effects. Experiments of this type were carried out with legumes by WILSON and FRED (1937), and by BOND (1951) and FERGUSON and BOND (1954). The two latter investigations provided some evidence that over the range 1 to 20 per cent. increasing oxygen supply exerted a specifically beneficial effect on nodule fixation, a finding also reportes by MACCONNELL (1959) in similar experiments with Alnus.
Investigation of the effect on nodule fixation of factors such as oxygen tension has been greatly facilitated by the 15N technique, by means of which fixation can be measured in short-term experiments with detached nodules, as first demonstrated by APRISON and BURRIS (1952) for legume nodules and subsequently by BOND (1955, 1957b) for nodules of AZnus, Myrica, Hippophae and Casuarina. These non-legume nodules retain the faculty of fixation more strongly after detachment from the plant than do those of legume type (BOND 1958) and are thus particularly well-suited for use in experiment.