Biological nitrogen fixation has been the subject of a number of reviews in this series during the last 10 years (35,36,40,110,128). During this period, beginning with the first successful experiments with cell-free extracts of nitrogen-fixing bacteria (41), rapid advances in the understanding of the process of nitrogen fixation have been made. It is now clear that the enzyme complex in all of the organisms studied is similar in nature, in the factors necessary for activity, and in being able to catalyze a number of energy dependent reductions. There are still many important problems to be solved. For example, the role of each of the two components of the complex is not known and the high requirement for energy from ATP is not understood. Most recent reviews, including those from authors other than the above (37, 62, 66, 106), have included references to symbiotic systems. It is now possible to devote an entire review to the biochemistry of symbiotic nitrogen fixation, although most of the information has come from the study of only four legumes-soybeans, lupines, serradella, and peas. Biochemical informa tion from nodulated nonlegumes is still slight and has been adequately re viewed recently (35); it wi1\ therefore not be included in this review. Evans (60) and Bergersen (28) have recently reviewed work done on soybean nodules in their respective laboratories.In this review, the functioning of nodules wi1\ be considered and, with the exception of a brief discussion of the induction of nitrogenase, matters pertaining to the biochemical events involved in the infection of roots and the development of nodules will be omitted. For convenience the material is divided into sections, although all are interrelated. Some of the interrelation ships will be discussed, but to avoid unnecessary duplication, the reader will be left to deduce most of these for himself. The scheme which is presented at the end summarizes the author's understanding of the present state of knowledge.
NITROGEN-FIXING PROPERTIES OF INTACT NODULESThis section introduces selected references which give a general account of the factors involved in the functioning of intact nodules.