The question of symbiosis of nitrogen-fixing bacteria with each other and with other organisms offers an inviting field for research. There is evidence that this factor may have considerable importance. Beijerinck and Van Delden[3] early recognised that _Azotobacter_ in mixed cultures fixed more nitrogen than in pure cultures. _Granulobacter_, an organism which they found to be commonly associated with _Azotobacter_ in crude cultures, appears to increase its nitrogen-fixing powers (Krzeminiewski).[41] It was also found by Hanzawa[31] that a greater fixation of nitrogen was obtained when two strains of _Azotobacter_ were grown together. A symbiosis between _Azotobacter_ and green algæ has been described, and will be further discussed by Dr. Bristol. It is likely that this association may be of importance under suitable conditions on the soil surface where the algæ are exposed to light. The combination of elemental nitrogen is an endothermic process which requires a very considerable amount of energy for its accomplishment. This fact is well illustrated by the various commercial processes in use for fixation of atmospheric nitrogen. The nitrogen-fixing bacteria obtain this energy from the carbon compounds in the soil. A number of compounds were compared as sources of energy by Löhnis and Pillai,[47] who tested their effect on the amounts of nitrogen fixed by _Azotobacter_ in culture. It was found that mannitol and the simpler sugars give the best results as sources of energy, but that other organic compounds can also be used. Mockeridge[51] has adduced evidence that ethylene glycol, methyl-, ethyl-, and propyl-alcohol, lactic, malic, succinic, and glycocollic acids could also be utilised. Since so large a part of the organic matter added to soil is in the form of celluloses, it is of great importance to ascertain how far these compounds and their decomposition products can be utilised in nitrogen fixation. Stubble, corn-stalks and roots, oak leaves, lupine and lucerne tops, maple leaves, and pine needles may all serve as useful sources of energy to nitrogen-fixing organisms in the soil. Pure cellulose cannot apparently be used as a source of energy, but when acted upon by cellulose decomposing organisms, it becomes available as a source of energy. Hutchinson and Clayton, at Rothamsted, found that a fixation of nitrogen could be brought about by mixed cultures of _Azotobacter_, and of the cellulose attacking _Spirochæta cytophaga_, when grown in cultures containing pure cellulose. It is not known how far cellulose decomposition must proceed to produce an effective source of energy, nor what are the substances thus produced that are utilised. This point will not be decided until something more is known of the course of changes in the breaking-down of cellulose in the soil.