These bacteria were finally isolated and studied in pure culture by Beijerinck. Since then a very great deal of literature has accumulated on the subject of the nodule-producing bacteria, which it is impossible to deal with in a small space. The nodule organism, _Bacillus radicicola_, when grown on suitable media, passes through a number of different changes in morphology. The most connected account of these changes is given in a paper by Bewley and Hutchinson.[4] In a vigorous culture the commonest type is a rod-shaped bacillus which may or may not be motile. As these get older they often become branched, or irregular in shape, the formation of these branched forms being perhaps due to conditions in the medium. These irregular forms, known as “bacteroids,” are a characteristic type in the nodules. Their production in culture media has been found to be stimulated by sugars and organic acids such as would occur in their environment within the host plant. In the older rods and bacteroids the staining material becomes condensed into granules, and finally the rods disintegrate or break up into coccoid forms. By suitable culture conditions, Bewley and Hutchinson obtained cultures consisting almost entirely of this stage. If such a culture be inoculated into a fresh medium rich in sugar, the swarmer stage appears in great numbers. These swarmers are very minute coccoid rods, ·9 × ·18 in size, that are actively motile. They apparently develop later into the rod stage. [Illustration: FIG. 6.--_Bacillus radicicola._ Stages in the life cycle. (After HUTCHINSON and BEWLEY.) Motile Rods Vacuolated Stage “Swarmers” “Bacteroids” “Pre-swarmers”] Very little is known as to the life of the organism in the soil. It is able, however, to fix nitrogen in cultures, and it has been claimed[35],[48] that it can do so in the soil outside the plant, so that it is possible that we must take it into consideration in this connection. More knowledge is needed as to the optimum conditions for the growth of the organism in the soil. It seems to be more tolerant of acid soil conditions than _Azotobacter_. The limiting degree of acidity has been found to vary among different varieties of the organism from P_{H} 3·15 to P_{H} 4·9.