414 
Journal of Agricultural Research 
Vol. XXITT, No. 6 
more detailed investigation; but in view of the experimental diffi¬ 
culties already encountered in such studies with the comparatively large 
yeasts, the outlook is not very promising. 
DIFFERENT TYPES OF GROWTH OF AZOTOBACTER—COMPARISON 
WITH OTHER BACTERIA 
If the seven cell types characteristic of the life cycle of Azotobacter 
are separated and fully stabilized, they naturally exhibit pronounced 
differences in physiological and especially in cultural behavior. But 
our experiments revealed that there may be more than one type of 
growth pertaining to one type of cell form, due to different pigmentation 
of the bacterial growth. The large nonsporulating cells may grow white, 
yellowish, or brown, and the large sporulating cells either white, or 
yellow, or brown; but in these two cases stabilization of the differently 
colored strains remained more or less incomplete. On the other hand, 
the coccoid, the dwarfed, as well as the fungoid growths of Azotobacter, 
may show white, yellow-orange, or red pigmentation; and all these 
strains could be brought to a very marked stability. The small non¬ 
sporulating rods grew usually white, more rarely yellow; and although 
the small sporulating rods did not exhibit any pronounced pigmenta¬ 
tion, they, too, showed differences of growth which thus far have been 
accepted as sufficient for establishing different bacterial “species.” 
But even if only the pronounced and fairly constant differences in pig¬ 
mentation are taken into account, we are confronted by the truly aston¬ 
ishing result that Azotobacter may present itself in not less than 14 
types of growth, all so different from each other that, according to the 
customary methods of defining bacterial species, they all would have 
to be accepted as separate species belonging to five or six different 
genera. 
Before entering upon the characterization of these different types of 
growth of Azotobacter a summary may be given in Table I, showing how 
often each type of growth was observed in our experiments, how many 
transformations were effected in each case, and in what direction these 
transformations took place. Most of the dwarfed type strains grew 
yellow; therefore all of them are listed in one column. 
These 188 transformations effected among approximately 2,000 trans¬ 
fers may not appear veiy impressive. However, several points have to be 
considered in this respect in order to reach a fair valuation of these 
results. First, it has to be kept in mind that under the microscope the 
seven different cell types have been seen in all cultures, although the 
number of cells of different shape was frequently not large enough to 
permit separation by plating. Secondly, it frequently happened that 
when plates were made the changed cells returned to the original type 
of growth, or their colonies were at first so similar to the others that they 
escaped detection. And the third point is that the new strains, once 
established, repeatedly proved themselves very persistent; it was men¬ 
tioned above that retransformations took place occasionally not before 
five or more years had elapsed, and after hundreds of transfers had been 
made. It might be assumed that single-cell cultures should have given 
better results, but as pointed out before we do not share this view. 
As most of our experiments were made with Azotobacter chroococcum and 
A. Beijerinckii , their types of growth will always be discussed first, 
and then the differences obtained with regard to A. agile and A. vitreum. 
