44 THE ROYAL SOCIETY OF CANADA 



Mounted in iodine-potassium-iodide solution, the azotobacter 

 stained golden brown, giving the glycogen reaction, a few of the rods 

 gave the bluish-black starch reaction, and the other forms did not 

 stain. 



When four weeks old, the growth on these cultures had become 

 variously coloured brown, and a thick, irregular, pasty ring was present. 

 There was still no growth or change of any kind in the old compost 

 culture, but a very light granular pellicle in patches had formed on the 

 surface of the unrotted compost, this being composed of fine fungous 

 mycelium, higher bacteria, and some small rod bacteria; body of medi- 

 um clear. 



Each culture was tested for presence of ammonia with Nessler's 

 solution, for nitrite with sulphanilic acid, and nitrate with phenol- 

 sulphonic acid, the result being positive but varying in strength for 

 ammonia and nitrate, and negative for nitrite in all cultures where 

 azotobacter had developed, and negative in each case for controls. 



Plate Cultures. 



An examination of the plate cultures made when they were seven 

 days old showed various types of bacteria colonies present, including 

 azotobacter, Ps. radicicola and others and some moulds. 



1. Surface loam plates showed an azotobacter count of about 

 400 per gram of soil. These colonies varied from 4-8 m.m. diameter 

 were raised, glistening, generally coarsely contoured, pasty-viscid, 

 colorless, and semi-transparent. 



Preparations examined under the microscope showed many of 

 these colonies to be mixed cultures of azotobacter and rod forms, both 

 varying in size and shape. 



Mounted in lodine-potas-iodide solution — Azotobacter stained 

 golden yellow; rods not stained. 



St. aq. fuchsin — Azotobacter appeared as irregular spheres, vary- 

 ing in diameter from 2//-10/^; some stained evenly dark; others evenly 

 light; most were encapsulated in a thick, slimy capsule and they occurred 

 in ones, twos, fours and irregular groups. 



Rod forms varied in size and shape, some being positive and others 

 negative to stain. 



2. Subsoil 18 inches. About 100 small azotobacter colonies on 

 plate, giving a count of 10,000 per gram of soil (abnormally large num- 

 ber when compared with other subsequent tests). Some were white, 

 pasty, raised, perpendicular at edge, contoured on surface in concentric 

 rings; the smaller ones, especially those most crowded together, were 

 dark Vandyke brown in color, but similar in shape to those described. 



