1/8 FIXATION OR ASSIMILATION OF NITROGEN 



All the organisms belonging to the genus Azotobacter 

 are extremely variable in size and shape. The typical 

 form is a short oval cell from 4 to 6 /^ long and 3 to 4 # 

 broad (Fig. 32). Under certain conditions these cells 

 become thicker and rounded into a coccus, two of which 

 are usually found united a diplococcus. They appear 

 also as streptococci, and can divide 

 in two or three directions so as 

 to form sarcina - like " packets " ; 

 the latter are especially abundant 

 in old cultures. Spores are not 

 known. The cell walls are often 

 thick and slimy and stain well with 

 methylene blue or gentian violet. 

 FIG. ^.-Azotob^ter chroo- The protoplasm within contains 

 small refractive granules and usu- 

 ally a vacuole. Iodine stains the organisms a yellowish 

 tint. Four or five species or varieties have been de- 

 scribed. Although they differ in nitrogen fixing power, 

 they are probably modifications of one organism. The 

 following are the best known : 



A. chroococcum, Beijk. This, the first species isolated, is very 

 widely distributed in field and garden soils. It produces after a 

 time a brown or blackish coloration on solid substrata and on 

 the surface of liquid media. In young cultures the cells are 

 motile, short, plump rods, 4.5 to 7 /a. long, and 3 to 4 /* broad, 

 generally in pairs. On agar-mannite plates (Ex. 91) at 30 C. 

 the colonies are pasty at first, round, white, usually with dark 

 concentric rings in the middle : later they become dark brown, 

 and may grow to be 5 or 6 mm. in diameter The cells in three 

 or four days are short rods with rounded ends, often in pairs, 

 4 to 5 // in diameter; others are longer oval rods, 3 to 6 /* 

 long and 2 to 3 /a thick. In a week or ten days they may 



