MORPHOLOGY OF THE NITROGEN-FIXING ORGANISMS 271 



in nitrates and alkalies which would dissolve the pigments from the 

 body of the organism. But Omelianski and Sswewrowa are of the 

 opinion that althought in some cases the dark color of vegetable 

 soil may be due in a measure to the action of these microorganisms, 

 it would be a mistake to attribute it to this factor alone. Further- 

 more, it has recently been proved that the brown color of the 

 "nitre spots" is due to solvent and decomposing action of the 

 nitrates on the colored organic compounds of the soil, for they may 

 be produced at will in a rich greenhouse soil with an excess of sodium 

 nitrate, and this too in soils which have been rendered sterile with a 

 saturated solution of mercuric chlorid. 



Morphology of the Nitrogen-fixing Organisms. Of the many 

 different bacteria which have been isolated and proved to have the 

 ability to assimilate free nitrogen, ( 1 lostridium pasteurianum may be 

 taken as a type of the anaerobic and Azotobacter chroococcum as a 

 type of the aerobic. 



Clostridium pasteurianum is a short thick rod from 1.2 to 1.3 /x 

 in diameter and 1.5 to 2/j. long in the young cells; the older spore- 

 bearing cells take on a spindle shape. The bacteria stain a violet 

 brown with iodin. The spores when ripe are l.b> long and 1.3/x 

 broad and often lie in a roughly triangular covering. The ripe 

 spore escapes through the wall of the mother in a longitudinal 

 direction. Their germination is polar. 



Azotobacter chroococcum occurs ordinarily as diplococci or short 

 rounded rods 1 to 2/x thick and 1.5 to 3/z long, and according to 

 Prazmowski the microorganism first presents itself in its vegetative 

 stage as a bacterium, in the fruiting stage as a micrococcus, and 

 possesses a nucleus which functions in the same way as that of higher 

 animals. In the resting stage the nucleus assumes a globular form, 

 having a strongly refractive nucleolus with clearly differentiated 

 boundary layers. The individuality of the nucleus appears to be 

 practically lost at times, because of its relation to the cytoplasm. 

 The division of the nucleus marks the first stage of cell division. 

 According to Bonazzi the organism shows peculiar granulations 

 apparently not related to reproduction. These take the basic dyes 

 and are neither fats, glycogen, starch nor chromatin, but appear to 

 be of metachromatic nature and seem to have their genesis in the 

 nucleus. Their disposition in the cells is not constant but changes 

 in different individuals. Involution forms occur and cell division 

 is preceded by a simple form of mitosis. Some, but not all, varieties 

 have been observed to form spores. The volutin bodies within the 

 organism increase in number and size when the organisms are grown 

 on media rich in nitrates. Hills suggests that they may have some 

 relation to nitrogen-fixation, but his results appear to oppose this 

 view; whereas the addition of nitrates to a medium greatly increased 

 the reproduction, it very materially decreased the physiological 



