MORPHOLOGY OF THE COLONIES 



299 



The zone of optimum temperature (Zipfel) is between 18 and 20 

 C.; the limits of growth are 3 and 45 C.; and the upper limits of 

 life are from 60 to 62 C. Burrill gives a considerably higher 

 optimum temperature 25 to 28 C. Grieg-Smith found the best 

 temperature for the production of slime to be 22 C. with most organ- 

 isms and 26 C. for one obtained from Robinia. The organism is 

 aerobic, and he found that diffused sunlight of the laboratory is not 

 harmful; even exposure to direct sunlight for several months with- 

 out transferring did not k'ill the organisms when grown upon favor- 

 able media with precautions to prevent evaporation. 



Ball found that the organism endures at least two years in dry 

 soil. Harrison and Barlow found that the limit of viability on ash- 

 maltose-agar varied somewhat, but in the majority of cases it was 

 about two years. How long the organism will exist in a soil under 

 field conditions is not yet known, but practical observations indicate 

 that it must be many years. 



FIG. 38. Young nodule magnified, showing affected root hair and same root hair 

 more highly magnified. (After Atkinson.) 



Morphology of the Colonies. Two types of colonies appear on agar 

 plates buried and surface colonies and are thus described by 

 Burrill and Hansen: 



