E. B. FRED AND A. R. C. HAAS 635 



graph of one of the pea plants after a period of 42 days is shown in 

 Fig. 1. 



Immediately upon opening the uninoculated tubes, samples of the 

 soil were inoculated in tubes of clear nutrient broth. After incu- 

 bating for several days, the broth tubes were examined for the 

 growth of microorganisms. Whenever any growth was observed, the 

 contaminated tubes were included with the inoculated ones. 



The plants were removed from the tubes by a gentle stream of 

 water. It was found that in nearly every instance the plants in the 

 inoculated tubes made a slightly better growth as regards the length 

 of roots and tops than the plants grown under sterile conditions. An 

 effort was also made to grow plants in agar, but it was found 

 unsatisfactory. 



In the preliminary experiments, water-proof drawing ink was 

 rubbed with a smooth cloth into the etchings to bring out the details 

 better in the photograph. Fig. 2, A, shows the effect of the roots 

 upon the marble in the absence of bacteria; Fig. 2, B, shows the 

 effect when the bacteria are present. In the later experiments the 

 etchings were better defined for photographic purposes by carefully 

 filling the etchings with lamp black. Fig. 3, A, shows the effect 

 when the bacteria were absent; Fig. 3, B, shows the effect when the 

 bacteria were present. 



The slabs were placed in a row with those from the uninoculated 

 scattered among those from the inoculated tubes. A person unac- 

 quainted with the numbers of the slabs was then called upon to ar- 

 range the slabs in the order of their etching. In choosing a criterion 

 of etching, the emphasis was put upon the depth to which the etch- 

 ing had taken place. The amount of etching upon the entire pol- 

 ished surface of the slabs was also taken into consideration although 

 here the possibility of some roots not coming into close contact with 

 the marble becomes a factor. 



It was found that in every case the marble slabs that had the 

 greatest degree of etching were those taken from the tubes that 

 had been inoculated. It was also found that in many cases it was 

 possible to predict from the order of the slabs whether or not an un- 

 inoculated tube had become contaminated before the conclusion of an 

 experiment, for in this event such slabs would usually be classed 

 among the pieces of marble from the inoculated tubes. 



