E. B. FRED AND A. R. C. HAAS 633 



made it possible to make a selection of the germinating seeds and to 

 avoid the use of those which showed contamination. It should be 

 pointed out in this connection that the uninoculated tubes were tested 

 again at the close of the experiments for the presence of micro- 

 organisms. 



Two bacteria-free pea seedhngs were transferred into each vessel. 

 The seeds were placed directly over the slanted marble slabs. This 

 made it possible for the roots, as they grew downward, to come into 

 intimate contact with the polished surface of the marble. The seeds 

 were covered over with soil. Preliminary experiments showed that the 

 roots of pea seeds, unless covered over with soil, have a tendency to 

 grow along the surface and are tardy in penetrating the soil. This 

 condition may be due in part to the effects of the sterilization upon 

 the soil texture. 



The tubes were placed in the greenhouse and were kept at about 

 60-70° F. When the seeds in the tubes had grown for 7 days, sev- 

 eral of the tubes were inoculated. In the first experiments one-half 

 the tubes were inoculated with a water suspension from field soil. 

 Because of the great number of bacteria as well as the different types 

 added in this inoculum, it is not surprising that the plants with bac- 

 teria far exceeded in growth the plants without bacteria. Perhaps 

 the nitrifying bacteria caused this increased plant growth. To avoid 

 the action of certain organisms, e.g. nitrifying bacteria, nodule bac- 

 teria, and molds, another experiment was carried out in which the 

 soil was inoculated with a mixture of pure cultures of bacteria instead 

 of soil extract. The nitrifying bacteria, therefore, were purposely 

 omitted from this mixture. The following organisms were used : 

 Azotobacter, Bacillus aerogcnes, Bacillus coli, Bacillus commniiior "C," 

 Bacillus communis "B,^' Bacillus hartlebii, Bacillus avaniensis, Ba- 

 cillus mescntericus, Bacillus fluorescens liquefacieiis, Bacillus fluo- 

 rescens iwn-liguefaciens, Bacillus pyocyaneus, Bacillus proteus, Bacillus 

 prodigiosus, Bacillus tumescens, Bacillus subtilis, Cladothrix dichotoma, 

 and Cladolhrix oderifera. 



5 cc. of sterile water were added to each of the uninoculated tubes 

 in order to keep the moisture content of the soils similar in each 

 tube. 25 days after the seeds had been placed in the tubes, the 

 tubes were opened and the slabs and plants examined. A photo- 



