84 BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. 



phytes had been added and those which received no saprophytes 

 showed a slight increase, proportionally as well as in the absolute 

 figures. At the end of the 36 days the survivals on all the pots were 

 counted separately. The average number of seedlings per pot were 

 as follows : 



Without Pythium : 



Without saprophytes, 42.S±2.3 ; with saprophytes, 52.1±1.1 

 With Pythium : 



Without saprophytes, 30.1±2.'4 ; with saprophytes, 4S.1±1.4. 



The difference in the survivals in favor of the pots with sapro : 

 phytes in the first case is 9.3±2.5, three and two-thirds times its 

 probable error. In the second case it is 18.0 ±2.8, more than six times 

 its probable error. 



In general, it appears that in this experiment the inoculation of ster- 

 ilized soil with saprophytes gave the seedlings some protection both 

 against damping-off due to accidental infection with unidentified 

 parasites and from the additional loss caused by light inoculation 

 with Pythium debaryanum. The indication is, as would be expected, 

 that only part of the favoring influence of heat sterilization of soil 

 on introduced P. debaryanum is immediately due to the elimination 

 of competition with other fungi. If a mixture of different bacteria 

 and fungi had been added to each of the pots instead of but one or 

 two organisms to each 5-pot unit, the effect might have been more 

 marked. 



It will be noted that for all the groups (fig. 18) , whether with or 

 without Pythium inoculation and with or without added parasites, 

 the frequency polygon is asymmetrical, indicating by its shape, as 

 did the frequency polygon of survivals in pots inoculated with Rheo- 

 sporangium (fig. IT), that with infections which do not kill all of 

 the seedlings the selection tends to be by pots rather than by seed- 

 lings. In other words, in pots in which the parasites succeed in kill- 

 ing any of the seedlings, they usually kill a considerable number. 

 The tendency is illustrated not only by inspection of the graphs, but 

 by the variability of the different groups. The greater variability 

 in survivals between different pots was in both cases in the groups in 

 which both the damping-off after emergence and the survival per- 

 centages indicated the largest loss. The percentages of seedlings 

 damped-off during the entire 36 days following emergence and the 

 coefficients of variability of the survivals of the individual pots at 

 the end of that time are as follows : 



Without Pythium : 



Without saprophytes, 15.5 per cent damped-off; coefficient of variability, 



39±4.2 iter cent. 

 With saprophytes, 11.1 per cent damped-off; coefficient of variability, 

 28±1.G per cent. 



