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



received equal quantities of seed. Three strongly parasitic strains 

 of Pythium were used, and a total of 12 units of jack pine and an 

 equal number of western yellow pine was inoculated with 12 inter- 

 spersed units of each species as controls. The mean results are as 

 follows : 



Pinus banksiana. — Inoculated plats : Emerged, 64.2±4.9 ; died during the next 



17 days. 25 per cent. Control plats: Emerged, 85.5±3.6; died during 



the next 17 days, 13 per cent. 

 Pinus ponderosa. — Inoculated plats : Emerged, 34.6±1.8 ; died during the next 



9 days, 39 per cent. Control plats : Emerged, 45.4±1.3 ; died during the 



next 9 days, 25 per cent. 



The difference in emergence apparently clue to the inoculation is 

 for the first species three and one-half and for the second nearly five 

 times its probable error. While, of course, 12-unit means are in- 

 sufficient to allow the calculation of entirely reliable probable errors, 

 they give some idea of the amount of variability of the results and 

 the confidence which can be given them. It is impossible to give any 

 such expression apptying directly to the damping-off percentages and 

 their differences, for the reason that averages for this item have 

 been made in the writer's work not by averaging the percentages for 

 the individual units but by totaling all the seedlings and the dead 

 seedlings on the plats to be averaged and recalculating the percent- 

 age from these figures. This seems the only safe method, as other- 

 wise units in which germination is low by accident or by the action 

 of parasites will be given an influence on the resultant mean entirely 

 disproportionate to the number of seedlings which they contain. 

 Average values for damping-off obtained by this method and by the 

 method of averaging the percentages of the individual plats or pots 

 are often very different; it not uncommonly happens that the units 

 in which germination is lower than the average also have especially 

 high damping-off percentages, both phenomena being caused by an 

 unusual activity of parasites. In such case to average the percentages 

 themselves usually gives a higher damping-off figure than to total 

 the seedlings for the different units and redetermine the percentage, 

 and the latter practice is considered the better. In the present case 

 the differences in the damping-off percentages obtained by the two 

 methods are not great. The figures obtained by averaging the per- 

 centages of the ultimate units are as follows : 



Pinus banksiana. — Inoculated, loss 30.9±5.0 per cent; controls, loss 13.2±2.8 



per cent. 

 Pinus ponderosa. — Inoculated, loss 40.0±5.1 per cent; controls, loss 24.1 ±3.3 



per cent. 



The differences between the inoculated and control plats in damp- 

 ing-off percentage were for the first species a little over and for the 

 second a little under three times their indicated probable errors. 



