32 
BULLETIX 934, U. S. DEPARTMENT OF AGRICULTURE. 
remaining, giving the mean of the results in experiments 71 and 72, 
are shown graphically in figure 13. together with the results of some 
of Peltier's experiments in which other strains were used. Per- 
centages of seedlings damped-off after germination are not included 
in these and most of the other data on pines because the most viru- 
lent strains often entirely prevent germination, and no value for sub- 
sequent loss is obtainable. The grouping of most of the writer's 
strains at the least virulent end of the register (that is, the one with 
the highest number of living seedlings) is of some interest. The 
distributions based on the two experiments considered separately 
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Fig. 12. — Diagram showing the relation between damping-off of conifers (broken line) 
and soil acidity (solid line). The acidity of soil samples from the different nurseries, 
determined by Dr. L. J. Gillespie, is reported as P H 7, indicating approximate neutrality 
while P H 6 indicates ten, and P H o one hundred times as great a hydrogen-ion concentra- 
tion as P H T ; therefore the lower the hydrogen-ion exponent line, the greater the acidity. 
The seriousness of damping-off at each nursery is on an arbitrary scale in which nurseries 
with negligible loss are rated as 1, and the nursery which suffered most is rated as 10. 
These values are estimates, though for some of the nurseries extensive counts were 
available on which the estimates were based. 
agreed very well in this grouping. The minor group at the end of 
extreme virulence is not taken to indicate. an actual grouping but. 
rather, an artificial one. due to the fact that both the strongest 
strains and some less strong were thrown into the same group by 
the lack of additional seedlings for the stronger strains to kill. This 
lack of additional seedlings constituted a limiting factor. In other 
words, conditions favored damping-off even in these two experiments 
too much to permit completely differential results for the more viru- 
lent strains. Despite this artificial limit preventing the full vari- 
ability becoming evident, the coefficient of variability of the survivals 
