26 BULLETIN 722, U. S. DEPARTMENT OF AGRICULTURE. 
in degree of injury with an increase in the degree of infection and with 
‘increased age. With increase in age the cumulative chances for 
infection due to injuries of all kinds, and especially to branch stubs, 
increase appreciably, and it is but natural that the older trees bearing 
many injuries and a high degree of injury should show an equally 
high degree of infection. 
The two types of stand compared on the basis of the amount of 
injury are found to vary but little in the general relation between 
the degree of infection and the degree of injury. In both types the 
groups of higher rot percentage also show a higher degree of injury, 
and similarly the groups of lower rot percentage show a smaller degree 
of injury. In both sites the infections traced to branch stubs bore 
the largest percentage and the frost cracks the smallest. Broken 
tops in both instances camé second in importance. 
In the southwestern-slope type (Table IV), the percentage attrib- 
uted to broken tops was larger than in the river-bottom type (Table 
IV), due to the more exposed location and to the older stand. Infec- 
tions traced to branch stubs in the slope type equaled 72.8 per cent 
of the total, while in the river-bottom type it equaled 91.5 per cent. 
It would appear that in spite of the younger age class the river- 
bottom type developed more infection-producing branch stubs than 
the other. A reason for this may be found in the fact that the 
crowded, suppressed condition of the river-bottom stand was much 
more favorable to the infection of branch stubs than the other more 
open type of stand. The high proportion of branch-stub infections 
to injury infections can be partly explained by the fact that the 
trees of the river-bottom type, bemmg younger, had fewer injuries. 
In the slope type the largest amount of injury was found in the 
oldest age class (201 years and older), and in the river-bottom type 
it was also found in the older age class (101 to 160 years). In the 
slope type 10 per cent of the trees were uninfected, and in the river- 
bottom type a much smaller percentage (3) was uninfected, showing 
by this comparison a more favorable environment for the attacking 
fungus in the river-bottom sites. 
The slope type exhibited more-frost cracks, broken tops, afd mis- 
cellaneous injuries to the stand than the river-bottom type, which is 
due partly to the older age and partly to the more exposed situation. 
The wind plays an important part in both the formation of frost 
cracks and in the broken-top condition of many of the trees. It was 
particularly interesting to note that most of the oldest and largest 
frost cracks were found to have formed in the hollows between the 
root spurs. This seems to be more general in the slope type, where 
the exposure to high winds and the height of the t.ees (in connection 
with low temperature) appears to play an important part in their 
