28 Nebraska Agricultural Exp. Station, Research Bui. 12 
aging in that of 42 apparently sound trees examined, which 
were from 12 to 16 years old, only 3 were free from the 
fungus. In a number of cases infection was traced to cracks 
and other wounds in the trunk, several to pruning and other 
wounds in the limbs and a smaller number to injury to the 
roots near the surface of the ground. 
This led to omitting from the data any infection which 
could not be followed from the inoculation to the end of infec- 
tions except in a few cases specifically mentioned later, such as 
stub inoculations and inoculations in four trees which had 
been root pruned. 
GROWTH OF MYCELIUM IN THE WOOD 
INOCULATION IN TISSUES OF DIFFERENT AGES 
The growth of the mycelium within the tree varies with 
the age and condition of the wood. In tissues which are still 
active the hyphae are confined largely to the tracheal vessels 
and the medullary ray cells, altho the adjacent sclerenchyma 
cells are slowly invaded as the tissue dies. The hyphae ad- 
vance rapidly along the tracheae passing from one to the other 
and to adjacent cells thru the pits in the cell walls. This ex- 
plains why recent infections appear as long brownish-black 
threads in the wood. The tracheae offer little resistance to 
progress of the fungus lengthwise of the branch, but lateral 
progress thru the medullary ray cells and the adjacent scle- 
renchyma is very slow. 
The most rapid progress occurs in that part of the annual 
growth made early in the spring when the tracheal vessels are 
relatively large and abundant. Here many of the parallel 
tracheae communicate thru pits so that the progress laterally 
is also relatively rapid. The annual ring of growth on either 
side is protected from invasion to some extent by the barrier 
of small, thick-walled, nonpitted wood cells which form late 
in the season's growth. Tracheae in this region are fewer and 
somewhat smaller. However, the regions of the different an- 
nual growths communicate by means of the medullary rays 
and thru these are finally invaded. As the fungus goes deeper 
into the branch each succeeding year's growth up to four-year- 
old wood is found more susceptible. However, from here on 
there is practically no difference in the susceptibility of the 
tissue of different ages altho there is the same relative rate 
of progress in the different cells. This difference in suscepti- 
bility of tissues of different ages which varies in different 
varieties and individual trees of the same variety is no doubt 
