550 
Journal of Agricultural Research 
Vol. XIV, No. 12 
around the stem and the illustration shows plasmodia in the cambium 
on the side of the stem opposite the point of original infection. 
By this time the disease has passed beyond what may be designated as 
the early stages of infection. We have seen that the parasite passes suc¬ 
cessively through the outer layers of the secondary cortex, through the 
phloem region, into the cambium and finally even into the undifferentiated 
tissues beneath the cambium. We have also seen that it spreads to the 
sides as well as downward from the point of original infection, so that by 
the time the cambium is reached this little plug of diseased tissue has 
become much broader than it was when infection started. Up to this 
point the invading organism has followed no special course, but has pene¬ 
trated with almost equal rapidity in all directions through the bark. 
After reaching the cambium the plasmodia no longer penetrate the 
different tissues with equal readiness, but follow what is undoubtedly the 
path of least resistance. The intruder now becomes what may be termed 
a “cambial parasite.” Its further spread up and down and around 
the stem is through the cambium and the layers of undifferentiated 
cells immediately adjacent. These cells are young and rapidly grow¬ 
ing; their cellulose walls are still very thin and probably offer little 
resistance to penetration. It may also be that in this region of most 
vigorous growth there is a more abundant supply of food materials than 
in the older tissues. It spreads through the cambium around the stem 
until it reaches the side of the stem opposite the point where it originally 
entered the bark. It also spreads up and down the stem, forming a 
cylinder of infected cambial tissue. The distance that the fungus travels 
in its spread through the cambium seems to depend largely on the con¬ 
dition of the host, especially as regards age and rate of growth. If the 
host plant is young and growing vigorously it may infect the cambium 
for a considerable distance from the point of original penetration. The 
writer has observed cases where the cambial infection has extended for as 
much as 6 inches up the stem from the point where it originally entered. 
Plate 67, B, shows a longitudinal section through a young stem having 
a diseased cambium. It should be noted that infection has extended far 
beyond the region of swelling. The plasmodia are either in the cambium 
cells or in the cells adjacent to it. The cortex is free of infection for a 
long distance up the stem. Plate 71, A, shows the distribution of young 
plasmodia in the cambium of an old stem. 
One might think that the disease would continue to spread until the 
cambium of the entire plant would be infected. This, however, is not 
the case. At first the spread is very rapid, but during the later stages 
of the disease it becomes slower and slower and finally almost ceases. 
Although, as above noted, the swelling does not always extend as far as 
the cambial infection; this infection, nevertheless, determines very largely 
the length of the spindle. If the cambial cylinder becomes infected for 
