9 
151 
transparent spot, which, under magnification of 1,000 diameters shows 
the colony of bacteria already formed. There is now manifest about 
the colony a hypertrophy of the elements which may become more or less 
profoundly altered. As the colony enlarges the hypertrophy increases. 
The tubercle grows until in time it cracks through the exterior bark. 
When the tubercle is formed its growth is not usually arrested, but it 
continues to increase more or less in size each year, often attaining a 
diameter of 0.01 to 0.02 meter (two to four fifths of an inch). The tu¬ 
bercle is formed in the spring* during the heat of the summer the 
hypertrophy is arrested, but the colony of bacteria increases consid¬ 
erably. Then, during the autumn renewal of growth the hypertrophy 
begins again. 
The irritation or stimulation caused by the presence of the bacillus, 
so far as our observations have extended, produces only a localized 
growth of tissue. There is scarcely more evidence of a general or con¬ 
stitutional disorder of the sap of the tree affected than is produced in 
the oak under the action of the Cynipidae. The stimulation of the af¬ 
fected branch scarcely extends beyond the node or iuternode where the 
swelling occurs. The impoverishing action of this growth, however, is 
often plainly observed on the entire twig beyond the tubercle. The 
limb sometimes shows a marked reduction in diameter, though perhaps 
green and healthy in other respects. In a majority of cases the en¬ 
largement only involves one side of the branch. It is not uncommon 
to find two centers of inoculation producing coalescing tubercles ) but 
the distinction of origin is rarely lost. So far as I am aware progress¬ 
ive death of the limb below the point of infection, as is the case with 
pear-blight, never occurs. There is no analogous and general patho¬ 
genic degeneration of the tissues as found in limbs affected by that dis¬ 
ease. 
From some of mv first observations, where I found the tubercle devel- 
oping at the node of the limb, I thought it likely that inoculation had 
been effected by means of the axillary buds. Later, however, many tu¬ 
bercles were noticed, located upon iuteruodes, and having no connection 
with the leaf axil. This has left the method of entrance of the Bacillus 
obscured, unless, perchance, it be through the growing point, and contin¬ 
ued growth has left it within the iuternode or at the node. This expla¬ 
nation seems more probable than that the organism has directly pene¬ 
trated the bark of the branch. It is also rather indorsed than other¬ 
wise by the fact that whenever mechanical injury has occurred to the 
bark, laying bare the cambium tissue, the tumors are often unusually 
numerous. They are most common where a bud or leaf or branch has 
been broken off', or where some injury or splitting of the branch has 
occurred. In one case observed, where a branch had been split for a 
few inches, three distinct centers of inoculation were seen at the edge 
of the ruptured bark within the distance of 2 inches. Undoubtedly, 
however, inoculation may occur through slight cracking or other injury 
of the bark. 
