50 
Journal of Agricultural Research 
Vol. XXVI, No. 2 
phloem tissue in the external excrescence is generally found in a rather 
advanced state of gummous disintegration. While gummous lesions 
have been observed in this region in various stages, the incipient stages 
had to be studied from the lesions deeper in the phloem where it is rela¬ 
tively free of cork. 
LESIONS IN THE PHLOEM 
It will be remembered that the bast lying immediately outside of the 
wood has many gum pockets scattered through it. Plate 8 shows a 
cross section through phloem parenchyma with a very small lesion in it 
(B) . This lesion is surrounded by healthy tissue, which is more deeply 
stained, and beyond the healthy tissue there is another lesion (A). An 
examination of the lesion B will show it bordered by a layer of flat cells 
containing many granules. The center of the lesion is occupied by gum 
(C) , while the tissue of this, as well as the other lesion, consists of thin- 
walled, irregularly shaped cells, apparently undergoing a process of 
gelatinization. 
What happens to the healthy tissue when it becomes diseased may be 
more clearly understood when Plate 8 is compared with Plate 9, which 
shows a different plane of the same lesion, illustrating a later stage of the 
disease. The band of healthy tissue which in Plate 8 is seen separating 
the lesions at A and B is missing in Plate 9, its place having been taken 
by the thin-walled gelatinizing cells. 
An examination of numerous lesions would seem to indicate that the 
progress of this phase of the gall is as follows: 
The first indication of the disease shown by the affected tissue consists 
in a thinning of the cell walls. 
It would seem that in some cases the surrounding healthy tissue tries 
to check the advance of the disease by increasing the food content of 
the cells bordering the lesions, as may be concluded from the denser and 
more granular state of their protoplasm. This, however, is by no means 
a constant feature. 
The bordering cells, thus richly provided with food, begin, in some cases, 
to proliferate rapidly. This may be interpreted as an attempt to protect 
the healthy tissue from the oncoming disease, or at least to delay its 
advance by throwing into the diseased area, tissue at the rate at which 
it is consumed. This attempt is never successful. 
The cell walls in the interior of the affected tissue continue in the 
meanwhile to grow thin and stretch, and the contents of the cells continue 
to gelatinize. Soon the cells collapse, and, with the disorganized proto¬ 
plasm and cell sap, constitute the “gum’ , which fills the lesions. 
The nuclei seem to be more resistant to the effects of the disease, 
persisting in the cells at a time when the cytoplasm and cell wall are in 
an advanced state of gelatinization (PI. 8 and 9). 
No difference has been noted in the way in which the various phloem 
elements behave under the influence of the disease. When a medullary 
ray is affected its course is stopped short and its cells undergo the same 
process of proliferation, gelatinization, and, finally, gummous disintegra¬ 
tion, as has been described above. 
lesions in The wood 
In the gummous lesions in the wood a similar process is taking place. 
Plate 10, representing a cross section through the xylem, shows a series 
of confluent lesions extending diagonally across the field. The black 
