144 Butler.— A Study on Gummosis of Prunus and Citrus , with 
In the trunk and limbs, besides the gum pockets just described, 
we also find, corresponding to the pustules, large almond-shaped areas 
of susceptible tissue being laid down, degenerating somewhat and then 
lignifying. Over this tissue normal xylem may be formed, or the cells, while 
regularly laid down, may remain comparatively thin-walled and show rows 
of gum pockets wherever an increase in the rapidity of growth takes place. 
It should be noted, en passant , that localized areas of rapid growth occurring 
quite close together may be frequently observed in the xylem without the 
accompaniment of gummous degeneration, thus proving that the diseased 
trees function most irregularly. 
Text*fig. 2. Cross-section through edges of an almond-shaped area, showing development of lig- 
neous parenchyma and one of a row of gum pockets beyond which lies the cortex (not shown). 
When the xylem appears discoloured to macroscopic vision this is due 
largely to infiltration, though it appears that, in some cases, the middle 
lamellae of the vessels show gummous degeneration, as likewise those'of the 
wood fibres. 
In the bark the tissues die from the periphery inward, the cells being 
largely occluded by a homogeneous yellowish brown mass, and are cut off 
by suber or a proliferation of the subjacent cortical cells. 
4. Cause of Squamosis. 
Gummosis, as we have seen, may be brought about by various agencies : 
in the case of squamosis, however, we are still ignorant as to the cause 
of the malady. It may be an autogenous disease, though it would appear 
from the manner in which it develops, especially on the trunk and limbs, 
that it is due rather to bark-binding, the particular symptoms of the malady 
being induced by the manner in which the pressure on the cambium is 
released, the release occurring at a time when the affected tree was growing 
rapidly and the tissues were quite sapid. This explanation may perhaps also 
apply in the case of the shoots, seeing that the irregularities of growth 
distinctly shown in the older parts of diseased trees may also be seen 
in them. Added the death of the bark from fungous or other injury intro- 
ducing changes of pressure on the cambium and we can conceive how the 
