Observations on Squamosis and Exanthema of the Citriis . 143 
epidermis apparently less brittle, for it rifts within the diseased areas and 
not at their boundary. The rifts are parallel to the axis of the shoot, more 
or less numerous, and inclined to be labiate. In these cases the pathogno- 
monic tissues form marked swellings on the shoots. 
When diseased limbs are observed in cross-section they will appear 
more or less zoned with brown, and rings of growth will be quite prominent. 
The dark zones indicate the beginning of a new growth annelid. In some 
cases, however, the zonation may be somewhat masked by a more general 
discoloration of the wood. The innermost zone of brown indicates when 
squamosis first affected the tree, but the innermost extent of wood dis- 
coloration should be considered as indicating a certain amount of centri- 
petal infiltration. In the youngest wood it will be observed that the pustules 
are composed of an outer layer of normal xylem enclosing a thin-walled 
xylem parenchyma ; the whole structure would remind a mycologist of 
a young sorus. When the shoots are examined in cross-section they may 
show a similar zonation to the older branches, but instead of many dark 
circles or partial circles, there will be generally only one, sometimes two, 
rarely more ; the bark shows infiltrated yellowish brown areas which appear 
to be cut off by suber, and, when the shoots are old enough, rings of growth 
are plainly discernible. 
3. Histology. 
I have mentioned that squamosis has characters in common with 
gummosis; we have also seen that it has characters particular to itself. In 
the following histological sketch, I shall consider squamosis in the former 
rather than in the latter aspect. 
Gum pockets are the histological characteristic of squamosis. As 
in gummosis, they are formed in a susceptible tissue, but do not develop 
to the same extent except in transitional stages. In squamosis the gum 
pockets are typically inextensive, and the medullary rays are not, as a rule, 
affected ; nor do the cells become totally destroyed to the same extent as in 
gummosis. The secondary and primary membranes are affected by gum- 
mous degeneration, but only a few cells suffer complete dissolution. The 
failure of the gum pockets to develop as in gummosis is undoubtedly due to 
the fact that conditions favourable to gummous degeneration are transient, 
as is shown by the gum in young pockets being largely insoluble and soon 
tinged with yellow. Furthermore, lignification soon begins in the susceptible 
tissue, and no sharp distinction between it and the xylem can well be 
drawn ; one might readily be deceived by the appearance into believing 
that it was xylem parenchyma that suffered gummous degeneration and 
not embryonic wood cells. 
Circular rows of the gum cavities are produced with considerable 
regularity at the beginning of every new annelid of growth, and it is, there- 
fore, to them that the zonation of the xylem is due. 
