Observations on Squamosis and Exanthema of the Citrus . 129 
and the extent of this tissue formation can always be traced by this character, 
as well as by the asymmetry of the cells due to the pressure exerted upon 
them by the gum, which occupies more space than the cells from which it 
is derived. 
Coincidently with the lignification of the susceptible tissues, starch 
may appear within the cells ; it will be observed also in the cells within and 
bordering the gum pocket. I have never observed any quantity of starch 
in these cells myself, though there is no reason for supposing that it cannot 
accumulate to a considerable extent in them. The starch is laid down in 
these tissues when the trees are accumulating food reserves ; its production 
cannot be considered, therefore, a pathological phenomenon. 
Before lignification sets in in the susceptible tissues the gum may undergo 
changes. I have already pointed out that it becomes yellowish in the more 
highly developed gum pocket and at the same time less soluble than when 
colourless ; it will also be remembered that at this stage it no longer stains 
with Bohmer’s haematoxylin. I may now add that this loss of power 
of fixing haematoxylin is brought about by a gradual increase of affinity for 
lignocellulose stains, and a further decrease in solubility. Finally, after 
lignification of the enclosing cells has taken place, the gum becomes appa- 
rently quite insoluble, and takes the lignocellulose stains as well as the xylem, 
or even better. 
The changes that take place in the gum parallel very closely those 
that accompany the lignification of the cell-walls, and we may draw the con- 
clusion that, whether we are dealing with the changes that take place in the 
cell-walls of the susceptible tissue that do not degenerate into gum, or with 
the gum that has originated from the cell-walls of certain of them, the 
ultimate changes are similar in both cases ; the cell-walls lignify and the 
gum assumes all the characters of lignocellulose. 
As soon as the susceptible tissue ceases to be affected by gummosis the 
conditions favourable for its development no longer exist, and the cambium 
assumes normal activity. Healthy wood cells are formed once more, and 
a certain development of xylem may have enclosed the gum pocket and 
surrounding tissues before lignification has set in in them (PI. X, Fig. 1). 
6. Cause of Gummosis. 
Primus and Citrus species, broadly speaking, are susceptible to gum- 
mosis whenever conditions are favourable for active growth of the cambium. 
It will be remembered that the disease may be produced either autogenously 
or by external agents. The autogenous form of gummosis appears to be 
confined very largely to the cherry and the lemon, and is induced when 
vigorous growth is accompanied by an excess of water in the substratum. 
In the lemon and cherry this form of the disease develops severely under 
these conditions, and is not induced by injuries of one kind or another, as 
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