Observations on Squamosis and Exanthema of the Citrus. 127 
said to begin? (Pl. X, Figs. 4, 7). Gummosis now spreads more and more 
deeply into the circumambient tissues. The cells bordering the pocket are 
sloughed off from the subjacent cells, which then become, in turn, convex 
on their free ends, and finally toosened and freed by a process exactly 
similar to that which brought about the first formation of the gum cavity, 
and which may continue until all the tissue capable of gummous degenera- 
tion has been destroyed. The growth of any given gum pocket is, there- 
fore, only limited by the amount of susceptible tissue (embryonic wood cells) 
laid down by the cambium, though it may develop to such an extent as 
to destroy the cambium, the medullary rays, and, in rare instances, the 
bordering cells of the xylem. 
Having described in general terms the method of growth of the gum 
pocket, I shall now consider at some length the changes that accompany 
and follow the sloughing off of the cells. 
The cells freed during the development of the gum pocket may be 
either totally destroyed or remain apparently unaffected, depending upon 
the rapidity with which gummous degeneration is taking place. If gum- 
mosis develops slowly or has nearly run its course, the tertiary membrane, 
now the only envelope of the cell, remains unaffected and life continues ; but, 
if the disease is progressing rapidly, it gelatinizes centripetally and the cell 
finally vanishes, completely absorbed by the gum surrounding it. 
_ I shall study at some length the cells sloughed off during the growth 
of the gum pocket. 
If one examines, in a suitable medium, sections cut from material fixed 
and preserved in 95: % alcohol, and in which there are young and actively 
growing gum pockets, he will observe that the gum pocket either contains 
homogeneous gum or is more or less filled with a mesh of thick strands, 
anastomosing in such a manner as to present an alveolar appearance (Pl. X, 
Fig. 3), which vanishes immediately when water is run under the cover- 
glass, being replaced by gum. I at first supposed that this structural 
pattern was a stage in gummous degeneration of the cells; but a careful 
study of a number of sections made it evident that this cellular appearance 
was due to an alveolation of the gum brought about by the fixative used, 
i.e. 95% alcohol. a 
- In young gum pockets, cells will sometimes be observed floating in the 
1 According to Delacroix, gummosis begins towards the centre of the pathognomonic embryonic 
wood cells, enframed between the cambium and the xylem, on the one hand, and two adjacent 
medullary rays on the other, in Pruxus, but rather nearer the cambium in the case of the C7trus. 
{have been unable to confirm this observation. Gummosis begins, I have found, now centrally, now 
more laterally.. When, however, it begins near one of the medullary rays the extension of the disease 
is necessarily more unilateral (the medullary rays only becoming affected in severe cases), and, when 
more fully formed, the gum pocket may give the false impression of having originated near the 
entre. Again, if gummosis began somewhat mildly and gradually grew worse as new tissues were 
laid down, we might obtain the equally false impression that the disease had worked centripetally 
when, in reality, it had worked centrifugally. 
