Observations on Squamosis and Exanthema of the Citrus . 125 
difficult to say what effect it has upon the already yellowish gum. Lastly, 
if we cut a section through a gum pocket already old, and in which gum 
formation has not taken place for some time, and in fact is no longer 
capable of taking place, it will be found that the gum no longer stains with 
Bohmer’s haematoxylin or chloriodide of zinc, but, on the other hand, is 
brought out more or less distinctly by lignocellulose stains— orcein and 
hydrochloric acid, methylene blue, Bismarck brown, fuchsin. 
Other stains useful in the study of the early but not nascent stages 
are Congo red and aniline blue 2 v. Congo red may be used as a counter- 
stain with haematoxylin as it does not stain the gum ; the cell-walls and 
protoplasm are coloured red, the gum blue. Aniline blue 2 v. stains the 
protoplasm only and may be used, therefore, more or less as a check upon 
the other stains. 
Suitable material (fruit, non-ligneous stems) may be embedded in 
paraffin or celloidin — I think the latter preferable as it hardens less — cut 
and mounted as usual. 
Canada balsam or Venetian turpentine may be used as mounting 
media, but for unstained sections their indices of refraction are somewhat 
high, and I am inclined to prefer a mixture composed of castor oil 2 pts. 
and of oil of thyme 1 pt. When, however, the sections to be mounted 
will bear passage through water, I use Farrant’s medium. 
5. Histology. 
The anatomy of gummosis has been studied (I refer to Primus in 
particular) from time to time during the last fifty years, and our knowledge 
of the various appearances that pathognomonic tissues may present is now 
very complete as regards the later stages of the disease. Of the initial and 
post-initial stages, however, practically nothing is at present known, — hence 
very largely the lack of agreement between authors regarding the nature 
and origin of gum. In taking up in turn the histology of gummosis, I have, 
therefore, devoted considerable time to the study of these important stages 
with the view of answering definitely, if may be, the question : Where and 
how does the gum originate ? 
In my description of the gross internal characters of twigs, branches, 
limbs, or trunks affected by gummosis, I mentioned that, when the disease 
was not sufficiently intense to completely encircle the stem, the patho- 
gnomonic tissues were fusiform in outline. In these cases, it will be re- 
membered, I described gum formation as abundant at the centre, but 
decreasing in amount and finally becoming unobservable as the apices were 
approached. The tissues at the centre of the sickle have been longest 
affected by the disease and may have reached the ultimate stage of de- 
composition, while those nearer the apices only show young, if not the 
initial, stages of gummosis. One might, therefore, readily imagine an ideal 
