122 Butler . — A Study on Gummosis of Primus and Citrus , with 
The cortex remains healthy except where the gum has accumulated 
sub-epidermally, or broken out on the surface of the bark. In these 
places, as I have already mentioned, it becomes more or less infiltrated and 
discoloured. 
Within the wood, if the branch examined is old enough, brown 
maculations and fusoid areas may be observed amidst the healthy tissue ; 
they are due to attacks of gummosis at some previous period, when the 
now dead tissues were a part of the meristematic zone. 
When the attack of gummosis is very severe the branches or trunk, 
as the case may be, may become completely ringed with gumming tissue, 
and the fusoid character accompanying mild infection is lost. The wood 
is more or less deeply infiltrated and discoloured and the disease gives the 
false impression of working centripetally. 
Gummosis is characterized, as we have seen, macroscopically by a more 
or less copious gummous exudate on the surface of the bark, and by a zone 
of pathognomonic tissue in the outermost region of the xylem, which, 
however, is only recognized by the unaided eye when gum flows or pearls 
from it. The gum is, therefore, an important symptom of the disease, and, 
as such, is deserving of separate study. 
3. Chemistry of the Gums of Prunus and Citrus. 
The gums 1 of Prunus are derivatives of the hemicelluloses and repre- 
sent, to use the illustration of Griiss, the homologous stage in their hydration 
to that of the dextrins in the hydrolysis of starch. The changes taking 
place during the hydrolysis may be illustrated thus: — 
araban . . , arabin . . . arabinose 
starch . . . dextrin . . . maltose 
The gums of the Citrus do not appear to have been closely studied 
up to the present, but, as will be seen from the following table, they behave 
in essentially the same manner as the gums of Prunus towards the various 
reagents, and like the latter are hemicellulose derivatives. As type of the 
Citrus gums I have taken lemon gum. 
1 The chemistry of the gums is still imperfectly known and consequently not entirely free from 
inaccuracies. Vide Wiesner, J. : Die Rohstoffe des Pflanzenreiches, vol. i, 2. Bd. Czapek, F. : 
Biochemie der Pflanzen, vol. i, 1905. The literature is cited in these works. 
