120 Butler. — A Study on Gummosis of Prunus and Citrus . 
be found more or less permeated with gum. The death of this infiltrated 
bark is but a matter of time ; it becomes extremely hard, cracks, curls 
more or less, and sloughs off. 
Severe cases of gummosis are always accompanied by chlorosis and, 
when the malady recurs from year to year, by a marked decrease in vigour. 
As the disease indirectly, by dehiscing the bark, destroys the avenue 
of food supply to the roots, death from starvation must ultimately 
result. With the reduced root development consequent on the failure 
of the proper supply of elaborated material, there follows a decrease in 
the absorption of mineral nutriments, and it does not appear to me at 
all doubtful that it is to this reduced absorption that chlorosis of the 
diseased trees is due. A number of analyses of healthy and diseased Citrus 
have been made at various times with the idea of throwing some light on 
the cause of gummosis. Diseased and healthy trees show considerable 
difference in ash content, but this difference is purely a relative one, and 
the affected trees show not, as has sometimes been thought, selective 
absorption, either forced or otherwise, but rather reduced normal absorption. 
The following curves drawn from the data of Ricciardi and Silvestri clearly 
show that relative absorption is normal in diseased as well as healthy trees 
(Text-fig. i). 
Gummosis having been detected by means of the pustulated epidermis 
or the exudate, the extent of the development of the disease within the 
tissues may be readily followed by a series of cross-sections taken at intervals 
above and below the place of gum accumulation or outflow. 
The internal characters of the disease are best shown on slightly 
affected stout branches two or more years old. A cross-section cut through 
the swollen epidermis or the exudate will show that the pathognomonic 
tissues are situated in the young wood and young wood in formation, and 
extend to a greater or less extent around the xylem. The diseased area 
is fusoid in form from the greater development of the diseased tissues near 
the point of gum accumulation and their lesser and lesser development as 
one proceeds further away. The gum exudes en masse from the centre 
of the sickle, but as one proceeds towards its extremities it will appear in 
droplets of decreasing magnitude, and separated with larger intervening 
spaces of apparently healthy tissue. The gum in the centre of the sickle 
is more or less tinted yellow, depending on its age, whereas that pearling 
from the tissues at its extremities is always colourless (PI. IX, Fig. 3). 
If sections are now cut at various distances above and below the centre 
of disease, it will be found that gum formation proceeds downward to 
a much less extent than upward. Furthermore, if we imagine a line drawn 
through the middle of the pathognomonic tissues, it will be found, as one 
proceeds upwards and downwards, that the gumming sickle diminishes 
in size. 
