146 Builer—A Study on Gummosis of Prunus and Citrus, with 
A luxuriant growth, a deep green colour of the foliage, and thick- 
skinned fruit, though not in themselves specific, precede the first symptoms 
of the disease sufficiently frequently to possess the value of an indication. 
Exanthema, however, cannot be diagnosed until the shoots become more 
or less stained sub-epidermally by a yellowish brown material and begin to 
die back; or the fruit becomes similarly stained and the epidermis so 
indurated that it cracks and splits, due to the pressure of the developing 
pulp-cells. Either one or the other of these symptoms must be observed 
before exanthema can be accurately diagnosed. 
When conditions are not very favourable for the development of 
exanthema, an affected tree may live for years without developing any 
further pathognomonic symptoms ; when conditions are favourable for its 
development, however, further changes take place, especially in the shoots 
and branches. When young the shoots swell at the nodes (PI. VIII, Fig. a), 
infrequently on the internodes—the homologues of these swellings are 
sometimes observed on the fruit, but as they become more mature linear 
erumpent pustules break out on the internodes (Pl. VIII, Fig. c); these 
pustules may even alternate with nodal swellings. On the older branches 
the nodal swellings are not observed, but the pustules may become exceed- 
ingly numerous, and a small amount of gum may be observed in them. 
Gum may also be observed exuding through the bark in small amount. 
On shoots and branches that are not covered with swellings and pustules, 
there frequently occurs a marked proliferation of young buds (Pl. VIII, 
Fig. D), which may develop into short branches with chlorotic foliage, 
thus producing a pseudo witch’s-broom effect (Pl. VIII, Fig. B). 
When diseased shoots are examined in cross-section it will be observed : 
(1) that the swellings are due to an accumulation of gum and are entirely 
similar to those sometimes met with in gummosis; (2) that the pustules 
are produced by a proliferation of the cortical tissues, accompanied by the 
formation of a sickle of susceptible tissue containing gum pockets. It will 
also be observed that rings of growth are extremely marked, much more so 
than in squamosis, and altogether too numerous to be considered variations 
in growth due to seasonal changes. 
3. HISTOLOGY. 
A cursory histological study of the swellings and erumpent pustules—in 
other words, the most striking and typical characters of exanthema—is 
sufficient to show that this malady is very closely related to gummosis; in 
fact, were it not for the erumpent pustules, it would be impossible to point 
out any essential anatomical differences between the two maladies. The 
swellings are nothing more nor less than well-developed gum pockets in 
which the gum has accumulated to a considerable extent, it not having 
been able to break through the epidermis. These gum pockets are not only 
