148 Butler—A Study on Gummosis of Prunus and Citrus, with 
From the brief sketch of the histology of exanthema that I have just 
given it appears evident that were it not for the erumpent pustules this 
malady would be undifferentiable from gummosis. I am inclined to think that 
the proliferation of the cortex may not be a differentiating character of very 
great importance. May it not be simply due to the epidermis becoming 
inelastic and preventing for a time normal cellular division, which is able to 
proceed with vigour the moment a rupture occurs? At the time the 
rupture in the epidermis takes place there is every reason to believe that 
metabolism is unusually active, and the tissues at the same time very sapid 
—gummous degeneration which only occurs, as we know, when the cambium 
is active, and when the plant has an abundant supply of water at its com- 
mand, is an evidence of this—and that owing to this concurrence the cells 
of the cortex become meristematic the moment pressure is released. With 
the release of pressure due to the rupture of the epidermis the growth 
stimulus should be transmitted centripetally and only to a slight extent 
tangentially if my hypothesis were correct, and we have seen that this is 
exactly what happens. 
4. CAUSE OF EXANTHEMA. 
The conditions favourable for the development of exanthema, as well 
as an histological study of the tissues of diseased trees, indicate that the 
malady is induced, like gummosis, by the concurrence of active growth and 
sapid tissues. In the case of the latter malady these conditions prevail for 
a period of time, but in the case of the former they are extremely transient. 
The soils in which exanthema occur are typically dry soils, which, 
when saturated by irrigation water or rains, promptly become dry once 
more when the weather clears or irrigation is discontinued. The rings of 
growth, which, as we have seen, are very marked in diseased shoots and 
branches of trees affected by exanthema, could not be caused except by 
a more or less rapid succession of maxima and minima of growth, and such 
an alteration could only be accounted for by synchronous changes in the 
amount of available water present in the soil, or a like succession of favour- 
able and unfavourable climatic conditions. Obviously climatic changes 
cannot be considered as in any way favouring or inhibiting the development 
of the disease, and we have no alternative but to assign the unusual and 
marked development of rings of growth to changes in the water relation. 
Exanthema has another character in common with gummosis : trees 
are ceteris paribus much more severely affected in rich soils than in poor 
soils. In Florida it is even considered that heavy fertilization with organic 
manures, such as cotton-seed meal and dried blood, is alone sufficient to 
cause the appearance of the disease in healthy trees. This opinion is in 
a certain sense well founded, for nitrogen is a growth stimulant, and would, 
therefore, tend to widen the range between the maxima and minima of 
