312 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY, 
effect aid and abet each other, and the result is a chronic invalidism 
of an enormous number of trees that are attacked by this disastrous 
disease. - 
A similar process is frequently observed as a result of bacterial 
invasion. Erwin F. Smith has described a number of leaf-spot 
diseases in which the parasite penetrates through stomata in the 
unbroken leaf and stem surface, and multiplies in the substomatic 
chamber, causing a local destruction of tissue. Thus Bacterium 
Phaseoli is responsible for the spot disease of beans, Bacterium maculi- 
colum for the spot disease of cauliflowers, and other examples have 
been observed. Leaf spots are often slow in their progress and con- 
fined to small areas, the reason probably being that the vascular 
system is not invaded. Nevertheless, the bacteria weaken the cells 
of their host or destroy them, altogether. With increasing supplies 
of food and a nidus rendered alkaline by their own excretions, the 
bacteria multiply more and more, unless indeed, as sometimes happens, 
The Perpetuation of Disease. The Destruction of Organs. The Termination of Life. 
Fig. 49. — Plant Disease and the "Vicious Circle." 
the host can arrest the morbid gyration by separating the diseased 
from the healthy tissues by the formation of a corky layer. 
II. — The Destruction of Organs. 
Another result of an injurious circular reaction is the destruction 
of an organ either by a non-living or a living agency. 
The action of a non-living agency is illustrated by the premature 
shedding of leaves or of twigs which may be thrown off in great numbers 
as a result of organic tissue changes. 
The disorder is most often observed as regards leaves, and is 
initiated by an impairment of assimilation and transpiration processes, 
as a result of which the suction force by which under healthy con- 
ditions the sap is drawn up is lost. Consequently the sap accumulates 
in the basal leaf zone through which the line of cleavage ultimately 
passes, and which becomes excessively turgid with osmotic materials. 
This turgor stimulates to premature activity the dormant cells of 
the abscission layer ; the partition wall between adjacent cells swells 
and a process of dissociation is inaugurated. The result is a further 
impairment of vitality in the leaves, culminating in complete detach- 
ment. To quote Sorauer : " Every premature shedding of leaves 
