150 Dale. — A Bacterial Disease of Potato Leaves. 
workers. That B. Tubifex is able to cause fermentation is shown by its 
behaviour on culture media. 
The tube itself is doubtless formed, as in the Leguminosae, by the 
union of the individual organisms into a zoogloea by means of the gelatinous 
envelope surrounding them. This envelope is only developed in subaerial 
cultures and not in liquid media. That the tube is not formed from the 
dissolved middle lamella of the cells of the host plant seems to be indicated 
by the fact that the tubes can be formed in, and cross over, large intercellular 
spaces in the mesophyll of the lamina. 
The tube is therefore a kind of zoogloea, which penetrates the host by 
means of a ferment action which dissolves the middle lamella of the cells of 
the potato leaf. Apparently the Bacteria pass out of the tube into the cells 
of the host plant, leaving the matrix as a resistant, deeply staining tube, 
since all attempts to differentiate Bacteria by means of staining failed in the 
old tubes, though the same methods were used which were successfully 
employed by Miss Dawson in the case of the nodules of the Leguminosae. 
Whether there is any relation between bacterial leaf-disease and leaf- 
curl is uncertain. The constant symptoms of the former disease are 
a yellowing of the leaves accompanied by the formation of brown patches 
in the lamina and of a brown colour in the veins. These symptoms may 
or may not be accompanied by leaf-curl or leaf-roll. On the other hand, as 
stated above, leaf-curl may occur when no organism can be found in the sub- 
aerial shoots. Reinke and Berthold 1 observed this fact, and came to the con- 
clusion that curl is due to the presence of Verticillium albo-atrum , which 
may, however, be confined to the subterranean parts of the host and to the 
lower parts of the subaerial stem, though it may eventually grow upwards 
along the whole length of the shoot, and kill it by blocking up the vessels. 
7. Conclusions and Summary. 
Bacterial leaf-disease is due to Bacillus Tubifex , n. sp., which attacks the 
leaves of the potato plant by piercing the cuticle where this is thin enough 
for it to penetrate. The bacilli form a kind of zoogloea and dissolve the 
middle lamella of the host plant by means of a ferment, and so pass 
between, or in some cases across, the cells of the host plant by means of 
infection tubes like those present in the nodules of certain leguminous 
plants. Infection generally takes place near the edges of the leaves or 
along the veins either on the upper or under surface. The infection tubes 
grow at right angles to the surface of the lamina and sooner or later end 
in cells of the host plant, which become filled with Bacteria. 
The disease appears to be more of theoretical than of practical impor- 
tance, as in a fairly dry, hot summer the cuticle of the potato leaves would 
1 Reinke and Berthold : Zersetzung der Kartoffel. Die Krauselkrankheit der Kartoffel, 1879, 
p. 67 et seq. 
