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6 BACTERIA IN RELATION TO PLANT DISEASES. 
potato, the spot disease of beans, the black-spot of plums, and the olive-tubercle. Bacteria 
frequently occur in the vascular bundles of plants suffering from these diseases especially 
-after considerable destruction of the parenchyma, but they are not vascular diseases in any 
such specific sense as the wilt of cucurbits or the black rot of cabbage. Occasionally also 
tumefactions, or the stimulation to growth of dormant organs, may be induced by organisms 
which appear to have little in common with those which regularly produce tubercles and 
tumors, e. g., the normal action of Bact. solanacearum on the tomato, inducing the premature 
developmnent of adventitious roots and the occasional swelling of tissues when inoculated 
with cultures which have lost their virulence. The true tumors, 7. e., the crown-galls, 
appear to be an exception to this rule. In these I have not seen any bacterial occupation 
of the vessels or intercellular spaces. Various subdivisions of these three groups, especially 
of the second and third, will become necessary and will be made use of in the proper place. 
With these explanations and qualifications, we may proceed to the subject in hand, 
noting, in conclusion, that in this volume as in the preceding the illustrations, so far as 
possible, have been drawn from the writer’s own material and were made under his personal 
supervision, mostly by James F. Brewer. In case of drawings from sections the writer not 
only selected the part to be illustrated but also checked up the finished drawing line by 
line under the microscope, so that a fair degree of accuracy may be assumed. In most 
instances, however, the slide number is given under the figure, and, in case of doubt, these 
slides are on file for reference in the collections of the Department of Agriculture. 
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