6 BACTERIA IN RELATION TO PLANT DISEASES. 



potato, the spot disease of beans, the blaek-spot of plums, and the olive-tuberele. 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 Bad. solanacearum on the tomato, inducing the premature 

 development of adventitious roots and the occasional swelling of tissues when inoculated 

 with cultures which have lost their virulence. The true tumors, *. 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. 



