RESEARCH IN PHYTOPATHOLOGY 197 



brown rot extends along the medullary rays, thus creating in 

 the root a characteristically radial structure, with a central 

 hollow in which alternating portions of the woody strands of 

 the vascular cylinder alone persist. Externally the root may 

 appear quite sound, the disease producing a kind of dry rot 

 internally. 



The Potato and Tomato Disease — Bacillus solanaceariim — was 

 investigated by Smith in 1896. This is early recognised by a 

 shrinking of the stem and wilting of the foliage, due to the 

 penetration of the bacilli through the vessels of the xylem, 

 which become filled with an extraordinary number of these 

 organisms. This bacillus is at first confined to the vascular 

 system but ultimately the parenchyma becomes invaded and 

 broken down. The organism makes its way through the 

 stem into the tuber, where again the vascular cylinder is first 

 traversed and only in very advanced stages does the starch- 

 bearing parenchyma become affected. 



Each one of the diseases enumerated above had been con- 

 clusively established, the evidence resting upon much carefully 

 conducted work, based upon Koch's four premises, in which 

 the organism had been studied in pure culture ; repeated 

 inoculations from pure cultures produced always the character- 

 istic pathogenic symptoms and the reappearance in the tissues 

 of the plant of the same specific organism. 



In 1896 E. F. Smith published, in the American Naturalist, 

 " A Critical Review of the present state of our knowledge upon 

 the Bacterial Diseases of Plants." He drew attention to the 

 need for full descriptions of the various forms, including a study 

 of both morphological and biological peculiarities, at the same 

 time emphasising the importance of the strictest cultural 

 technicalities and rigid tests of pathogenesis, which have too 

 often been disregarded. His review of thoroughly investigated 

 examples up to that date leaves no doubt that certain well- 

 marked plant-diseases owe their origin solely to a specific 

 Bacterium. 



Migula, in his System det Bakterien (May, 1897) still con- 

 sidered that the impenetrable cell-wall of plants presents great 

 difficulties to the entrance ot bacteria and that stomatal 

 infection is generally impossible, yet he allowed that these 

 objections were not of universal application. He admitted that 

 a number of bacterial diseases had been established and devotes 



