RESEARCH IN PHYTOPATHOLOGY 195 



terial intrusion, owing to its peculiar structure and the absence 

 of circulatory channels which would serve for the distribution 

 of micro-organisms, and that serious obstacles to their passage 

 were presented by the impervious character of the non-nitro- 

 genous cell-walls. Further, that the acid reaction of the cell- 

 sap operates unfavourably for their growth. This latter view 

 was also shared by de Bary, The reasons advanced by Hartig 

 are merely theoretical and when submitted to actual experiment 

 have been shown to break down. The citation of recent work 

 upon the secretion of a cytolytic enzyme by bacteria and their 

 penetration through the softened cell-wall, that showing the 

 entrance of the bacteria through the water-pores and their 

 power of living and travelling in the xylem vessels, is sufficient 

 to indicate how completely his conception was at fault. Though 

 the influence of the cell-sap has an important bearing upon the 

 tendency to disease, it is now well known that the nature of 

 the cell-sap offers no absolute resistance to the active growth 

 of bacteria. It has been proved that the reaction of the 

 parenchymatous tissues is by no means always acid, and 

 moreover certain bacteria have been found to flourish best in 

 distinctly acid media ; w-hile others possess the property of 

 producing alkaline secretions which assist their penetration 

 into the cells. 



In 1882 Hartig summed up his convictions in the statement 

 that there was no such thing as diseases of plants due to 

 bacteria. In 1884 de Bary asserts that they have scarcely 

 ever been observed; again, in 1885, in the Lectures on 

 Bacteria^ he assumes that present knowledge justifies him in 

 regarding " parasitic bacteria as of but little importance as the 

 contagia of plant-diseases." The whole subject is dismissed 

 in some two pages with the mention of Wakker's Hyacinth, 

 Burril's Pear and Apple Blight, Prillieux's Wheat Disease and 

 Wehmer's Wet Rot of Potatoes ; and while admitting that 

 saprophytic bacteria may, under special conditions, attack the 

 tissues of living plants as facultative parasites, he concludes by 

 a repetition of the statement that bacteria are not objects of 

 great importance in diseases aff'ecting plants. 



In spite of the weight of such authority a mass of literature 

 gradually accumulated in favour of bacterial parasitism. 



Pear Blight. — Burril's results were subsequently confirmed 

 by the more definite cultural experiments of Arthur (18S5) and 



