^O PRESIDENTIAL ADDRESS SECTION C. 



cxiulation of gum (5b) ; this is a conspicuous symptom in the 

 bacterial gummosis of stone fruits caused by Bacterium cerasi, 

 an organism which kills the blossom buds and spurs, and then 

 invades the branches, where large cankers are produced. The 

 affected areas, however, can be detected before the bark splits to 

 form the canker by the discoloration of the bark, and the copious 

 exudation of gum from the invaded tissues. 



In the early stages of the angular leaf spot of the cucumber, 

 a bacterial exudate collects in drops on the lower surface of the 

 leaf during the night, hence the name Bacterium, lochrymans 

 (54) applied to the causal organism; and there is frequently a 

 gummy exudate from fruits of the mango affected with the 

 bacterial blight (14). 



Storage organs, such as potato tubers, and the fleshy roots 

 and stems of certain vegetables are particularly susceptible to 

 the attacks of soft rot bacteria. In the case of the soft or 

 wet rot of potatoes, the tissues become softened within a few 

 days, and the tuber soon becomes a shapeless, slimy mass with 

 a particularly unpleasant smell. In tropical regions the coconut 

 is seriously affected by a bacterium which causes a rotting of the 

 large terminal bud (28). 



Pathological Plant Anatomy. 



The bacterial diseases of plants may be roughly divided (51") 

 into three classes, according to their effect on the host. 



(i) Diseases of the parenchyma without hyperplasia; these 

 include the so-called leaf spot diseases, which produce discoloured 

 areas on leaf, shoot, and fruit, and often cause dropping of 

 half-formed fruit and premature defoliation ; the soft rot must 

 also be included here. 



(2) V^ascular diseases, where there is a marked occlusion of 

 the vessels ; the majority of these cause wilting of the host plant 

 as from drought, owing to the stoppage of the -water- ways of the 

 plant bv masses of bacteria. 



(3) Diseases in which there is more or less distinct hyper- 

 plasia, resulting in the formation of cankers, tubercles, and 

 tumors. 



In diseases of the parenchyma, infection is, 'in general, 

 through the stomata ; the organism enters the leaf tissues through 

 a stoma, and begins to multiply in the sub-stomatal cavity. The 

 bacteria invade the neighbouring intercellular spaces, and t^'2 

 tension caused by the raoidlv multiplying organism causes a 

 splitting and crushing of the cells. The organism is inter- 

 cellular, and does not penetrate into uninjured cells. Apart 

 from the mechanical tearing and crushing of the cells, due to 

 the multiplication of the bacteria in confined spaces, the solvent 

 action of the products on the middle lamella separates the cells 

 and produces large cavities in the tissues, which become pockets 

 of bacteria. The leaf <;oot lesions do not extend indefinitelv. the 

 nntlti|)lication of the bacteria in the intercellular spaces being 



