120 The South African Mulberry Blight 



plasmolysis takes place and the contents appear as a contracted disin- 

 tegrated mass in the centre of the cell which stains deeply with carbol 

 fuchsin. The bacteria then invade the cells themselves and complete 

 their destruction. In this way the palisade cells and the parenchyma 

 of the mesophyll are entirely destroyed at the points attacked. The 

 bacteria also enter the vessels of the fibrovascular bundles. 



The conditions described above are those found in infected spots 

 on the leaves of Morus nigra. In this species the mesophyll is very 

 loose in texture and the intercellular spaces large, so that the bacteria 

 have no difficulty in penetrating these tissues. The leaf of the common 

 variety is much thinner, the tissues more compact and the intercellular 

 spaces correspondingly small. Sections through leaves infected by 

 pure culture in which the water-soaked spots were just visible on the 

 under surface showed a slightly different state of affairs (Plate XXIII). 

 Here the bacteria have multiplied enormously in the substomatal 

 cavity and for some distance have levered away the epidermis from 

 the adjacent cells. 



In the stem if infection takes place, as it frequently does near the 

 tip where the tissues are tender, all parts are equally affected ; if the 

 whole of the circumference becomes involved the end of the twig dies. 

 Frequently only one side of the stem is attacked and sometimes the 

 infection penetrates to the pith, which shows a yellow discoloration. 



In older parts of the stem the bacteria are for the most part restricted 

 to the cortex and to the vessels of the wood. Tyloses are frequent in 

 the latter. A fairly deep crack forms in the middle of the infected 

 area owing to the strain on the dead cells caused by the living parts of 

 the stem which continue to grow. Round this a cork cambium is 

 formed and the dead cells are cut off by a layer of cork. 



The dead tissues in stem and leaf assume a bright brown colour and 

 do not stain with ordinary aniline dyes. 



Morphology. 



The cause of the disease is a long rod with rounded ends, usually 

 occurring singly or in pairs, less frequently in long or short chains. 

 The latter are found in the pseudo-zoogloea formed on the surface of 

 beef-broth and other liquid media. 



No spores or capsules were observed. The limits of she wore found 

 to be 1-5 to 5ju, by -8 to 1*2 p, the majority being from 2-5 fco 3*5 p in 

 length. 



The organism is actively mobile in a hanging drop culture made 



