E. M. Doidge 119 



August 27th (Plate XIX, fig. B), the young leaves showing by far the 

 greater number. The number of spots varied from 20 to several hundred 

 on each leaf, and many of them were in the neighbourhood of the veins. 

 The spots did not begin to discolour for more than a week. Leaves 

 which had been infected in the neighbourhood of the veins before the 

 leaf had attained its maximum size were wrinkled and distorted and 

 frequently curved over to one side (Plate XX, fig. A). After two months 

 most of the infected areas had dried up and fallen out, leaving the 

 leaves torn and ragged (Plate XIX, fig. A). 



Infections were also obtained on the stems as described above, the 

 affected area forming a streak 1-4 cms. long. Numerous small stem 

 infections also occurred in parts of the stem which had not been pricked. 



On August 28th the organism was again plated out from one of the 

 recently infected leaves and a pure culture immediately obtained, and 

 with this infections were again obtained on a young tree. 



In the above experiments the common variety of the mulberry was 

 used as no trees of the black mulberry (Moras nigra) could be obtained 

 in Pretoria which were not blighted. Controls were kept in every 

 case and these remained perfectly healthy. 



Morbid Anatomy. 



In addition to the fact that infection can take place through an 

 unwounded surface and that infection is first evident on the under side 

 of the leaf where the stomata are situated, the distribution of the 

 bacteria in the leaf tissues also points to the probability of stomatal 

 infection. Unfortunately, up to the present I have been unable to 

 obtain slides showing very early stages of infection but sections through 

 leaves bearing a few small spots still in the water-soaked stage included 

 a large number showing the conditions depicted in Plate XXII. A dense 

 mass of bacteria crowds the intercellular spaces near the stoma and 

 occupies the substomatal cavity: spaces more remote from the stoma 

 are also occupied but the bacteria in these are not so numerous. This 

 infected area, however, was in rather close proximity to a portion of 

 the leaf in a more advanced stage of infection, and therefore did not 

 furnish any conclusive evidence that the bacteria had entered through 

 the stoma as there was a possibility they had travelled through the 

 intercellular spaces from the neighbouring infected tissues. At this 

 stage the bacteria are entirely limited to the intercellular spaces and 

 have not invaded the cells, which are still intact. 



Later, the increasing mass of bacteria wedges the cells apart, 



