246 



branches or superficial roots which have been killed by the 

 fungus, and where there is an indication of the fructifications 

 beginning to pierce the bark, the hyphae are naturally present 

 in practically all the tissues. The hyphae are septate, and 

 clamp connections frequently occur (see Fig. 1). The brown 

 material on which the discoloration depends is found mainly in 

 the xylem vessels, but some cavities and the walls of the 

 medullary ray cells are also stained, as well as certain cambium 

 cells. 



(2) Prunus lusitanica. — Among the evergreen members of 

 the Pruneae, this is the- only species which up to the present has 

 been found to suffer from Silver leaf disease. The material 

 selected for the present investigations was secured from the 

 Prunus collection in the Royal Botanic Gardens, Kew. On 

 individual plants several branches showed silvering, the youngest 

 leaves of the current year's growth seeming to be most severely 

 attacked. The curling over of these leaves rendered them very 

 noticeable at a distance. The upper and lower epidermis were 

 found in many instances to be loosened and even hanging in 

 shreds as a result of contact with other branches and leaves (see 

 Plate). The previous year's leaves and also those of early 

 spring growth, though severely affected, did not display any 

 curling or blistering. The upper and lower epidermis of such 

 diseased leaves could be peeled off with a penknife with com- 

 parative ease. On cutting off branches from one to five years 

 old a conspicuous discoloration was to be observed in the region 

 of the outer xylem; only in branches five to ten years old, 

 however, did the discoloration extend into the older xylem. A 

 branch which had its origin below ground was jeut off im- 

 mediately above the soil level; three weeks later, towards the 

 end of September, fructifications of S. purpureum developed on 

 the cut surface of the part of the branch still attached to the 

 plant, the fructifications taking their origin from the region of 

 the young wood. Another 8 in. was then cut off and in turn 

 fructifications were produced 23 days later on the surface so 

 exposed. 



Satisfactory hand sections of the curled leaves were clearly 

 unobtainable; and even from less infected leaves no suitable 

 sections could be cut, without the cells of the tissue falling 

 apart. In order to avoid the separation of the cells, the material, 

 after fixing in strong Flemming's fluid, was imbedded in 

 paraffin. When staining with E. E. Smith's method, I per- 

 ceived bacteria-like structures in a few of the lower epidermal 

 cells of the curled leaves. As the epidermis of these leaves was 

 detached from the other tissue, these bacteria are most likely 

 saprophytic and to be considered secondary in nature ; they were 

 absent in those epidermal cells where no dissolution of the middle 

 lamella had taken place. No hyphae could be seen in any part 

 of the leaves or petioles either in cross or longitudinal sections. 

 In one-year-old twigs a few threads occasionally passed from the 

 cambial layer into the bast parenchyma, causing discoloration 

 of the latter, but the hyphae were most prevalent in the primary 

 wood vessels, which in turn showed distinct discoloration. 



