F. T. Brooks and M. A. Bailey 191 



Percival (7), Pickering (8), Giissow (5), and Broolcs (1,2), including many 

 pure culture inoculations performed by the last named, this statement 

 is surprising. Later in the present paper it will be shown that all Koch's 

 postulates have now been carried out and fructifications of Siereinii 

 pKrpiiretun have developed on trees which were inoculated with ])urc 

 cultures of the fungus. The suggestion of Smolak that silver-leaf may 

 he due to bacterial infection is not supported by definite evidence. 



As regards the physical reasons for the appearance of silvering, 

 there seems to be little disagreement between Smolak's views and ours. 

 We still consider that by far the greater part of the peculiar appear- 

 ance of plum leaves in the earlier stages of silvering is due to the forma- 

 tion of abnormal air spaces below the upper epidermis, whereby the 

 character of the light normally reflected is changed. If the upper 

 epidermis of a plum leaf which has not been long silvered is stripped 

 off, the underlying mesophyll is dark green in colour and is indistin- 

 guishable to the naked eye from the mesophyll of a normal leaf. The 

 mesophyll of plum leaves which have been silvered a long time on the 

 other hand does appear lighter green in colour than that of healthy 

 leaves viewed in the same manner. The individual mesophyll cells of 

 young silvered leaves examined freshly under the microscope, cannot be 

 distinguished from corresponding cells of a normal leaf, although leaves 

 in which the silvering is of long standing eventually show a considerable 

 number of cells in which the chloroplasts are fading and disorganising. 

 It is noteworthy that even in a normal plum leaf a few of the cells often 

 show disorganised chloroplasts long before the onset of autumn. Smolak 

 did not examine the contents of silvered cells in the fresh condition. 



2. FURTHER OBSERVATIONS UPON THE OCCURRENCE AND 

 INCIDENCE OF SILVER-LEAF. 



The leaves of a plum tree affected by Sfereniii purpureum often show 

 silvering both at and directly after the time of unfolding in the spring, 

 but sometimes only the first formed leaves on a particular shoot exhibit 

 the affection. The same is often true of shoots belonging to second or 

 " midsummer" growth later in the year. Although leaves are sometimes 

 completely silvered at the time of unfolding many leaves subsequently 

 become silvered which were normal while expanding. Smolak states 

 (loc. cil. p. 141) that "the spreading of the silvering always began first 

 of all on and around the vascular bundles (veins)." As the network of 

 veins in a plum leaf is very intricate and permeates almost the whole of 

 the leaf, this statement does not appear to be clear, but if the principal 



13—2 



