150 Contribution to Knowledge of Silver -Leaf Disease 



The cytoplasm forms a peripheral layer in the healthy mesophyll 

 cells of Prunus leaves and is thin and clear, but in affected cells it is 

 clouded. The cell contents of the diseased area are very striking 

 because of a uniformly stained, granular deposit. This fine granulation 

 occurs in the palisade and spongy parenchyma and epidermis alike and 

 sometimes in the intercellular spaces. It is always in the silvered 

 "Victoria" plum leaves a certain indication of a diseased area. It 

 was never found in the cells with entirely normal content. An attempt 

 to determine the nature of this substance by microchemical methods 

 was not very successful. The tests for sugar (Fehling's solution) and 

 for protein (Millon's reagent) gave negative results. Usually the test 

 for tannin (ferric chloride, potassium bichromate) gave negative results 

 also, although occasionally very slight reactions were obtained. 



It would appear that the vascular bundles (veins) play a certain 

 part in spreading disorganisation in the mesophyll. It can often be 

 noticed in the sections that the vascular bundles limit the diseased 

 area; cases were observed where on one side of vascular bundle the 

 tissue was more or less profoundly altered, whilst on the other side 

 the cells did not show any symptoms of attack. It is possible that this 

 fact is connected with the observation previously mentioned of the 

 spreading of silvering from the veins. 



III. 



Discussions and Conclusions. 



The facts above described show clearly that the changes occurring 

 in "silvered" leaves of the plum are not confined to the development of 

 air spaces and the separation of cells but are far more profound than 

 was supposed. There is no doubt that in the diseased leaves markedly 

 abnormal physiological conditions exist. 



It is true that some of the phenomena described take place only 

 in pathological tissue, while others are known occasionally in healthy 

 tissue also. For let us consider briefly a few features of the silvered 

 leaves which are similar to those in healthy tissue. The abnormal 

 increase in size of the nucleus is — to judge by the manner of its occur- 

 rence — connected with the increase of metabolism just as it is for instance 

 in the large nuclei in the glands of animals or in healthy tissue of the 

 seaweed Antithamnion (Schiller, 14), where the surface of the nuclei is 

 much increased in size by enlargement and change of form or develop- 

 ment of lobes. In the diseased Primus leaves the hypertrophy of the 



