142 Contribution to Knowledge of Silver- Leaf Disease 



methods, however, the starch grains appear as structures shining through 

 the thin transparent substance of the chloroplast. The nucleus stains 

 intensely and is more or less spherical or oval and is often found in a 

 somewhat central position in the cell. It usually contains one or two 

 nucleoli and plenty of chromatin substance (Fig. 1). 



If we compare with these the mesophyll cells of the "silvered" 

 leaf — either the palisade or spongy parenchyma — we cannot be in 

 doubt as to the difference between the cytological structure of the 



Fig. 1. Normal palisade cell from leaf of Prunus domestica var. " Victoria." 

 Figs. 2, 3, 8. Cells from an affected region of the same leaf, showing a hypertrophied 

 nucleus (fig. 2), a nucleus with depressions corresponding to the surface of the chloro- 

 . plasts (fig. 3), and a nucleus with filamentous projections (fig. 8). 



healthy and the infected tissues. There is often some difficult y in 

 distinguishing the relations in the diseased mesophyll because the 

 intensely stained chloroplasts make observation difficult. But careful 

 study and suitable methods will reveal the true state of affairs. 



I am unable to agree with Percival (12, p. 391) in his statement: 

 "The peculiar light grey colour of the leaves is due to these air-rilled 

 spaces, and not to any alteration' in the chloroplasts; the latter 

 structures are of the same size and appearance as those in healthy 

 leaves." On the contrary I have found changes in the nuclei, cyto- 

 plasm and chloroplasts of the affected mesophyll. In the same slide 



