256 



plum, such as "Yellow Pershore Plum" (sometimes called 

 " Yellow Egg Plum") and "Early Eivers," is highly to be 



recommended. 



8. The soft cooking varieties of Apples are more susceptible 

 to Silver leaf than others, the latter should therefore preferably 

 be planted. 



9. Certain stocks, such as the " Brompton " and " Common 

 Plum," are highly susceptible to this disease, whereas the 



i i ~XT 



Yellow Pershore " is more or less resistent, and, in addition, 

 possesses other valuable qualities. 



10. Pruning of ornamental trees and shrubs known to be 

 affected by this malady should be discontinued, unless the cut 

 surfaces can be protected immediately after the operation. 



11. When replanting infected orchards precautions should 

 be taken to avoid the varieties of trees mentioned on page 242, 

 since these varieties have proved susceptible to " true Silver 

 leaf." An alternative to this is the interplanting in Plum 

 orchards of Pear trees, or Cob-nuts and Filberts. 



General Discussion. 



The results described in the present contribution clearly sup- 

 port the view that Stereum purpureum is the main cause of 

 Silver leaf. Cases of Silver leaf, however, are here described 

 in which no trace of the fungus could be detected in any part of 

 the plant. 



It is suggested that silvering caused by Stereum purpureum 

 should be called " True Silver leaf/' and the term " False 

 Silver leaf " he applied to those cases where the fungus is absent, 

 the cause being apparently some physiological weakness. 



In July, 1918, a plant of Camellia Sasanqua var. was 

 observed in a pot in the Temperate House, Kew Gardens, which 

 exhibited marked silvering of the leaves. The roots, branches, 

 and twigs of this plant were examined, but no trace of hyphae 

 were found. Local discoloration in the cells of the various 

 tissues was observed here and there, but a continuous streak was 

 never perceptible. Hand and microtome sections, both longi- 

 tudinal and transverse, of petioles and leaves, failed to reveal 

 any fungal threads. In hand sections no separation of the tissues 

 occurred, indicating that there had been no dissolution of the 

 middle lamella. In comparison with normal leaves, a waxy 

 deposit of the colour of white lead is secreted by the affected 

 leaves, and this gives a silvery appearance when the leaves are 

 seen from a distance. There was also a reduction of chloroplasts 

 in these leaves as compared with the healthy ones. 



" False Silver leaf " is also to be found on a few varieties of 

 Apples, the variety " Grenadier," already mentioned, and 

 '* BramleY's Seedling." It has also been observed on Koelreu- 

 teria paniculata, Saxm., on cultivated varieties of Cherries, 

 Peaches and Plums. False Silver leaf is probably due to some 

 physiological disturbance of the plants, possibly induced by un- 

 suitable treatment. Plants of Koelreuteria which showed this 

 false Silver leaf during one or two seasons, were observed to 

 regain their normal colour under improved cultural treatment. 



