258 



The fact that 110 hyphae are to be found in the leaves of plants 

 suffering from True Silver leaf has no doubt led these authors 

 to the non-pathogenic theory of the disease. At that time also 

 the fungus Stereum pur pure urn was considered a saprophyte, and 

 it is apparently only during recent years that its virulence as a 

 parasite has increased and has spread to plants belonging to 

 various natural orders. In the case of Neviusia alabamensis and 

 Spiraea japonica var. glabrata, described here, the hyphae would 

 have certainly been overlooked, if the examination had been 

 confined to the apical shoots. 



In addition to the literature on Silver leaf mentioned in this 

 paper I have been unable to find any evidence as to bacteria bein: 



in 



the cause of Silver leaf, as referred to by Blackmore (T), in his 

 report. It is obvious that soil and atmospheric conditions or 

 other environmental factors of a greenhouse are capable of 

 causing physiological injury to the plant, but up to the present 

 there is no evidence indicating parasitic bacteria as the cause of 

 either True or False Silver leaf. 



It is interesting to note that the partial disorganisation of a 



tissue, as a result of the dissolution of the middle lamella, in 



which fungal hyphae are not present — which is normal to the 

 leaves of silvered plants — is also found in the silvered twigs of 

 Prunus triloba, var. ft ore plena. There the bast shows partial 

 disorganisation, although the hyphae are confined to the cambial 



region. 



Since enzymes are apparently colloidal substances which 

 would diffuse with difficulty through cell-walls; the most satis- 

 factory explanation of the changes in the leaf, seems to be the 

 one suggested to me by Professor V. H. Blackman, that some 

 diffusible poisonous substance is produced during the metabolic 

 processes of the fungus or of the invaded cells of the host; this 

 substance is then conveyed to the leaves by the water conducting 

 channels, where it causes a change in the activity of the enzymes 

 capable of acting on the middle lamella of the wall. A similar 

 reaction apparently takes place in the bast of Prunus triloba var. 

 flore pleno. 



In the experiments already described where pieces of 

 mycelium, or fructifications or spores, were placed on injured 

 leaves of Prunus lusitanica in petri dishes, the hyphae were 

 found to have penetrated the leaves in about 12 days, without, 

 however, causing a falling asunder of the tissues. The disso- 

 lution of the middle lamella therefore does not seem to be due 

 to direct action of the fundus, at least in its early stages, but 

 rather to the result of the interaction of the metabolisms of the 

 fungus and bast cells of the twig. Whether infection can also 



take place in nature in injured leaves is yet unknown. 



The appearance of the discoloration varies with the different 

 host plants. In all the cases examined, the discoloration of the 

 host tissue extended beyond the limit of the hyphae. Brooks' 

 observations on the discoloured wood of plum trees were confirmed 

 by placing pieces of the fructifications or ordinary mycelium 

 from artificial cultures on sterilized blocks of plum wood. In four 

 days a definite discoloration of the wood was to be observed; when 



