259 



three weeks later the block of wood was examined no hyphae 

 could be found towards the edge of the discoloured area. To 

 avoid confusion between the discoloration produced by the fungus 

 and that resulting from oxidases, the material was steeped in 

 boiling water for ten minutes before being cut into sections. 



With regard to the general characteristics of Steremn yur- 

 pureum, it was found that this fungus varies considerably in 

 colour and in external characters. It differs apparently 



according to whether it has been growing parasiticallv or 

 purely saprophytically, for the fructifications on Salix have 

 distinctly smaller spores than those on Plum and Apple. How- 

 ever, for the present, the fungus must be treated as a single 

 polymorphic species, as only definite evidence afforded by a series 

 of careful investigations would justify the division of the species 

 into distinct forms. S. purpureum is found growing as a sapro- 

 phyte on the dead wood of the following genera : Betula, Corylus, 

 Fayus, Ulmus, Genista, Salix, etc. There can be no question 

 that the fungus was less detrimental in past years, and it is only 

 during the last twenty years that its virulence has gradually 

 increased to such an extent that the disease is now of almost 

 epidemic nature. Whether the purely saprophytic form of 

 S. purpureum growing on the hosts mentioned above are able to 

 cause " Silver leaf " remains to be proved,* but it is certain that 

 mycelium or pieces of fructifications from Betula, Genista and 

 Salix when inoculated into Plum and Apple trees will cause dis- 

 coloration of the wood, indicating that the hyphae have developed 

 in the tissues. 



The fungus can apparently only enter a living plant by open 

 wounds, and is capable of attacking a great number of woody 

 species, belonging to various natural orders, infection taking 

 place both through the sterna and superficial roots. It is 

 facilitated by pruning, particularlv when the wounds are left 

 unprotected , and also by any mechanical injury of an exposed 

 part of the tree, or as a result of injury to superficial roots. 



With regard to the distribution of mycelium, no hyphae have 

 been detected in the lamina or petiole of leaves in any " silvered " 

 plant, either by the author or any previous workers. In the 



bast of affected twigs of Primus triloba var. fore plena, abnormal 

 air spaces are to be found, and a dissolution of the middle lamella 

 which leads to a falling asunder of the tissues, similar to that 

 found in the leaves of plums. No hyphae, however, were 

 observed. It is evident therefore that under some circumstances 

 the fungus is able to exert in the stem the same injurious action 

 at a distance which is common in the leaf. 



VI. Sl'MMAKY AXD CONCLUSIONS. 



The present investigations lead to the conclusion that there 

 are two types of Silver-leaf, namelv, (1) True Silver leaf, by far 



* The inoculation experiments which are in progress at the Royal 

 Horticultural Society Gardens, Wisley, definitely prove that the forms of 

 S. purpureum found growing as a pure saprophyte are capable of producing 

 Silver leaf, and with the same virulence upon various host-plants as those . 



of S. purpureum growing parasitically. 



B — 



