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the most common, and caused by the fungus Stereum purpureum, 

 and (2) False Silver leaf, which is comparatively rare, and due 

 apparently to the result of some physiological weakness. It is 

 difficult to distinguish the two forms by external examination, as 

 the appearance of the foliage is practically identical. True 

 Silver leaf however is characterised by a dark discoloration of the 

 wood when seen in cross-section, and a ready peeling off of the 

 upper epidermis of the leaf. 



The principal features of the True Silver leaf are as follows : 

 The normal green colour is hidden by the presence of air spaces 

 under the cuticle, which causes a reflection of the light, and 

 brings about the silvery appearance. There is usually no reduc- 

 tion in the number of chloroplasts. As a result of the dissolution 

 of the middle lamella of the cell walls of the leaf, the leaf tissues 

 easily fall apart when the leaf is sectioned. The dissolution of 

 the middle lamella is perhaps b brought about by the production 

 of some diffusible poisonous substance during the metabolic pro- 

 cess of the fungus and 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 enzymes capable of 

 dissolving* the pectie substances of the cell-wall. Affected plants 

 moreover nearly always show continuous streaks of dark brow r n 

 discoloration in the woody tissues of the stem, branches, twigs or 

 roots, which when examined microscopically, reveal the presence 

 of exceeding fine filaments of S. pitrpureum. Branches 

 suffering from True Silver leaf, when placed in water for 48 hours 

 cause the discoloration of the water, the degree of discoloration 

 depending on the severity of the attack. 



Initial infection takes place above ground and also immediately 

 below the ground level, and so far as the present investigations 

 are concerned, the fungus can only gain entrance through open 

 wounds. Recent inoculation experiments (the results of which 

 w^ill be published later) confirm the view that injured superficial 

 roots can be infected; the frequency of infection probably depends 

 on the variety of the stock used for grafting. Localised silvering 

 of a branch results from local infection, which lias not yet spread 

 to the main stem. Silvered suckers originating from a healthy 

 tree indicate root infection, whereas the occurence of healthy 

 suckers from diseased plants points to crown or stem infection, 

 since the mycelium has not yet readied the root system. The 

 following plants have been examined, all of which suffered from 

 True Silver leaf: "Victoria Plum " (and the stocks upon which 

 it is usually grafted, e.g., the " Brompton 7 ' and "Common 

 Plum "), Prunu$ luritanica, P. triloba var. fore piano* among 

 Apples the varieties "Lord Grosvenor " and " Bramley's 

 Seedling," Spiraea japoniea var. glabrata, Nevivsio alabamensis, 

 Ribes cereum, Laburnum alpinum and L. vulgare. Although 

 fructifications of S. purpureum have not been noticed in Ribes 

 cereum and Spiraea japoniea var. glabrata, yet the hyphae ob- 

 served in the tissues were identical with the hyphae of that fungus 

 grown in artificial cultures. In view of the frequency and the 

 disastrous effects of True Silver leaf, and its increasing world- 

 wide importance, the author of this paper recommends the ex- 



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