257 



Silvering is more common, according, to my observations, wliere 

 tlie drainage is bad and where there is a deficiency of lime in 

 the soil. If the trees are top-grafted on an unsuitable stock, 

 the chances of the occurence of false Silver leaf appear to be 

 increased. 



The following table indicates the distinction between true and 

 false Silver leaf: — 



True Silver Leaf. 

 (Cause. — Sterenm pitrpureum.) 



1. Easy separation of the tissues of 



the leaf, resulting from the dis- 

 solution of the middle lamella; 

 usually no reduction in the 

 number of chloroplasts. 



2. Discoloration present in the xylem 



or,other tissues of the branches, 

 stem and roots, appearing as 

 continuous streaks, very seldom 

 in scattered patches. 



3. Branches placed in water for 48 



hours discolour the water after 

 48 hours. In those cases where 

 normal branches discolour the 

 water, the coloration is differ- 

 ent in the case of diseased 

 branches. 



4. Fungal hyphae always present in 



the stem or root, but so far not 

 discovered in the leaves. 



False Silver Leaf. 

 (Cause. — Physiological Weakness ?) 



1. No separation of the tissues of 



the leaf; the chlorophyll is 

 reduced in amount. 



2. No regular discoloration in the 



stem, but it may occur as small 

 localised areas, never in con- 

 tinuous streaks. 



3. Branches placed in water for 48 



hours do not discclour the 

 water as compared with normal 

 branches. 



4. No fungal hyphae in any tissue of 



the plant. 



It should be noted that it does not necessarily follow that in 

 every part of a true silvered plant all four distinguishing 

 characters would be met with. When dealing with plants which 

 in normal conditions discolour water, care must be exercised that 

 branches or twigs of equal size are taken and the capacity of the 

 vessel considered. 



been 



of 



False Silver leaf ** by any writer on the subject. Brooks, 

 however, observed a plant of Lamium album exhibiting Silver 

 leaf with a shrivelled epidermis, which could easily be separated 

 from the mesophyll cells, but he concluded that the silvering in 



;*> omild not be attributed to Steven rm vurvureum. Brooks 



ca 



also draws attention in his second paper to the confusion of 

 Silver leaf with Chlorosis when dealing with applications of iron 

 compounds to restore normal growth. It is therefore very 

 difficult to comment on the work of those who hold the physiolo- 

 gical theory of the cause of Silver leaf, but according to the 

 work of Prilbuix (1), Sorauer (2), Alderhold (3), and Massee (5), 

 it is obvious from their descriptions that they were dealing with 

 True Silver leaf, although these writers invariably interpreted 

 the disease as of non-parasitic nature. A considerable amount 

 of material is required for successful investigation, particularly 

 fair-sized twigs and branches, since the hyphae only in extreme 

 cases approach the terminal bud r£ the current year's growth. 



B 



