140 Contribution to Knowledge of Silver- Leaf Disease 



part of these leaves so that these changes remind us here and there of 

 the structure of the gall in pear leaves caused by the mite Eriophyes 

 piri, where the strong growth in length of all the mesophyll-cells together 

 with the great enlargement of the intercellular spaces is the well-known 

 symptom of this phytoptosis. But the cells of the spongy parenchyma 

 in the silvering leaves do not remain together a long time; they fall 

 asunder easily, as Giissow and Brooks state. Whole groups of the cells 

 are disintegrated in this way which is the result of the dissolution of the 

 middle lamellae. As is known a marked increase in size in the epidermal 

 cells may also be observed. This is shown in the bulging of the walls 

 of the epidermis towards the palisade cells. The volume of these cells 

 is therefore greater. The tearing off of the epidermis from the palisade 

 cells is very striking. Either the epidermis is simply detached and 

 somewhat elevated so that a closed cavity is formed in this place, or 

 the epidermis is quite torn asunder and the free portions are slightly 

 lifted up from the palisades. Both this formation of cavities and the 

 tearing of the epidermis are very common but not necessarily always 

 present on all the areas attacked. A similar case was described by 

 Miehe (vide 8, p. 478) as follows : " Epidermiszellen, die von Synchytrium 

 Taraxaci infiziert worden sind, wuchern gegen das ihnen anliegende 

 Mesophyll vor und drangen es beiseite u. dgl. m." 



As for the cavities in the walls of epidermal cells which Percival (12) 

 describes as a symptom of Silver-leaf disease I can state that I have 

 looked for them in vain throughout an extensive series of prepara- 

 tions. In this respect my results agree with those of Giissow and 

 Brooks. 



The phenomenon of silvering of foliage which is the only external 

 symptom of this disease is said to be due to the accumulation of air 

 in the above mentioned subepidermal cavities, which interferes with 

 the normal reflection of light. For instance it causes the white colour 

 of the young Bryum argenteum leaves and of the white flower-petal, 

 etc. I have had reason to doubt the adequacy of this theory and so 

 I endeavoured to decide its value by the following experiment. The 

 question is not so simple as it seems at first sight. The following 

 experiment should dispel any doubts: the strongly attacked portion 

 of a leaf was cut into small pieces which were then immersed in water 

 in a vessel connected with an air-pump. Into the same vessel were 

 put small healthy pieces of another shape — so as to distinguish them. 

 These tissues were then completely injected with water. If the white 

 colour is solely due to the air in the subepidermal cavities, it should 



