J. Smolak 141 



disappear after injection. The leaves were cut in quite small pieces 

 (about the average size of 2-3 mm. square) so that the water had easy 

 access to the cavities. After the injection all the segments were carefully 

 dried between filter paper and arranged in line, the attacked injected 

 with the attacked non-injected and similarly the healthy injected with 

 the healthy non-injected. Of course all the leaf-segments swelled up 

 in the water. The white colour of the attacked segments was seen 

 to have disappeared somewhat but not entirely so, for the green colour 

 of the injected segments never equalled the bright green of the normal. 

 The former always were a little dim. And moreover a piece of an 

 attacked leaf loses its grey colour somewhat even without injection, 

 viz. when it has merely been immersed for a short time in water. It 

 became less and less till after about 25 minutes. It seems then that 

 the existence of abnormal air spaces will not account completely for 

 the silvered appearance of the leaves. It is very striking also that the 

 spreading of the silvering always began first of all on and around the 

 vascular bundles (veins) and from that it spread over the surface of 

 the leaves. We can perceive in nature itself from the leaves only slightly 

 attacked and also on looking at the photograph of infected twigs given 

 by Gussow (3, p. 386) that the silvering of the leaves very often starts 

 from the region of the veins. 



On the above and the following grounds one must then be sceptical as 

 to the "silvering" being really due to the accumulation of air in special 

 subepidermal cavities : (a) it is a striking fact that the phenomenon 

 of "silvering" spreads over the blade very often from the veins, above 

 which the epidermis is very seldom separated; (b) the subepidermal 

 cavities are not necessarily always present everywhere in the silvered 

 leaves ; (c) the contents of the epidermal cells and a certain disorganisa- 

 tion in the mesophyll (see below) can hardly fail to have an effect on 

 the coloration of the foliage. 



II. 



Before dealing with the cytological changes in the mesophyll cells 

 of the diseased leaves, one may consider a typical cell of the healthy 

 mesophyll of a leaf of Prunus domestica var. "Victoria." Here we 

 notice the simple normal relations of the cell content which can be 

 observed in every healthy leaf. In the clear transparent cytoplasm 

 lie the chloroplasts, ellipsoid in form, most commonly close to the cell- 

 walls. They usually contain many starch grains which we can easily 

 recognise in preparations stained by the "inverse" method. By other 



